Friends of the Pleistocene Field Trip 1996 South-Central Cell

April 19 - 21, 1996

Upland, Lowland, and In Between - Landscapes in the Lampasas Cut Plain

Co-Sponsored by

Texas A&M University and

Baylor University

EXPLAN"nON

AthNiat lJn~5 Age Iy, e p I FO Ford 400·plesenl WA West Range 4200·600 FH For1 Hood 8000-5000 OT GeOfgetown 11 ,OOO·BOOO JA Jackson 15,000

TA Tlench T1 Landlorm ~ Rovally paleosol I"'TIT'I Othel soil

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Table of Contents

Lampasas Cut Plain: Episodic Development of an Ancient and Complex Regional Landscape, Central Texas o. T. Hayward, Peter M. Allen, and David L. Amsbury ........................ 1-1 - 1-97

Soils and Geomorphology of Cowhouse Creek, Fort Hood, Texas Lee Nordt ..................................................... 2-1 - 246

Amino Acid Racemization Analysis of the Chronology and Integrity of Archaeological Sites in Central Texas Glenn A. Goodfriend. G. Lain Ellis. and James T. Abbott ....................... 3-1 - 3-13

Archaeology at Fort Hood David L. Carlson ................................................. 4-1 - 4-21

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L C P 1 Day 96

Episodic Development of an

Ancient and Complex Regional Landscape

Central Texas

O. T. Hayward, Peler M. Allen, David L. Amsbury

FRIENDS OF THE PLEISTOCENE

South Central Cell 1996 Field Trip-Central Texas

April 19, 1996

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Publlshed by Friends of the Pleistocene, David L Carlson, Department of Anthropology, Texas A & M University, College Station, nc, 77843-4352

AUTHORS

O. T. Hayward, Department of Geology, Baylor University, Waco, TX., 76798-7354, (817) 755-2361; e-mail, O_T_Hayward@baylor.edu

Peter M. Allen, Department of Geology, Baylor University, Waco, TX., 76798-7354, (817) 755-2361; e-mail.Dr.Alien is illiterate.

David L. Amsbury, NASA Johnson Space Center, Code SN 15, Houston, TX., 77058, (713)-483-5160; e-mail, amsbury@snjsc.nasa.gov

We solicit your Interest, comments, observations, condemnations, vituperation, whatever.

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THE CRETACEOUS PRAIRIES OF TEXAS

While the principal interest of this trip is in the geomorphic region of the Lampasas Cut Plain, it is also important to appreciate that the Cut Plain is but a subprovince of the of the far larger Grand Prairie of Texas, itself only a part of the Cretaceous Prairies of Texas (FIG. A-1, A-2).

To the earliest visitors, Texas was known as " ...... a land of prairies .... : (Roemer, 1848), grasslands and savannah extending from the coast to the most distant reaches then explored. But among the most beautiful and productive of all of these seemingly-endless grasslands were the Cretaceous Prairies of Texas, developed on the outcrop of Cretaceous Rocks, a widening band of distinctive landscapes extending from the Rio Grande to the Red River (Fig A-1)

Two major subdivisions further divide the Cretaceous Prairies; on the west the Grand Prairie, on the east the Black Prairie. On a smaller scale as north-south trending belts within the Grand and Black Prairies are yet smaller subprovinces, each differing from those adjoining, each on different geologic foundations.

THE BLACK AND GRAND PRAIRIES The major determinants in the geomorphic differentiation of these Cretaceous Prairies from all the other Texas prairies, and also for the existence of subprovinces within the Cretaceous Prairies, are structural and lithologic differences in underlying bedrock. The Grand Prairie, westernmost of the Cretaceous Prairies, is developed on Comanchean (Lower Cretaceous) rocks, mostly near-horizontal beds of limestone with lesser sands at base and top, exposed on the Central Texas Platform. The Black Prairie, the eastern subdivision of the Cretaceous Prairies, exists on Gulfian (Upper Cretaceous) rocks, principally great thicknesses of near-homogeneous mudstones with subordinate chalks, dipping gently eastward into the western margin of the East Texas Basin (Fig. A-3, 0-8)

On a smaller scale, and within both the Grand and Black Prairies, lesser subdivisions (Fig A-4), are distinguished one from another on the basis of landform, soils, and vegetation, each reflecting differences in underlying stratigraphy and geologic structure.

Since the emphasis of this trip is on the Lampasas Cut Plain, a subprovince of the Grand Prairie, the remainder of this introduction is devoted largely to the Grand Prairie Subprovince of the Cretaceous Prairies of Texas. Of the Black Prairie subprovinces, the Eastern Crosstimbers, the common border of Grand and Black Prairies, is given added consideration (Fig A-4, 0-4).

THE BLACK PRAIRIE The Black Prairie, at its widest some forty miles across, extends from the Red River to San Antonio, continuing as a narrow band to the Rio Grande. In the area of its greatest width, it consists of four distinctive subprovinces, each reflecting a lithologic difference In the underlying bedrock. Of these Black Prairie subprovinces the only one of immediate Interest to us on this trip is the westernmost, the Eastern Crosstlmbers (Fig. A-2, A-4).

I.he Eastern Crosstlmbers We encounter the southern terminus of the Eastern Cross Timbers at Waco. At its southern end the dark, narrow band of Crosstimbers woodland extending northward from Waco to the Red Rivers is developed on sands and clays of the Quaternary alluvial deposits of the Brazos River. From Hill County northward, it is formed on the outcrop of the sandy Woodbine Group (lower Gulfian). Throughout this length, it forms the common border of Grand and Black prairies (Fig. A·4).

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Figure A·1 The Cretaceous Prairies of Texas are developed on the outcrop belt of Cretaceous rocks. Thus they comprise one of the largest of Texas geomorphic subprovinces, and in early day Texas perhaps the most distlnct/ve of Texa; landscapes.

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Figure A-Z The Cretaceous Prairies are further divided into Black and Grand Prairies, the Black Prairie on Gulfian rocks, principally muds; and the Grand Prairie on Comanchean rocks, prinCipally limestones. The Black Prairie is further divided into five smaller subprovinces, and the Grand Prairie into four, each subdivision reflecting differences in the underlying bedrock.

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Figure A-3 The landscape of the Cretaceous Prairies is dependent upon composition and structure of the Cretaceous bedrock. The Grand Prairie is on near-horizontal limestones and sands of Comanchean age. The Black Prairie is on Gulfian muds and chalks dipping eastward into the East Texas Basin. Where each differing lithic unit crops out, it forms a band of recognizable landscape, unique to that unit and different from those to either side.

WICHITA PALEO PLAIN BLACK PRAIRIE

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Figure A-4 Subdivisions of the Grand and Black Prairies reflect differences in the underlying bedrock. From Waco northward, the Black and Grand Prairies are separated by the Eastern Cross-timbers, on sands of the Woodbine Group, basal unit of the Gulfian section, and hence westernmost of the Black Prairie subprovinces. From east to west within the Grand Prairie, the subprovinces are (1 )the Washita Prairie on rocks of the Washita Group; (2) the Lampasas Cut Plain on rocks of the Fredericksburg group; (3) the Glen Rose Prairie, a narrow band along the westem margin of the Lampasas Cut Plain, and as windows within the Cut Plain; on rocks of the upper Trinity Group, and (4) the Western Crosstimbers, on sands of the basal Trinity Group.

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To early visitors the densely wooded Crosstimbers was so different from the near-treeless open grasslands on either side that it became the principal marker on all early maps and emigrant guides (irving, 1835, Ikin, 1841; Kendall, 1844). It remains a conspicuous geomorphic element to the present day.

The Eastern Crosstimbers separates the Grand Prairie, a subdued but angular landscape of mixed-grass and savannah on Lower Cretaceous "hard lime-rock", from the Black Prairie, gently rounded "jello-mold" landforms of treeless tall-grass prairie on soft Upper Cretaceous blocky "joint clay".

Throughout its length the Crosstimbers is veneered by deep, peach-colored, sandy soil derived from the sands and muds of the underlying Quaternary Brazos Alluvium and the Upper Cretaceous Woodbine Group. It supports a dense post oak forest, with lesser amounts of blackjack oak and other deciduous trees, ani:J an almost impenetrable deciduous undergrowth. The result is a narrow wall-like band of woodland in the midst of the Cretaceous Prairies. For those immigrants of the last century, mostly from the southeastern states, this woodland was a momentary return to the forests of all their previous experience. It recalled home and fireside, while the prairies to east and west were strange, inhospitable, woodless, waterless, threatening worlds.

THE GRAND PRAIRIE Abruptly abutting the Eastern Crosstimbers on the west is the Grand Prairie (Fig. A-2, A-4). The common denominator for all Grand Prairie landscapes was the underlying lower Cretaceous rock foundation. Yet within this common inheritance were sufficient lithologic variations to create distinctive geomorphic subprovinces. .From east to west, the Grand Prairie is divided Into four geomorphic subdivisions, (1) the Washita Prairie, (2) the Lampasas Cut Plain (the landscape of our major interest), (3) the Glen Rose Prairie, and (4) the Western Crosstimbers (Fig. A-3, A-4). Each of these subdivisions is briefly described in the following paragraphs

The Washita Prairie We see the Washita Prairie at Stops 1, 2, and 3 of the present trip. Formerly it was in mixed-grass prairie, now it is in farm and grazing land. The Washita Prairie consists of broad, gently rolling upland grasslands with streams in shallow but angular valleys marked by gallery forests. Typically the prairies are also dotted with isolated trees, live oak mottes, and small mesquite savannas, associations early described as the most beautiful of the Texas Prairies. This landscape is developed on limestone and shale of the Washita Group (Comanchean, Cretaceous), lower Washita limestone (Georgetown) cropping out mostly on the western Washita Prairie, upper Washita shale (Grayson Marl) exposed mostly on the east. Therefore from west to east the landscape varies in relief, and the soils vary in depth, in fertility, and in water holding capacity, depending on whether their inheritance was from limestone or shale.

The eastern part of the Washita Prairie was originally mixed-grass prairie dominated by Little Bluestem and was almost devoid of trees (Kendall, 1846; Roemer, 1848; Poole, 1964). Though tie richest parts were in farmland from early in the era of settlement, a great deal more was broken to the plow during the First World War and into the early 1920s. It remained largely in farmland until the late 'twenties and early 'thirties, to be abandoned as marginal land during the farm programs of the pre-World War II New Deal. Much of it has since reverted to secondary grassland and savannah. The best of the soils, those of the eastern Washita Prairie, on Grayson Marl (Del Rio Clay), remain in farmland today.

The western Washita Prairie was also originally grassland, but the soils were generally thin and rocky, formed from the limestones of the lower Georgetown Formation (Washita Group, Comanchean), and the vegetation included juniper savannah and even dense juniper brakes

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along rocky hllisiopes. Still, within this limestone-dominated environment are several significant clay horizons, and on the outcrop bands of each of these, following the contour of the land, one can often see bands of Little Bluestem marking these more hospitable soil-environments. Thus over the western Washita Prairie the grass tended to be shorter and less abundant, savannah was far more common, tree bands and mattes marked the poSitions of perched water tables, and bands of Little Bluestem followed the outcrop lines of thin but persistent clays. Because of limited productivity, the western Washita Prairie has tended to remain more as it was in the beginning, though each year woodlands expand at the expense of prairie.

The Lampasas Cut Plain The Lampasas Cut Plain is the geomorphic subdivision to the west of the Washita Prairie, and the central and largest subprovince of the Grand Prairie (Fig. A-3, A-4, 0-4). It is also the landscape that will dominate the view at all the remaining Stops of this trip, Stops 4 through 17 (NOTE For reasons later explained, Stops 9 through 15 have been omitted from this trip). The Cut Plain is a far more rugged land than the Washita Prairie, of buttes and mesas with distant and dramatic views of broad grassland valleys separated by narrow wooded mesa-like divides, almost the whole carved in rocks of the Fredericksburg Group (Comanchean, Cretaceous).

The term 'cut plain", coined by Robert T. Hill in 1900, describes landscapes everywhere that have characteristics he saw so clearly demonstrated here in the Lampasas Cut Plain;

A cut plain (dissected plain) is a stratum plain of any kind which has been so dissected Into remnants by erosion that the level of the overall stratum plain is still recognizable in the summits of the dissected members (Hill, 1900, p. 6).

Within the Cut Plain, the dramatic views of diverse and distant landscapes were, from the beginning of European exploration, a common experience and an attractant to settlement. This subprovince clearly reflects the stratigraphy and structure of the underling Fredericksburg rock­foundation (Fig. A-3, 0-2, 0-9). The flat-topped mesas and divides are defended by the near­horizontal, thin but massive, highly erosion-resistant Edwards Limestone (uppermost Fredericksburg Group). On this flat upland surface soils are rocky and thin, and vegetation is dominated by juniper and postoak. Still, on top of larger divide lands, particularly near the eastern margin of the Cut Plain, remnants of the Washita Prairie remain, veneered with productive soils derived from the Kiamichi Clay (lowermost member of the Georgetown Formation, Washita Group). Where these clay-derived soils are present even divide-crests, once upland prairie, have long been in farmland.

Slopes from the Edwards caprock to valley-floors are carved In the white, chalky, nodular limestones of the Comanche Peak Formation, next underlying the Edwards caprock (Fig 0-6) .. Soils on the steep Comanche Peak slopes are stripped by erosion almost as soon as they are formed, the slope-microclimate is arid, and juniper brakes typically surround almost every mesa and divide.

The valley floors, far the largest part of the Cut Plain landscape, are formed on the Walnut Clay, the formation that next underlies the Comanche Peak Limestone. While the Walnut Clay is not all clay, it is far more clay than anything else, and that has had a marked effect on landform, soils, native vegetation, and land use. In the eyes of the earliest beholders, both Indian and European, the Cut Plain valleys provided an almost "Garden of Eden" aspect. These broad valleys constitute an area of major interest on the present trip, and the locus of most towns, farms, roads, barbecue joints, and coffee stops in the whole of the Grand Prairie.

The Glen Rose Prairie But not all Cut Plain valleys are formed entirely in Walnut Clay. Centered in the broad basins of the western Cut Plain, occupying low, rounded in-basin divides, and existing also as long southeastward-narrowing tongues within most the entrenched valleys

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of major drainages (particularly in the northern and western parts of the Cut Plain) are rugged, inner-valleys carved into Glen Rose Limestone (of the Upper Trinity Group, Fig. 0-9). These landscapes on the outcrop of Glen Rose rocks constitute the Glen Rose Prairie, the smallest subprovince of the Grand Prairie though in reality most of it is surrounded by Edwards-capped buttes and divides within the region typically described as Lampasas Cut Plain.

This more rugged, rockier, somewhat stair-stepped landscape of basin-center and river­entrenched valley systems was originally in mixed-grass, savannah, and thin oak and juniper woodland, all developed on the thin soils and varied rock foundations of the Glen Rose Limestone. Thin, rocky soils and highly dissected landscapes generally precluded agriculture over most of the Glen Rose Prairie, and it has therefore tended to remain as grazing land. With the control of fire, the area of prairie has diminished and savannah and woodlands have encroached, but the broad view from uplands still shows a clearly delineated Glen Rose Prairie, within the broad expanse of the Lampasas Cut Plain.

The Western Crosstlmbers The western boundary of the Glen Rose Prairie, and generally also of the Lampasas Cut Plain, is the westernmost subprovince of the Grand Prairie, the Western Crosstimbers (Fig A-4). This is a more extensive, and more diffuse counterpart of the Eastern Crosstlmbers, gentle wooded landscapes of sandy soils, in marked contrast to the lands, vegetation, and soils of the remainder of the Grand Prairie. The Western Crosstimbers is developed almost entirely on Twin Mountains and Antlers sands of the lower Trinity Group (Fig 0-9) These earliest Cretaceous sand, silt and clay beds underlie the "hard lime-rock" formations of the Upper Trinity, Fredericksburg, and Washita Groups of the rest of the Grand Prairie and are the substrate for the sandy soils of the Crosstimbers.

In the years of exploration and settlement, the western Crosstimbers was more open than the Eastern Crosstimbers. Because the underlying rocks contained substantial sections of clay and sandy clay, there were numerous glade-like prairies within the belt of woodland and savannah. These islands of prairie within the sea of woodland became early targets of immigrants who appreciated the combination of wood, water, and easily tilled soil typical of these glades.

For those western-bound from the southeastern states, as most Central Texas immigrants were, the Western Crosstimbers was a second reminder of a distant, eastern, wooded and watered home. To those continuing on westward, it was also the westernmost and last of the Cretaceous Prairies. The contrast was enormous. Beyond the Crosstimbers lay the Red Rolling Plains; of sparse grassland, of brush land, and of streams with grand names but little water, mostly gyp and mostly bad. And mile by mile to the west prospects became ever more grim.

From the western margin of the Western Crosstimbers almost to the Great Valley of California, lands became more arid, wood less plentiful, soils less rich, water less abundant, and life more difficult (Marcy, 1854, p. 85). The effect on westward-bound immigrants was profound. This was noted particularly in letters from women, wives and daughters of westering families, who remembered with longing the distant watered world so many would not see again. It would be Interesting to know how many of those continuing westward ultimately turned 'round to return to this favored land-this last reminder of the eastern world they had left so far behind (Hayward, Dolliver, Amsbury, Yelderman, 1992, p. 8-9)

THE CUT PLAIN FIELD TRIP The present field trip consists of ten stops extending from Waco through McLennan, Bosque, Hamilton, Coryell, and Bell counties, to terminate at Lake Belton Dam, near Belton, Texas.

The stops are arranged in two major "packages". Stops 1 through 3 show the processes and landform of the initial establishment of Cut Plain drainage as an inheritance from the

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disappearing Washita Prairie. Streams on the Washita Prairie followed an apparently "normal" and orderly evolution. from (1) valley Inception on the Washita Prairie (Stop 1). (2) entrenchment through Georgetown (Washita) rocks into the Edwards Limestone (Fredericksburg) to Impress an incipient valley-pattern onto a Lampasas Cut Plain yet-to-be (Stop 2). (3) valley deepening and widening through entrenchment and slope retreat through the Comanche Peak Limestone (Fredericksburg) (Stop 3) prior to (4) the greater valley widening in the less erosion-resistant Walnut Clay (not yet encountered. but to be seen as Stop 4) that will typify the Lampasas Cut Plain. The processes of entrenchment and valley-widening that resulted in these varied landscapes were originally perceived as continuous. and the evidence of their activity should be visible on the time-scale of a human lifetime. This perception is questioned in the later stops.

Stops 4 through 17 are examples of processes and landforms In a Cut Plain that fails to fit easily as the next logical step in the orderly. simple sequence outlined above. The reality of Cut Plain landform leads to a series of questions and speculative answers indicating a far more complex and far longer history than that suggested by our first "package" of stops. and brings Into question the traditional evolutionary sequence previously described ..

The sequence of landforms and the tentative chronology of landscape evolution resulting from that sequence describes the grand design of the Lampasas Cut Plain. A particularly interesting peripheral aspect Is that a number of landscape elements. and the tentative landscape chronology developed from the Lampasas Cut Plain. appear to be represented far beyond the boundaries of the Cut Plain. and across the full width of the Cretaceous Prairies. Speculatively. the major elements of that landscape and chronology may well exist in landscapes of much of the southern Midcontinent. and even into the eastern ,Basin-and-Range of Texas and northern Mexico (Amsbury and Hayward. 1996) ,

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THE LAMPASAS CUT PLAIN NOTE: The following guide to the Lampasas Cut Plain is an adaptation of an earlier Geological Society of America Guidebook (Hayward, Alien, Amsbury, 1990), designed for a two-day trip across the full width of the Cut Plain. This present guide for a one-day trip across the eastern half of the Cut Plain we believe "covers" most of the points emphasized In that earlier trip. In addition, In the "Conclusions" section of the present guide we attempt to bring you up to date on accomplishments (mostly speculative) that postdate the earlier volume. When you read these latest conclusions, you will readily see why we encourage your observations, your questions, and even your enthusiastic participation In a geomorphic project that has vastly outgrown our own capabilities and calendars.

AN INTRODUCTION At the turn of the century, Robert T Hili, the first to work extensively in this unique region, and the one who gave it its name, best described its landscape:

"The Lampasas Cut Plain is the modified northern extension of the great Edwards Plateau. It is a greatly dissected dip plain, now recognized by the general level of its many remnant summits, which dominate all of the country south of the Brazos, between the Western Cross Timbers and the Balcones Fault Zone. These summits, which are called mountains by the inhabitants, are numerous remnantal circular flat-topped buttes crowning the divides between the drainage valleys. Life and industry are mostly found in the valleys scored below the summit level." (Hill, 1901, p. 78)

Hili, the first to study the Cut Plain, had as his principal interest the Cretaceous stratigraphy, but he never lost sight of the land. Few since Hill have appreciated the landscape of the Cut Plain more than did Hill, who attributed his interest in geology largely to that landscape. No one since Hill has thought quite so much about the evolution of that landscape, though most of Hili's comments on geomorphology were reserved for his newspaper columns in his later years.

Hili worked without adequate maps or aerial photographs, and in his broad reconnaissance, time and Inclination did not permit detailed assessment of landscapes, processes, and rates of change. His interpretations were largely intuitive, but it was an intuition guided by immense knowledge and a lifetime in the field, the most productive years of which were spent in the Cut Plain. Later authors (proctor, 1969; Mikels, 19n) have generally agreed with Hill's model of landscape evolution, and have thus confined their interests largely to problems of process in the Cut Plain. Hill's concept of Cut Plain evolution-and the one generally accepted-can be summarized under five major conclusions.

First, the Cut Plain is an intermediate stage in the evolution of the Grand Prairies (Fig. 0-4)

Second, the shape of the Cut Plain is dictated by the contrast in physical properties between Edwards Limestone and underlying and overlying formations (Fig. 0-5).

Third, the processes of slope retreat and landscape evolution in the formation of the Cut Plain involve the infiltration of precipitation through the Edwards, its emergence as seepage at margins of Edwards uplands, the undercutting of Edwards by spring sapping

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Figure 0-4 The Lampasas Cut Plain lies between the Brazos and Colorado Rivers, and Is drained largety by the Brazos River and its tributaries. Cut Plain landscape, dependent on the geomorphic effect of Edwards Limestone, is confined to the outcrop belt of that formation. Similar Edwards-dominated landscapes include those of the Callahan Divide, and of the Edwards Plateau (for which the Brady Mountains mark the northern margin). Other ancient Edwards-dominated landscapes are buried beneath the Southern High Plains (from Hayward and Allen, 1987, 1988).

and subsequent failure by slumping, leading to almost continuous slope retreat and valley widening (Fig. 0-5).

Fourth, the Cut Plain has migrated eastward through time, as the Grand Prairie landscape moved slowly down dip from some distant westward margin of Edwards deposition (Fig. 0-6). Northward, beyond the margin of Edwards reef-like deposition, the Cut Plain merges with the Fort Worth Prairie (Hill, 1901, p. 77), a gentler, more rounded landscape, but with numerous reminders of Cut Plain physiography

Fifth, stages in Cut Plain evolution include (a) initiation of dissection onto the Edwards Limestone, (b) Initial entrenchment through the Edwards Limestone, (c) rapid entrenchment and valley widening through the Comanche Peak Limestone and into the Walnut Clay to create the typical profile of narrow divides and broad valleys, (d) the ultimate stripping of Fredericksburg rocks and the consequence disappearance of the Cut Plain as the Grand Prairie retreats eastward

These original conclusions have survived, largely Intact, again chiefly because they are intuitively acceptable to one seeing the Cut Plain for the first time, or seeing Cut Plain evolution as an aside in the progress of other work.

On this trip we will examine each of the widely accepted concepts of Cut Plain evolution to see how they hold up to a more analytical view, with emphasis on landscape rather than stratigraphy. More recent interpretations are based on observations and data unavailable to earlier workers, or unnoticed because to casual view they are concealed by the far more obvious elements of the landscape.

Toward that end, this trip is presented as a series of vignettes which together form a panorama of episodic Cut Plain evolution. At each stop, the panorama widens, and at the end of the trip there will be a summing-up describing what has been seen, and what It appears to mean in terms of landscape evolution.

The trip is designed to emphasize five points; (1) the geological foundations of the Cut Plain; (2) some details of modern processes of stream behavior and underground water, that must also have contributed to the formation of the Cut Plain landscape; but that in significant ways fail to explain all, (3) the formation of the Cut Plain from an earlier regional Washita Prairie landscape; (4) a marked change in landscape formation midway in the orderly history of the Cut Plain; and (5) a proposed sequence of stages, and a very tentative chronology for that sequence In Cut Plain evolution, based on field relationships.

GEOLOGICAL FOUNDATIONS perhaps nowhere in Texas are stratigraphy, structure, and landform more closely Interrelated than they are In the Cut Plain (Fig. 0-2, 0-6, 0-9). So obvious are these relationships that for many years they worked as an impediment to real understanding of Cut Plain evolution. Appearances suggest that each landform element reflects absolutely the simple stratigraphy and the near-horizontal structural attitude of specific stratigraphic units so that no other explanation is needed or justified. Evidence presented on this trip Indicates a far more involved history. To understand the Cut Plain we must first understand the general Interrelationships of structure, stratigraphy and landscape.

Structure and landform Geologic structure in the Lampasas Cut Plain is principally that of the Trinity Shelf, the most stable part of the Central Texas Platform (Parrish, 1995; ; Hayward, 1988; Lemons, 1987).

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An exhumed aub-Crelaceous landscape I~ eastward, down dip, as erosion lowers the landscape. An observer (of enormous exposed 8S Crelaceous rocks ere slrlpped patience) at any point on the eastern margin of the Washita Prairie would see a away succession of landscapes pass in review, first the Washita Prairie, then the Lampasas Cut

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Plain, the Glen Rose Prairie, the Western Cross Timbers, and eventually landscapes in Paleozoic rocks (from Hayward , Allen, AmsbUlY, /990).

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FIgure 0-6 The generally accepted model of Cut Plain evolution, Note the close correlation of stratigraphy and landform. In this model, the landscape is graded to present drainage, slopes retreat by failure of the Edwards Limestone as seepage erosion undermines this massive caprock, the divide-to-basin slope break occurs at the Comanche Peak/Walnut contact, and slope retreat is a continuous process (from (from Hayward, Allen, Amsbury, /990).

Near its eastern margin, the Cut Plain shares structural characteristics of the far western margin of the East Texas Basin (Fig. 0-2, 0-7, 0-9).

From the western margin of the Cut Plain to the eastern divide of the Leon River Basin (our Stop 7, a distance of more than fifty miles, the dip increases from ten fUmi to twenty five fUmi. Recent studies indicate that even in the areas of gentle dip there are small, step-like descents in the regional near-homoclinal dip. Detailed studies of small areas also show minor crenulations in structural contours (Amsbury, 1995; Parrish, 1995).

A major increase in dip from twenty five fUmi to fifty fUmi occurs in the transition zone between Trinity Shelf and East Texas Basin, over a distance of less than twenty miles near the eastern margin of the Cut Plain. The disappearance of the Cut Plain to the east is a product of this increase in dip, as the Fredericksburg plunges beneath younger Comanchean and Gulfian sections (Fig. 0-8).

The Cut Plain province is almost free of visible surface faulting, but there are several "belts' and regions in which lineaments are conspicuous on Landsat imagery, aerial photos, and on drainage patterns on topographic maps. Lineament zones appear to have been localized by older structure in Paleozoic rocks, possibly reactivated by subsidence of the East Texas Basin. These appear to have had significant effect on the orientation and position of major drainages and Cut Plain divides, while having almost no influence on minor tributaries. Other lineaments, more distant from the known fault zones but equally localized, show little relationship to recognized structure or existing landform (Parrish, 1995).

Because the gentle eastward regional dip is essentially' equal to the mean eastward gradient of the Cut Plain (measured from divide elevations), the Cut Plain is both very broad, and

remarkably uniform in landscape over the whole area of gentle dip. Near the east margin, where the dip Increases, the Cut Plain disappears to be replaced by landscapes developed in younger rocks as the dip continues to steepen into the East Texas Basin (Fig. 0-8), In succession, the Washita Prairie, Eastern Cross Timbers, Black Prairie. and East Texas Timber Belt (Fig. A-3).

Stratigraphy and landform The Grand Prairie is developed in rocks of the Trinity, Fredericksburg and Washita Groups (Fig. A-3, A-4, 0-2, 0-9) This Comanchean section thins westward through depositional thinning and erosional truncation, from about 1160 feet thick in Central Hill County where it disappears beneath Gulfian rocks to 300 feet thick near Goldthwaite at the western margin of the Grand Prairie (Fig. 0-8) (Lemons, 1987, p. 37).

The Trinity Group: Over most of the area of the Cut Plain, the Trinity Group (lowermost Comanchean) consists of two formations, (1) the Twin Mountains Sand, and (2) the Glen Rose Limestone. Along the northern and western margins of the Cut Plain where the Glen Rose formation is missing through pinchout, Trinity rocks are represented by the Antlers Sand, of combined Trinity and lower Fredericksburg deposition (Fig. 0-9). The outcrop belt of Trinity rocks is represented by two geomorphic provinces; (1) the Western Cross Timbers, and (2) the Glen Rose Prairie (Figs. 0-2, 0-3, 0-6, 0-9).

The Western Cross Timbers (Fig. A-4, 0-2, 0-4), is the broad belt of woodland and savannah, of low relief, that is veneered in sandy soil derived from sands and sandy muds of the Twin Mountains Sand or the Antlers Sand. Where the Glen Rose Limestone is present in the section the Western Cross Timbers is separated from the Lampasas Cut Plain by a belt of Glen Rose Prairie, developed on Glen Rose Limestone (Fig. 0-2, 0-9). Along the north and west margins of the Cut Plain, where the Glen Rose is absent through pinchout, the Western Cross Timbers adjoins the Cut Plain (Fig 0-8, 0-9).

1-18

The Glen Rose Prairie is generally included in the Lampasas Cut Plain, where it forms a conspicuous dissected lowland landscape of prairie and savannah within the upper basins of larger southeastward-flowing streams. These inner valleys of the northern and western Cut Plain are cut into Glen Rose rocks, and veneered in shallow stony soil formed from the alternating dense limestones, chalky limestones, and marls of the Glen Rose Formation.

The Fredericksburg Group: The Cut Plain is the remarkably conspicuous geomorphic expression of the Fredericksburg Group, consisting of four formations, (1) Paluxy Sand, (2) Walnut Clay, (3) Comanche Peak Limestone, and (4) Edwards Limestone (Fig 0-9). Buttes and divides are capped by the thin but resistant Edwards Limestone. Upper slopes are formed in Comanche Peak Limestone. Upper valley floors are developed in Walnut Clay-and in the northern Cut Plain, in Paluxy Sand. Inner valley entrenchments are formed in Glen Rose Limestone-and to the margins of the Cut Plain, in Twin Mountains sand. The lower three formations of the Group thin to the west, so Cut Plain total relief also diminishes westward. Larger areas of the westernmost basins are in Glen Rose and older formations, yet the Cut Plain landforms are highly distinctive all the way to the western margin of the province (Figs. 0-2a, 0-2b).

Washita Group: In the area of the Grand Prairies, the Washita Group, youngest of the Comanchean Series, consists of the Georgetown Limestone and the Grayson Marl (Del Rio Clay) (Fig. 0-9). The Washita Prairie, the geomorphic expression of the outcrop of the Washita Group, is therefore subdivided into the Georgetown and Del Rio Prairies. Only the Georgetown Prairie is significant to our study of the Cut Plain, since the Del Rio Prairie is largely confined to the Eastern Washita Prairie, and therefore distant from the eastern margin of the Lampasas Cut Plain. The Georgetown Prairie, formed on the alternating chalky limestones and marls of the Georgetown Limestone, is typically gently rolling prairie, cut by shallow by angular, rock-walled streamways. It was originally almost treeless prairie of generally thin and stony soils, capping the divide areas of the eastern Cut Plain, and forming the foundation of the Washita Prairie geomorphic province that bounds it to the east (Fig. 0-2)

1-19

Wlchlta-Arbuckle-Ouachlta Trend

Zone of gentle subsidence. Bay floor frequently exposed, shoreline rapidly oscillating.

~~ /'-'----

/ /1'. _- r!

/ S Fort Worth ~'f'

Shelf st~ble and mostly '/.,~<c I generally submerged . non-manne. / i/'«; I / /

Gj I ~ '" r--t'J'-;I---,--;\-' L._.L----, ~ I

Brownwood 'lit

Llano "Island" or "peninsula"

Miles

East

f 50

Figure 0-7 Map of the Trinity Sh elf at the time of deposition of Trinity rocks. The Shelf lies to the west of the dashed line, the western margin of the East Texas Basin lies to the east. The western margin of Trinity marine deposition Is deeply crenulate, a product of the rldge-and­valley topography on the sub-Cretaceous surface at the time of Trinity deposition (From Hayward, 1988).

1-20

WEST EAST

High Plains Callahan Divide Grand Prairie Black Prairie

--------

\ Gentle east dip of Trinity Shell, 2 to ~ fUmi. Dip increasing from 10 to 100 ftlmi into western margin East Texas Basin

Callahan Divide·llke landscape buried. beneath High Plains deposits

Figure O-S Diagrammatic profile and section, showing the gentle regional dIp and regional thickening of Comanchean section, from the High Plains, along the Callahan Divide, across the Cut Plain, and into the East Texas Basin. There are questIons about the correct depositional model for Edwards Umestone (one would result in a very gentle southwestward initial dIp, the other an essentially horizontal initial dip), but this disagreement would not significantly change this picture. In either case, the gentle eastward dIp is not explained by deposition, but is a product of post-Edwards structural warp (from Hayward, Allen, Amsbury, 1990),

w E

< ~

i ., < ~

" • ., EDWARDS

" 2", COMANCHE PEAK

"'II: Z w:> < ffiOl w

:: '" u ... z

< ~ 0 u

~, : ~

.... :.;;::-:;; ,-t-PALUXY-'-' -? _ •..

.?,::' .. ::. U. GLEN AOSE

~NTLEA8-- _,OLE.N ROlE THORP PRINOS ::;-- =="'- -, -, >-:~:". • OLEN ROSE i

ii: ~ TWIN MOUNT AINI ',:' .

.;:: . .. . .--' .. .. ..... III!NH-TAIAIIIC ,

FIgure O-g Comanchean stratIgraphic nomenclature on the Trinity Shelf. The landscape of the Cut Plain Is developed In rocks of the Fredericksburg Group. Rocks of the Washita Group are confined to uplands in the eastern Cut Plain, and to the Washita Prairie, which bounds the Cut Plain on the East. Trinity rocks are exposed in the inner valleys of the central and western Cut Plain. Beyond the pinchout of the Glen Rose Limestone is the physiographic province of the Western Cross Timbers, developed on Antlers Sand (Fig, 0-4) (from Hayward, 1988),

1-21

STAGES AND PROCESS IN EARLY CUT PLAIN FORMATION

STOP 1--BRAZOS-BOSQUE DIVIDE--The "Time Zero" Surface (31 0 41' 44" Nj 970 23'19" Wj Valley Mills Quadrangle, 1/24,000, 1979) (Fig. 1-1, 0-1, 0-2)

Along our line of traverse from the Brazos River to this point are occasional exposures of mid· Cretaceous rocks (Georgetown Limestone, Washita Group (Fig. 0-9) in small roadcuts along the highway, and as rubble in fields and pastures to the right and left of the road. Near the Brazos River the Washita Prairie is seen in a degree of dissection encountered only near major streams. Terrace veneers, common near the Brazos River, have been absent from the highlands the last few miles to Stop #1.

Here on the divide between the Brazos and Bosque rivers (Fig. 1-2), where the gentle landscape is typical of much of the Washita Prairie highest divide-lands, terrace lags are again evident, but these are different from the better· represented terrace veneers along the Brazos. Here on the highest divide is a record of river history which antedates the highest terraces of the Brazos (see Fig 17-3), and which also antedates the Lampasas Cut Plain, the principal emphasis of this trip. The landscape upon which these gravels were initially deposited by streams unrelated to modern drainage has been called the "Time Zero Surface" (Montgomery, 1986) of Cut Plain evolution. It was the surface immediately antecedent to major incision into Fredericksburg Rocks.

This is an important point.

The history of the Cut Plain clearly began before there was a Cut Plain. Drainage nets, established on Washita Prairie (such as we see here) or even earlier landscapes, were the first to be incised into Fredericksburg rocks. With that incision, the Cut Plain first became a distinctive landscape. About eight miles north-northwest of this locality is the headwaters of Childress Creek, which flows southeastward to join the Brazos just north of Waco (Fig. 1-2). its basin is almost entirely in Georgetown rocks, while in its lower reaches the channel floor is just now beginning to cut into Edwards Limestone, the first stage in formation of a Cut Plain landscape.

Of critical importance to the history of the Cut Plain is that major tributaries such as Childress Creek tend to be dip-controlled, and to elongate rapidly up dip by processes which involve both groundwater and overland flow, a point to be discussed later. Obsequent tributaries tend to be short, giving larger basins an obvious asymmetry. Trunk streams tend to migrate down dip on rock surfaces, and to a lesser degree, to be driven down dip by clastic input from dip-controlled, steep-gradient tributaries. Valley lengthening by streams is thus accomplished both by major headward migration of tributaries, and by lesser, lateral (generally down-dip) migration of trunk streams.

The Brazos River is the major trunk stream toward which flow almost all Lampasas Cut Plain steams In the region of today's trip (Fig. 0-4). Prior to Cut Plain incision, drainage was radically different in both organization and bedload from present systems. But by the beginning of Cut Plain inciSion, an ancestral Brazos In about the same pathway as the present river,

1-22

))

Figure 1·1 (Valley Mills Quadrangle, 1124,000; 1979) Location of Stop 1 on the high divide between the Brazos and North Bosque rivers. This locality is in the Washita Prairie, of thin rocky soils, and mixed grassland and savanna. It is on the outcrop belt of Georgetown Limestone, and typical of many high divide localities in the Washita Prairie, the soil surface is covered by a thin veneer of alien lag gravels, coarse, vel}' well rounded, siliceous, apparently remnant from the "Time Zero Surface", an ancient surface from which was formed the landscape that now surrounds us. Streamways we see present on this Washita Prairie surface will be impressed into the rocks that will ultimately form the Lampasas Cut Plain, the next stage in the slowly-changing landscape. of the Grand Prairie.

1-23

':Z.

) \. Washita ~ '1 ,/,

STOP 1.... \.- . ' .. ' Prairie 1 t.... -......... , 1 )... -"'\ \

Whitney

'\. '. I " 'I." \ \ \ · .... ,l'I.-______ -tQuartzose pea gravei in auger 'to ~,,\ .\ '1 ¢. hoies all aiong divide

.,. -':, \ '\ ~\ "~. ,p,' I I,' '0:

..... ,;:,~, \'-\" :1 _' ... "' ........ &

\ ; ·--_.'-.c:;"':.·" ~ " -'''''~~'' '\ ~ : \ '. ~" ...... '-..

\ '-'-"" .". Eastern margin \ .~ c;;., ' ..... \., e""p.f.

Cut Plain <1>' -,,,,::::_, -, 7'-:."!I...._....j ' ... ::::'-.1". ~-..... ltI.' ..

Valley Miffs ., -. ,-0-I

miles

5 I

Figure 1-2 Basin of Childress Creek in relation to the Brazos and Bosque Rivers, and the eastern margin of the Cut Plain. Childress Creek flows generally down dip into the Brazos River. Where the stream is on the Edwards Limestone, gradient and dip are equal. Exotic, quartzose gravels on the divides are alien to Central Texas, and mark an earlier episode in landscape--the "Time Zero" surface of Cut Plain evolution. The eastern margin of the Cut Plain, here marked by the eastern divide of the North Bosque Basin, is everywhere abrupt. There is no transitional landform (from Havward. Allen. Amsburv. 19901 WEST

"' ~ e " ., 'tI ;; is c .. .c .!! a; u.

=

c a; ii:

= u

"' .. "' .. a.

--------------==== 3

"' Gi > .. -CD ..

-.: a; -a. .! :c "' ..

== '7Z-- ------ ---..-_ ~-- c- . -

1-24

EAST

"' Gi > .. -a. CD -.. a. u -."' .. u ... .'" u

0 0 -.!! !! :c --;:: ..

u

-------~-----,

EAST TEXAS BASIN

apparently flowed at a level from two to four hundred feet higher than at the present time, on Late Comanchean and Gulfian rocks which have since been partially stripped by erosion.

Initial Cut Plain dissection was by streams similar to the North Bosque River and Childress Creek (Fig. 1-2), inherited from an earlier Washita Prairie landscape. Thus the factors controlling drainage evolution on the Washita Prairie were important to the beginning Cut Plain geomorphology. The surface upon which ancestral Washita Prairie drainage was established was apparently this "Time Zero" surface, a remnant of which is yet present here at Stop 1. In addition to the exotic gravel lag seen here, several shallow auger holes along the northern divide of Childress Creek Basin have encountered quartzose pea-gravel lags in stony, caliche soils (Fig. 1-2), always at elevations far above the highest dated terrace of the Brazos. Similar quartzose gravels have been described from major Cut Plain divides to the west and southwest (Smith, 1984); on the crest of the Callahan Divide between Colorado and Brazos headwaters drainages (Byrd, 1971; Knapp, 1990); and eastward on high divides in the Blacklands and East Texas Timber Belt (Knapp, 1990; Amsbury, 1990; Spencer, 1991) (Fig. 1-4).

Sampling has been too limited to establish clearly the topography of the original Time Zero Surface (or surfaces) on which these fluvial sediments were deposited. The high gravels show considerable diversity in size and petrology, and occur at various elevations above modern drainage. The Time Zero Surface may have been an alluvial plain; but if so, there were also favored trunk routes across that plain (Spencer, 1990), and each of these had local tributary networks, reflected in differing ratios between locally-derived and exotic gravels. West-to-east drainage was accomplished by a few larger streams. These originated far west, beyond the present High Plains, and were served by local tributary networks that supplied abundant Fredericksburg cherts, and farther east, abundant Woodbine orthoquartzites (Flowers, 1987; Tharp, 1988; Montgomery, 1986). This surface, whatever its configuration, was the "Time Zero" surface of our discussion--the landscape that immediately preceded the Lampasas Cut Plain.

The Cut Plain was formed after reorganization of drainage originally on this ancient surface. Drainage reorganization was In response to southward structural tilt during Tertiary episodes of subsidence in the East Texas Basin and its northern borderlands, or to other factors as yet unrecognized. The pattern of reorganized drainage, somewhat askew with present regional dip, may in part be a key to older drainage yet visible in the Cut Plain.

The age of these highest gravel lags of Stop 1 is uncertain, but they are clearly older than the highest terraces of the Brazos, and they reflect a time when the pattern of Central Texas drainage was radically different than it is today. Attempts to date this beginning of Cut Plain evolution are tenuous at best, and call for the correlation of these gravels with similar gravels in other areas, but the problem is that we do not know whether we deal with one gravel or many. There are gravels atop the Callahan Divide far to the west, and to the east detrital gravels on high divides in the western margin of the East Texas Basin (Fig. 1-3). Both are quartzose, highly mature with similar but undistinctive cobble petrology, and both have ferruginate matrices. There the correlation ends.

The high gravels of the Callahan Divide are clearly much older than oldest Ogallala, for they are encountered more than 200 feet above the feather edge of Ogallala deposits where the eastern margin of the High Plains onlaps the toe of the Divide, at Roscoe (Nolan County).

When the Callahan gravels were deposited, the Callahan Divide, then the lowest part of the landscape, and the Time Zero surface, was the site of deposition of coarse fluvial gravels from far to the west (probably from the Southern Rocky Mountains). The present gradient from the

1-25

Ancestral Washita Prairie or alluvial plain

Time Zero Surface

NORTH SOUTH

Time Zero Surface

_____ ?~ __ ~Qaa9~O~9"Q~O~_\~ _______ r'-------______ ~S?Q~~C?~a~e~CM~ ___ ?-

Georgetown

Edwards I

Comanche Peak ;4 \ "

Figure 1·4 Block diagram and section across an ancestral Washita Prairie, showing prominent west-east drainage avenues. The profile shows an almost-undissected surface, and the suggestion of an alluvial veneer. Whether the streams of the pre-Cut Plain era flowed in gentle valleys cut into the Time Zero surface, or on an alluvial plain is not yet determined. Stop 1 is on a remnant of the Time Zero Surface, the landscape from which the Lampasas Cut Plain would ultimately be formed (from Hayward, Allen, Amsbury, 1990).

1-26

Callahan Divide deposits to the Central Texas deposits is about 10 ft.lmile., a gradient somewhat greater than that of the present High Plains surface ( ± 8 feeV mi.).

In East Texas no rocks of appropriate composition and texture, and predating Ogallala deposits (late Miocene/Pliocene) of the High Plains have been recognized. Additionally, there is ccnsiderable question whether the high gravels of the Cut Plain are truly correlative with those of the Calahan Divide, though their elevations seem to be about right (Fig. 1-3). A pre-Late Miocene, dissected "Calahan Divide" (or perhaps even mature Cut Plain) appears to be preserved beneath the Ogallala Formation along the eastern margin of the High Plains (Cronin, 1969; Seni, 1980; Gustavson and Finley, 1985 Fallin, 1989)). suggesting that a mature Cut Plain in that location is at least as old as mid-to-Iate Miocene.

To clarify a point that may be thoroughly ccnfusing by this time, you must remember that the Ogallala of the Southern High Plains is an enormous fan-like deposit. It pinches out where it onlaps the lower slopes of the Calahan Divide near Roscoe. At that point the Calahan divide rises 200 feet above the surface of the High Plains. From that point the Ogallala thickens to the north and west, and at Fluvana (about 50 miles northwest of Roscoe) it has completely buried a Callahan Divide-like landscape, now exhumed by retreat of the Caprock Escarpment, the present eastern margin of the High Plains.

In summary, the Time Zero surface is of uncertain age, though clearly older than late Miocene. By late Miocene there existed a Cut Plain-like landscape In the area now occupied by the eastern High Plains. It was later buried by the late Miocene/Pliocene Ogallala deposits. The Calahan Divide jOins the Cut Plain north of Brownwood (Brown County) where marked similarities clearly Indicate that at that point both are one, and both participated in the same pedimentation episode that led to the formation of a pediplain (King, 1962. p. 161). the one that we have called the Comanche Pediplain ..

Visualize this pediplain not as an areally extensive surface, but as a complex of local in-basin pediments, each individual pediment confined to a single drainage basin, but all together forming a stem-and-ieaf-like Joined-pediment surface. Each individual basin pediment was graded to a Junction with the next higher-order stream. The common base level for all individual pediments was the major trunk stream . At the time of pedimentation the ccmplex surface thus formed was the Comanche Pediplain, at a base level then much above than that of today's drainage network.

EARLY EVOLUTION INTRODUCTION TO CUT PLAIN DEVELOPMENT

Stops 2, 3, and 4 (Fig. 2-1) The next three stops (Stops 2. 3, and 4) illustrate stages in the earliest development of the Cut Plain. Assuming non-competitive systems with uniform climate. lithologies and available relief. such a sequence appears reasonable (Faulkner. 1974).

Two orders of major drainage are apparent in the Cut Plain.: (1) the main trunk streams. such as the Brazos. Leon. Bosque, and (2) iower order tributaries to this main system. such as the Middle Bosque River. Hog Creek. and Coryell Creek. among others. Streams of the first category In the Cut Plain flow at angles to the regional dip (Fig. 1-4). These streams presumably established their ccurses long before Cut Plain dissection began.

The streams of the second category align along the dip of the Edwards Limestone (Fig. 2-2). Their patterns may have been determined by sapping. i.e. groundwater control of drainage (Fig. 2-3). A combination of weakening by solution. followed by removal during overland flow is

1-27

believed responsible (Bunting, 1961): (1) stratal dip and regional groundwater flow patterns parallel stream flow (Yelderman, 1987); (2) the dominant direction of drainage dissection seen on maps and in the field is up-dip and up-gradient; and (3) amphitheater-shaped areas with associated seep zones are present at the headwaters of many streams (Fig. 2-3).

Valley cross-profiles In the basins of Hog Creek (Stop 2) and the Middle Bosque River (Stop 3) the major characteristic of the landscape is the dominance of divide areas over valley areas. Most of the land is in interfluve on Washita Prairie. Only in the immediate vicinity of stream courses is the deeply dissected, Lampasas Cut Plain landscape dominant. Thus, for these stream basins the principal landscape is that of the Washita Prairie, in marked contrast to much of what we will see in the remainder of the Cut Plain (Brown, 1988)

At Stops 2 and 3, valley width and side slope formation are clearly related to stream downcutting. But at Stop 4 (Neils Creek), the side slopes have retreated far from the stream, and the vastly greater distance between the Edwards capped hillslopes is apparent. This marks a major break with the simple model suggested at the beginning of this guidebook.

Past theories of Cut Plain evolution explain valley widening solely by lithologic control; that is, once the resistant Edwards Limestone is removed, the much-less resistant underlying rocks are quickly eroded to form broad valleys. But the transition from narrow valleys (Stops 2 and 3) to wide valleys (Stop 4) is everywhere abrupt. There are no intermediate examples. We believe that the present landscape of the Cut Plain was formed in part, if not largely, during a major period of side-slope retreat by pedimentation along streams that previously had cut through the Edwards Limestone. Evidence for this conclusion will be presented throughout the field trip; some evidence can be seen within the Cut Plain and evaluated in the field by the participants, but other evidence comes from surrounding regions and can best be presented during discussions.

A plot of valley width versus drainage areas (Fig. 2-4) shows two major populations of Cut Plain streams. "Type I" streams were still flowing on the Georgetown formation (Washita Prairie) during the period of slope retreat.. "Type II" are those streams which had breached the Edwards prior to the period of slope retreat. Pedimentatlon in the valleys of Type I streams would therefore be less apparent, owing to the behavior of the more easily eroded Georgetown Formation.

A second comparison of basin parameters, valley width versus width-to-depth ratios of main stem channel incision (Fig. 2-5) indicates: (I) a remarkable correlation of slope retreat with degree of incision and (2) that the two groups (I and II, identified in the previous figure) are stili apparent. These relationships (Brown, 1988) support the hypothesis that Cut Plain drainage and landscape modification Is not simply the effect of the work of streams on lithologies of varying resistance. The evolution of the Cut Plain is perhaps more appropriately tied to three major factors: (I) major controls on local stream base levels that Influenced the degree of Incision and stream extension, (2) the comparative level of incision of these streams with respect to the Edwards Limestone during a period of equilibrium in stream grades and major valley side-slope retreat and pediplaination, and (3) groundwater flow direction and volume, and its Influence on stream incision and slope retreat (Fig. 2-3).

Phases of Cut Plain Evolution The substitution of spatial variations for temporal ones has been a popular geomorphic strategy to solve problems in the long term evolution of landforms. Patterns of landform evolution have been inferred either directly or Indirectly on the assumption that time and space are interchangeable under certain circumstances (Knighton, 1984).

1-28

'I • I

V' \ v:

/~) [

I ( ('" . )

I 1(. K~~:;. ! l/

/",,-\.~

Iw· . , • If mdm.1I

J~. "" ) L /

,/

, •

\

/' /-<

';". ~

"')\' ' . . .':

1'-

".

;vl ,I 865>:.

,.,-. /( \ \\

'.'1' l. (

, ,

Figure 2-1 (Coryell [1955] Quadrangle, 1124,000) Stops 2 and 3, Hog Creek and the Middle Bosque River, two stages of ,a' incision. antecedent to development of the Cut Plain. While these localities are in the Washita Prairie, the streams are

-;, entrenched through the Edwards Limestone, These are progressively-later examples of Washita Prairie incision than that ", N' I

shown at Stop 1.. While, locally, stream valleys have taken on some of the attributes of the Cut Plain, the regional landscape I ) I

is still Washita Prairie. and will remain so for a very long time. However, stream networks inherited from the stage of \) \' Childress Creek, here amplified, are the first to be incised into the forming Cut Plain. Notice that while these two localities j'

\ apparently show significant dissection of the region, the Washita Prairie is still the dominant landscape, encompassing all but ~l I' tile immediate areas of the valleys. '!P. /

\ \ ~ /-----~I.. /1' \/ !~!"rill / ) r;(,Jtrr '~\ --- ~ ~?~ I I ) \ I ( ",<> II

~ .. ~

- •. $I,;.

f ... - .• - ... ~ ., . -"0 -"-..

.,.~ _.'

Strike of Edwards

~ I

I I ,. \ I T __ I t-

-"" I

-;.~" 1/ I

l­I \

-··-r I Major streams at angle to dip. Tributaries flow downdip, probably controlled by groundwater flow.

Figure 2·2 Comparison of directions of flow of trunk and tributary streams of the Lampasas Cut Plain. Notice that while the tributaries flow in dip·directed valleys, trunk streams flow in directions at significant angles to the regional dip. Tributary orientation is believed to be largely a product of dip-contrOlled, groundwater flow (Fig. 2-3). Trunk stream direction was apparently inherited from consequent flow at the time of establishment of modern drainage (from Hayward, Allen, Amsbury, 1990).

Flow Paths

/11 I 'I I I I I ' , I, I , I I I I I" I I I I I I I I I I ' , ' I I Time 1

I I I I I I I I II l"~! I I I, .. \..'-.. .t.ol.:;' .lLU.

Edwards Free Fae

- .... ~

Ordered reentrants mark spring exits

I I I I ' I ' ; I I I I T I I I I I I I I : II I : : I I I : " Time 2 I I I I ' ,If

.LU. ,t;-*. "LtLl Downdip flow perturbed by permeability variations

Valley elongation by spring action

Figure 2·3 Valley entrenchment by spring sapping. Groundwater flow is generally down dip (Time 1). However, locally subsurtace flow is perturbed by permeability differences. On margins of divides, this creates a series of orderly minor reentrants along exposed Edwards faces, with local more prominent, permeability-favored paths (Time 2). These favored pathways divert flow, leading to slow headward (up dip) erosion along selected routes, ultimately determining, at least in part, the pattern of later streams (Time 3) (from Hayward, Allen, Amsbury, 1990)

30

S c c - 20 )( -:::. .c: 'Ci

10 ~ >-.! OJ > a

• • • • .. E • ... -• ~.----- I .. ........ ........

/~ , / ",

I \ !! \

I .. , I z I

/ 1/ I :; /

\ ~ /'

'i 0

0 U //

\ . • /' G

" -'" ... , ..

'---~--- tI =. : o " > :z:.m-~ ,:;-.. . . ")

50 100 150 Basin Area (mi 2)

200

Figure 2-4 Plot of valley width versus basin area shows two families of Cut Plain valleys, Types I and II. While Type I and II valleys have similar basin areas, they have radically different valley widths, suggesting that basin area was not a significant factor in determining valley type (from Brown. 1988).

300 -I

u. Lampasas -:::. .c: ii.

200 .. 'D

Coryell --------+" --- • Neils Owl

>-.! OJ >

100

I I Valley wldlh

r Cave ~:]:?:6 M. Basque Bluff vary deplh

Hog

a 10 20 30 40

Valley widlh (II X 1000)

Figure 2-5 Plot of Cut Plain valley width versus depth. Again valleys of both Type I and Type II are clearly evident, and again they form two distinct populatlons--Type I with narrow valleys, and Type II with wide valleys. While all the valleys are of the same general age, Type II valleys went through a phase of valley widening in which Type I valleys did not participate (from Brown, 1988).

1-31

Following this logic, and based on flume studies and analysis of topographic maps (Glock, 1931; Parker, 1976), stream networks are believed to go through several stages of development:

(1) initiation (of a skeletal pattern) (2) elongation (by headwater tributary growth) (3) elaboration (through addition of small tributaries) (4) abstraction (as tributaries and relief are lost throughout time).

This process of drainage development, which apparently describes much of the reduction of Edwards-capped uplands during the valley-widening phase of Cut Plain evolution, is also the present sequence in which modern dissection is destroying the Comanche Pediplain (Brown, 1988).

STOP 2--HOG CREEK--The Cut Plain Is Born--(31 0 37' 22" N; 970 30' 44" W­(Coryell [19551, and Mosheim [19791, quadrangles, 1124,000) (Fig. 2-1)

Stop 2, on Hog Creek (Fig. 2-1) illustrates a stage in which the area in Washita Prairie upland is far greater than the area in entrenched valley. But throughout much of its upper basin, Hog Creek is now incised through the Edwards and into the Comanche Peak Umestone, in a drainage pattern inherited from the Washita Prairie stage of landscape evolution.

Recall that the earliest drainage network to be incised into the Edwards Umestone to form the Cut Plain was that which already existed on the Washita Prairie. Once Washita Prairie drainage had entrenched"into the Edwards, the stage was set for the formation of a Cut Plain, though the ultimate mesa-and-valley landscape was still concealed in a distant future. Initial incision and slope retreat probably were slow, so that the landscape (even adjacent to the deepening valleys cutting into the Edwards) remained Washita Prairie for a very long time.

Stratigraphy: Initial incision through the Edwards Umestone; the Georgetown limestone has been stripped from the immediate valley, but caps the interfluves (Figs. 2-6).

Relief: From top of the interfluve to stream level, relief is about 75-100 feet; the valley is dominantly v-shaped in cross-valley profile (Figs. 2-6).

Drainage: Drainage density (miles of channellsq mi of basin) is about 3.0 mllmi2. (Fig 2-7); drainage density is greater on the south side of the river than the north side; streams appear to fall within "initiation and elongation" stage (Fig. 2-7).

Slope Retreat: Slope retreat is minimal; cross valley distance between the Edwards free­faces is 200 feet or less (Fig. 2-6).

Soils: Flood plain soils are Frio Series silty clay loams; overlying the Edwards are terraces capped by Krum and Searsville clayey soils.

Drainage History: We infer that the river flowed just above (not on) the resistant Edwards Limestone prior to incision, based on the presence of extensive terraces,. Regional mapping suggests that Hog Creek and the Middle Bosque River flowed eastward across the White Rock Escarpment during that time to discharge into an ancestral Brazos River far below Waco (Fig. 4-6). If so the elevated Austin Chalk would have provided a local base level. Hog Creek and the Middle Bosque would thus have been more isolated from Brazos River Incision than the North Bosque, which discharged into the Brazos at Waco, or which for an extended period may have been the major pre-Brazos trunk stream at Waco.

1-32

If we visualize the valleys of Hog Creek and the Middle Bosque as early stages in Cut Plain evo­lUtion, it is natural to think of them as somehow younger than the more typical broad Cut Plain valleys we shall see on the rest of this trip. But Hog Creek and the Middle Bosque are "divide streams", flowing along the "divides" between the more deeply incised North Bosque and Leon rivers. Their patterns were established at the time of Cut Plain incision, and they were part of an original stream network, draining at least the basins they now drain. Bifurcation ratios clearly indicate that Hog Creek has lost basin to the encroaching North Bosque network (Proctor, 1969). The Middle Bosque may yet have much of its original basin area.

The lack of major incision in comparison to adjacent basins was apparently a product of lower gradient, and of inadequate basetlow. Stream basin area apparently had no significant effect (Fig. 2-4). For example, Hog Creek flows more than forty-five miles at an average gradient of about 13 ftlmi to fall some 600 feet before it joins the North Bosque River at Waco. At the time of formation of the Cut Plain, stream gradient between the headwaters divide and the White rock at Waco was about 9 ftlmi, and stream length may have been as much as fifteen miles greater than it is now. In addition, almost throughout the history of landscape evolution, Hog Creek has lost surface basin area to the larger and growing North and Middle Bosque Rivers, and consequentiy, surface water flow was reduced through time relative to those streams (Fig. 4-6), a point to be considered in greater detail at Stop 4.

Finally, the Middle Bosque was in a favored position to intercept groundwater flow directed down dip. Groundwater, the source for basetlow, was apparently essential to both incision and slope retreat in the developing Cut Plain. The reduction in baseflow to Hog Creek, resulting from interception of groundwater by the up-dip Middle Bosque River, very early limited the ability of Hog Creek to deepen or widen its valley. Similarly, baseflow to the Middle Bosque was intercepted by the updip Coryell Creek, and base flow to Coryell Creek by the Leon River. This factor may have had major effect, though the actual mechanics of basetlow solution-erosion are only vaguely understood. The role of groundwater in Cut Plain evolution will be pursued throughout this trip.

An increase in either gradient or volume could also radically change landscape and stream organization. At one point near Stop 2, Hog Creek is within one mile of the valley wall of the North Bosque basin, where it could fall one hundred fifty feet to the North Bosque drainage in a distance of about two miles. Capture seems imminent, and under conditions in which power will Increase greatly. Yet this situation has remained essentially unchanged for perhaps the past half-million years. Capture under similar conditions during very active slope retreat, aided by equally active groundwater flow may have been a principal mechanism of drainage reorganization in the then-evolving Cut Plain.

1-33

SOUTHWEST

Drainage reoriented, forms pattern for initial Cui Plain incision

NORTHEAST

High gravels Time Zero profile " I ___ ~-- ---;coo

~",qe.=-.!~! *~' .~-~ :7 ~~! .....--'-Washita Prairie

- -Georgetown

"'*. ~=f

Edwards -'---T-<--r--'--,.- ) ---, ! !

Comanche Peak

High gravels Beginning Cut Plain Washita Prairie

Washita Prairie profile f --------=~-~--J_ ~-

Georgetown - .. - -- '- ---- .e!~~= ~-

•

Edwards

Comanche Peak

~L 100 «

Figure 2-6 Block diagram. and section across Hog Creek valley at Stop 2. Notice That while In the immediate vicinity of the creek the profile is somewhat reflective of the Cut Plain. the region is still clearly Washita Prairie. However. once stream paths were clearly established on the Washita Prairie. only major structural tilt. or major incision in adjacent valleys could induce significant change in the principal stream-net (from Hayward. Allen. Amsbury. 1990)

1-34

_~ .... ~_........ __ J.. \ \ \ _ ... ~ \

'1~~i, ,,\uL~ ::!l!'-~ .- I I .. /.,:"\, 1 f \ :, """')\"', ~ I i .{ :' t: h \ i\ \ I (. I I I _----,

!! \ i. I ....... --

e\/i\,..O '-+--, .... '! ,

- 't-4'W\cS.e Miles

Figure 2-7 Drainage map and block diagram showing the effect of headward elongation of Hog Creek on tributary formation (see also Figure 2-3), Drainage density of a segment of Hog Creek Basin (taken from a 1162,000 scale topographic map) is about 3 mi/mf!· The difference in tributary length on north-facing and south-facing sides of the valley is a product of difference In antecedent moisture and slope direction (and hence insolation), On the block diagram, the unit cell is the minimum first-order basin area. notice that as the main stem elongates, the tributary network also elongates, This illustrates the stages of initiation and elongation (from Hayward, Allen, Amsbury, 1990)

\-35

STOP 3--MIDDLE BOSQUE RIVER--Drainage Elaboration And Extension, (31 0 35' 53"N; 97 0 33' 55"W; Coryell, 1955; Moseeim, 1979, quadrangles, 1/24,000) (Fig. 3-1)

A later stage in dissection··though not an 'older" stream .. is represented by the Middle Bosque River at Stop 3. Here drainage, locked into position when this was Washita Prairie, has not only cut through the Edwards Limestone, but almost through the underlying Comanche Peak Umestone. Valley widening has been extensive, almost entirely as a product of slope processes, but the width·to·depth ratio is not greatly different than that of Hog Creek at Stop 2 (Fig. 2·5). The landscape is still dominated by Washita Prairie, and entrenched streams, while far more deeply entrenched than at Stop 2, are yet minor elements in the Washita Prairie landscape.

Stratigraphy: The Edwards has been breached, as has most of the Comanche Peak, and the stream is flowing on the basal Comanche Peak Formation (Fig. 3·1).

Relief: It is about 120 feet from the top of the Edwards·capped interfluve to stream level (Fig. 3·1).

Drainage: Drainage density overall is about 2.95 mi.!mi2.; drainage density on the south side is less than on the north side (Fig. 3·3). This is interpreted as elaboration of drainage, cessation of extension on the north side; on the south side the drainage continues to extend (Figs. 2·2, 3·2).

Slope Retreat: With elongation and elaboration' of the drainage net, the Interfluves are being dissected from the east and west resulting in inner·valley widening to the north and south along the main channel. Total width of the main valley has now reached 1000 feet (Fig. 3·1).

Soils: Soils of the slopes consist of the thin Cranfils (formerly Bracket) Series, while the flood plain/colluvial apron consists principally of Frio silty ciay loams.

Drainage Evolution: Stops 2 and 3 form a very convincing time·sequence .. and a thoroughly acceptable model for the evolving Cut Plain, but their stream valleys are actually of about the same age. Valley deepening apparently progressed as a product of interception of groundwater flow and knickpoint migration from an entrenching Brazos River. Stream segments of greatest base flow, and during floods of greatest power, entrenched readily, leading to rapid upstream migration of knickpoints. Stream segments of lower baseflow and power were far slower to entrench.

1·36

Divides remain at original height

Washita Prairie

stili dominates landscape

Entrenched through Edwards and deeply through Comanche Peak

L ___ -- -_. __ .... Hog Creek profile

............ / -----;'

,----Georgetown .......... ----I

.L. Edwards

---.I'-~~~: r--, ~

-"

Comanche Peak

--'~ 100 "

Figure 3-1 Block diagram and section across Middle Bosque valley at Stop 3. The valley profile here is far more characteristic of the Cut Plain, but the broad divide regions are yet dominated by Washita Prairie. Hog Creek and the Middle Bosque have similar basin areas (Fig. 2-4), but the degree of entrenchment is markedly different. Basin area is not a significant factor in valley entrenchment, nor was time of capture (?) by the South Bosque River, since Hog Creek was the first encountered. The interception of groundwater flow by the Middle Bosque, up dip from Hog Creek, may be an explanation. Hog Creek, having lost basin to both Middle and North Bosque rivers, now has a vel}' long and narrow basin, also a factor (from Hayward, Allen, Amsbul}', 1990)

J!

o Milo. ~ ~ t--.... at",· ''I ......... 'a~ I 1 I \ 'f >-J 1,)(/\(1 \'1' 4\' 1 I ... ~ I . ., ,. \ '\ . i ( ~/; \

"'.1 y .. Y,l '" .. I 11/

'!i\UY1_ - .' :: :' \\ 0\ _ - 17.!" j j. ! I

\\tf\ ,... _ -"---r" :!... ,,,' J "" -........,. / .,..,. . '" .... _.- i ' .. ,. I

/" /"r _.'-i / ~._- .. -,- i - 7io- I ~ A l'/\: ' .. - I~ 1\ ' •• ,. .. \ { .." \ l' I ;; , A,", ....... J... _

\ ' ,:, .. -'. 10 -..,,;-__ ... "'JJ. _}

\ /" I J . ,-_.... :: , t' ....... - ..... ~ .... \08 I.§

'..... I 0'" ---..... I .... -

Figure 3-2 Drainage density of a segment of Middle Bosque Basin (taken from 1/62,000 topographic map) is again about 3 mi/mi2. This value appears to be a property of regional climate and stratigraphy. Here elaboration is accomplished by increase in valley width, not by increase in drainage density. Again the difference in tributary length on the north and south­facing sides of the valley is at least in part the result of differences in antecedent moisture as a product of slope direction. The block diagram shows the effect of elaboration on tributary formation and elaboration by headward migration in a part of Middle Bosque Basin. The unit-cell shown here is again the minimum first-order basin area, and has the same dimension as in the stage of elongation. Notice that as the main stem entrenches, the tributary network elongates and elaborates, with more complex tributary networks. This is clearly the stage of elaboration (from Hayward, Allen, Amsbury, 1990).

INTERRUPTION IN THE SEQUENCE

STOP 4--NEILS CREEK--Elaboratlon And Abstractlon--The beginning of the true Cut Plain--(31 0 42' 5" N; 970 37' 48" W; Hurst Spring [1956]. and Mosheim [1979] quadrangles, 1/24,000) (Fig. 4-1)

As streams approach grade with constant discharge, down cutting Is reduced. Valley widening continues independently, so width-to-depth ratios increase in an orderly sequence (defined by an almost hyperbolic curve) from initial incision to ultimate stripping. But here at Stop 4 that model fails. Immediately obvious is that while width-to-depth ratios have increased, it is not by the predicted continuous series, but by an enormous jump. There are no intermediate stages between that of Stop 3 (Middle Bosque River), and Stop 4 (Neil's Creek). Here the valley of Neil's Creek is typical of most of the Cut Plain. It is not an exception (Fig. 4-2). Here we are only about eight miles from Stops 1 and 2, yet we emphasize again that all these streams are about the same age.

The slow reduction of Washita Prairie to form Cut Plain (noted at Stops 1, 2, and 3) has suddenly become the total elimination of Washita Prairie. The landscape whiCh was dominated by uplands has become a landscape totally dominated by valley floor; Washita Prairie is confined to occasional small areas on narrow high divides, entirely in the eastern Cut Plain. Even there those small areas are insignificant. The Washita Prairie is gone.

1-38

Stratigraphy: The main channel is now flowing in the Walnut Clay; the Comanche Peak nodular limestone and Edwards limestone form a continuous slope, which rounds over at the top of the Edwards (Fig. 4-2).

Relief: Maximum valley relief from the top of the Edwards to the valley floor is about 160 feet (Fig. 4-2).

Drainage: Drainage density is about 3.5 milmi2.; drainage density on the south side of the main channel is considerably higher than that on the north side. The lower density on the north side is attributed to abstraction caused by reduced rates of downcutting and the presence of the Walnut Clay which has a different hydrologic response (Fig. 4-3). The greater drainage density on the south side is related to continued elaboration of the drainage in the Edwards dominated uplands (Fig. 4-3, 4-5).

Slope Retreat: Two phases of interfluve dissection are evident here; (1) to the north, the last remnants of the dissected interfluves are visible, representing the final phase of drainage dissection and slope retreat; and (2) we are presently standing on a retreating interfluve, similar to that seen at Stop 3. Total cross valley distance is now about 3,000 feet (Fig. 4-2, 4-5)

Soils: Soils are largely residual except along slope bases where Cranfils soils mark thick colluvial, caliche-dominated deposits, and on the elevated pediplain where Krum soils indicate old terrace remnants related to an episode of reduced downcuttlng or erosional still-stand.

Drainage Evolution: What we have seen at ·Stops 2, 3, and 4 are examples of the two families of stream valleys we have called valleys of Types I and II (Figs. 2-4, 2-5). They are clearly differentiated on the basis of width-depth ratios (Fig. 2-5). Type I valleys have width/depth ratios of 12 to 16. Type II valleys are characterized by width/depth ratios of 40 to 80 (Brown, 1988), and even higher (Dahl, 1987, oral communication.) (Fig. 2-5). The difference in ratios appears to be a product of stream history at the time of incision into the forming Cut Plain. Increased baseflow, and steeper gradient to major base-level, with its attendant increase in power, appear to have been the dominant control. Basin area was not critical, as indicated by field observations (Fig. 2-4, and Stop 8).

The broad valleys of Type II (Stop 4) have another common characteristic. Slopes leading down from Edwards-capped divides are not graded to present entrenchment, but to a surface much above present drainage and substantially below divide elevation (Fig. 4-2). Earlier this older surface was given the name "Intermediate surface", because that term had no genetic connotation, and we weren't sure what the surface represented. From habit, that name continued to be used (Hayward and Allen, 1987, 1988). In 1990 (Hayward, Allen, Amsbury, 1990) we reviewed the history of the Cut Plain (and by implication, far wider regions of the south central United States), and adopted as a newer name, "Comanche Pediplain", named for the rock Series on which it is developed, and the shape which it exhibits (King, 1962, p. 162).

Finally, as is evident from the profile of the far divide to the north, gaps appear for the first time as conspicuous elements in the landscape. The origins and Significance of these gaps to Cut Plain evolution is a recurring question throughout the progress of this trip.

1-39

,-..r,

l ,I

I i

/

'. . ,

-:

\ j

.'

/f l .. '_. __ ', .i") r. /.>~/' ( !/ /0

'\ . / ~( .'

Rgure 4·1 (Hurst Spring [1956}, and Mosheim [1979} quadrangles, 1/24,000) Neils Creek Valley, the Lampasas Cut Plain. In contrast to Stops 2 and 3, here the valley dominates the landscape, and the divides are reduced to remnants, Here for the first time we see the typical Lampasas Cut Plain--Type II broad valleys with narrow Edwards capped divides. Here, for the first time, we see an "intermediate surface" descending from the divides, but not graded to present drainage. This is the most conspicuous difference between Type I and Type /I landscapes, Hog Creek and Middle Bosque basins are entirely Type I valleys.

1-40

Plane of section

Cut Plain

Washita Prairie now gone

SOUTHWEST NORTHEAST

Streams entrench into pediment

Middle Bosque profile Comanche Pediplain

Edwards:t:;;;:=1>. ----1 ---- ) r-

, I \ ,

---- -

Colluvium ... ~

-""\" --Walnut --

\i ------~lQ~Q~Q~ft~--~

Figure 4·2 Block diagram and section across Neils Creek valley at Stop 4. The valley profile here is typical of the Cut Plain, and Washita Prairie is gone. The degree of entrenchment is slightly greater than in Middle Bosque basin (Stop 3) but valley width is far greater (Fig 2-5). Basin area (and hence flow volume) was apparently not a factor in valley-widening, for even very small tributary basins adjacent to our present location show similar profiles (from Hayward, Allen, Amsbury, 1990)

1-41

dissected pediment

Figure 4-3 Drainage density of a segment of Neils Creek Basin (taken from 1162,000 topographic map) is about 3.5 milmi2. The increase in density is a result of greater basin area in shales of limited permeability, of the Walnut Formation exposed on the Comanche Pediplain. This is the abstraction stage (note particularly the northern side of the valley) where increase in relatively impermeable valley area has resulted in loss of more permeable divide lands. Block diagram shows the effect of abstraction on tributary network as smaller tributary basins are lost to other more effective stream systems. The main-stem entrenchment is not greatly different from that of the Middle Bosque, but valley width is far greater, and the abandoned pediment is the dominant element of the landscape (from Hayward, Allen, Amsbury, 1990).

Groundwater In Cut Plain evolution In summary, evolution of the Cut Plain landscape was principally the result of groundwater sapping and drainage integration. The sequencing and spatial variability of these processes has been regulated by base level controls exerted on the system by the Brazos River and the range In permeabilities of the local stratigraphic sequence.

This hypothesis is based on three lines of evidence: (1) recent work on hydrogeology of this area (Cannata, 1988; Collins, 1989; and Myrick, 1989); (2) analysis of the evolution of streams and landforms in the region (Epps. 1973; Montgomery. 1986; Brown. 1988; and Tharp. 1988); (3) recent work on the Colorado Plateau (Laity and Malin. 1985).

Groundwater sapping is an erosional process that produces landforms with unique characteristics (Laity and Malin. 1985) (Fig. 1-5. Tables I and II). Perhaps the most important control on such processes is the dip direction of the beds relative to the direction of elongation of the stream valleys. since this controls the occurrence and distribution of groundwater seepage at the valley walls (Fig. 2-3) (Laity and Malin. 1985. p. 203).

The hypotheSized evolution of the drainage (Fig. 4-6a) involved three major steps; (1) a pre­Whiterock Escarpment stage of normal consequent flow of the Brazos and its major tributaries across the Cut Plain. in which the tributaries joined the Brazos below Waco; (2) a stage of capture of the major Brazos tributaries by the short. shale-subsequent South Bosque River along the northern face of the developing Whiterock Escarpment (Fig. 4-7b); and (3) the

1-42

subsequent "Isolation" of Hog Creek from groundwater supply through the interception of groundwater flow by the Middle Bosque River, up dip from Hog Creek (Fig. 4-7c).

Among the explanations for the unusual properties of Hog Creek Basin, this appears the most reasonable. Of the three streams (Hog Creek, The Middle Bosque River, and Coryell Creek) the Middle Bosque River has the largest drainage area, and the best developed tributary network, mostly on the up dip (western) side. Both Hog Creek and Coryell Creek flow in long narrow valleys, with very simple, largely first-order tributary networks (Fig. 4-6). It appears that the Middle Bosque River, near the up dip margin of the regional groundwater system was able to capture more of the available groundwater flow, and was more successful in extending its drainage for this reason. Coryell Creek, a stream which preserves much of the Cut Plain history, was apparently cut off from groundwater acquisition by the rapid entrenchment of Leon River, as the Leon cut through the Whiterock near Temple. Scarp retreat in the Leon Basin had already cut away the up-dip section that earlier supplied groundwater to Coryell Creek.

Evidence In support of this model of drainage evolution includes six components; (1) shapes, locations, and tributary networks of the three basins (Fig 4-6); (2) contrasting landforms of their inner basins, particularly the significant differences between the landforms of Coryell Creek, and those of Hog Creek and Middle Bosque (Figs. 2-6, 3-1, 4-2); (3) major springs entering the Middle Bosque from the southwest (up-dip) side, flowing from a diffuse aquifer (COllins, 1989) (Figs. 4-7, 4-8); (4) greater baseflow in the upper, more-dissected areas of the basin, where the aquifer is cut by numerous tributaries (Myrick, 1989); (5) basetlow contributions to the total flow than exceed storm runoff (Cannata 1988); (6) constant, main­channel valley-width with increasing drainage area, indicating that It is more dependent upon sapping than upon stream flow. Tables I and II summarize the attributes of basins of surface water origin and basins of groundwater origin. <

In the middle Bosque Basin (and contrary to intuitive expectations) baseflow In 1987 was found to comprise about 6.9 % of the total rainfall, whereas storm runoff accounted for only 5.3 % (Cannata, 1988). This indicates a major potential for groundwater sapping under the proper stratigraphic and geomorphic circumstances.

There Is no question that overland flow and related erosion is still a powerful mechanism in landscape modification of interfluves (Flowers, 1989) and stream channels (Baker, 1977) in the Central Texas region. But groundwater as a geomorphic agent in the Cut Plain has not previously been considered a major agent in landscape modification. Yet both geomorphic and hydrologic evidence seem to Indicate that it may well have been dominant during major episodes of scarp retreat and landscape modification.

The organization of early drainage The transition from Washita Prairie to Lampasas Cut Plain was originally thought to represent simple entrenchment through the Edwards Limestone to a generally graded profile. This entrenchment would be accompanied by rapid valley widening, until as incision diminished, the "normal" Cut Plain profile developed. By this interpretation the degree of development of a valley is one measure of its actual age (though other factors would, of course, be involved).

1-43

.., c "c c cOo

C Q= -.!:! in ~ U - c C) ~ ~ QI.Q U; - .CD .c .E--w "---<1:-

Hog Cr. M. Basque Neils Cr.

Figure 4-4 Stages in drainage network evolution and changes in drainage density in Hog Creek, Middle Bosque, and Neils Creek basins. Notice particularly the differences between north and south sides of the valleys, differences caused by factors related to slope direction. The slight Increase in drainage density during the abstraction phase is attributed to greater area of basin in relatively impermeable shales (from Hayward, Allen, Amsbury, 1990).

-'" E -! B ;e-o; 8 c " Q

4 II

'" .. 2 c C .. Q

0

c o 0; C II ;c W

c .2 iii .. o

.D .. iii

f---' Coryell Ct. I

lZ~ff C!.:. -. Owl Cr.

........... Cave Cr.

M. Bosque R. Hog Cr.

, I

c .2 U e iii .D <I:

INeils br. U~Lamp.

10 ro ~ ~ ~ ~ ro ~ ~ 1~ Valley width/Basin width

Figure 4-5 Stages in drainage development, Lampasas Cut Plain, as indicated by the ratio of drainage density versus valley width/basin width (valley width is distance between Edwards faces on opposite sides of the valley, basin width is distance divide-to-divide). Notice that as the basin matures and divides narrow, drainage density first increases and then decreases through stages of elaboration and abstraction (after Brown, 1988).

1-44

Hog Cr. M. Bosque

.' N. Bosque

, , . -N. Bosque

~og Cr.

...

• As

'.

;; -" ~ u " 0 :~ a: ::" M. Bosque .• , ., _ .... ' ....... ;: 3:

-", .... E .. ;::: .. ; . ,....... CJ "' ......

.:::i .....

,gl" .,.

Leon

Brazos

Leon

Brazos

Leon

Figure 4-6 Postulated early connections of Washita Prairie streams with the Brazos River. The South Bosque River, now an interceptor of Cut Plain drainage, may not then have existed, nor did the interposing ridge of the Whilerock Cuesta, developed on Austin Chalk. North Bosque River, Hog Creek, and the Middle Bosque River may have then flowed directly to successively lower points on the Brazos. Interception of this drainage by the South Bosque (a shale subsequent) affected first the North Bosque, and then In succession Hog Creek and the Middle Bosque. In the sequence of diagrams: Time 1, drainage antedates the formation of the Whiterock 'scarp; Time 2, at A the Brazos River cuts through the evolving 'scarp, at B the South Bosque River extends headward; at C the Leon River entrenches, cutling regional groundwater flow, and at 0 the favored Middle Bosque River elaborates; Time 3, at E, the Leon River continues to downcut, Coryell Creek extends and elaborates headward, Hog Creek, cut off from groundwater flow, extends without elaboration (Hayward, Allen, Amsbury, 1990).

1-45

TABLE 1

DRAINAGE EVOLUTION UNDER OVERLAND FLOW 1. Valley widths increase with distance from the valley head, and with stream order. 2. Valley heads are tapered, narrowest at the divides, widening downstream. 3. The longitudinal Profiles are usually concave. 4. Knickpoints in the profiles of tributaries match those of the trunk stream 5. A branched network of tributaries fills the drainage basin, evolving rapidly in the early

stages of development, and undergoing slow changes thereafter. 6. Direction of basin growth is related to surface landform, and fracture control. 7. Drainage lines may show prominent joint control where fractures are exposed at the

surface, but may show almost no structural control where such joints are deeply mantled with soil or debris.

TABLE 2

DRAINAGE EVOLUTION UNDER GROUNDWATER FLOW 1. Valley widths tend to remain constant, dependent upon sapping rate, and not on stream

volume. 2. Valley heads are steep, rounded, cuspate theaters, not tapered "vee" notches. 3. Longitudinal profiles tend to be linear, with the direction and gradient of the dip. 4. Runoff from the uplands is sufficient to account for large scale headfalls by waterfall

erosion. 5. Valleys extend slowly headward by sapping, ultimately to define a basin. There is large scale

variation in space-filling characteristics from basin to basin, depending on the relative ages of the systems.

6. Valleys grow in response to the hydraulic gradients of the groundwater flow. The direction of growth may mimic that of the joint-controlled streams where surface topography reflects subsurface flow.

7. Valley growth reflects the regional joint pattern even in debris-mantled basins. Joints at depth provide pathways for internally moving groundwater.

8. Stream flow derived from groundwater discharge increases steadily from the headwall to the midsection of the valley, and then increases only slightly to the outlet.

But recall that at the Washita Prairie (Stop 1, Hog Creek (Stop 2), Middle Bosque River (Stop 3), and Neils Creek (Stop 4), the point was made that all valleys are of the same true age, they flow in similar stratigraphic sections, and that basin-area is apparently not a major consideration (Fig. 2-4). The differences in development are attributed to different histories of origin, Involving base flow differences as a result of groundwater contributions, development of knick-­pOints, basin shape, and perhaps other factors as yet unrecognized. Thus rather than time being the dominant control on Cut Plain valley configuration, process was dominant, and time was secondary. This point was considered at each of the four stops.

Both Type I and Type II streams have similar ranges of basin area, but radically different valley widths. (Fig. 2-4). Even minor tributary basins of Type II streams show similar wide-valley profiles, with well-developed intermediate surfaces, and soil blankets similar to those of larger Type" basins. Thus the history of Type II trunk streams was shared by the total tributary networks of those streams. Type I streams remained largely in Washita rocks, and while they may have shared in a valley widening phase, the evidence was not preserved. Type" streams had already cut through the Edwards Umestone and participated in the valley widening episode, always in Fredericksburg section, which preserved the evidence of that event. (Brown, 1988)

Washita Prairie L.ampasas Cut I"ain

;~-~-~-:-~~~~~~tE~~ -----_ _ -".Sprlng "Edwards ,/ .

, , ' ....

~--::------Warer T.b'I""' .......... __ -----

Figure 4-7 Diagrammatic section showing transition between Washita Prairie and Lampasas Cut Plain. Perennial springs discharge near the margin of the Cut Plain where steep-gradient tributaries intersect the water table (from CoJlins, 1989). Runoff from the uplands and seepage from the margins of the uplands are particularly important mechanisms of scarp retreat, particularly on the downdip margins of the interfluves. Early in the history of slope retreat when the uplands were wide, both runoff and seepage were at a maximum. Antecedent moisture on the slopes was also at maximum Therefore, slope retreat was at a maximum. As divides narrowed through slope retreat, runoff and seepage diminished, as did antecedent moisture, and rates of slope retreat declined. Ultimately, as the divide narrowed to a width that no longer provided significant runoff or seepage,antecedent moisture diminished dramatically, and slope retreat then almost ended, sustained only by occasional intense rains falling directly on the slope faces. It was at that stage that, except for those periods of intense and frequent rainfall, landscape modification became almost dormant. This last stage is that of the present Cut Plain.

Also emphasized, a second feature of importance in the evolution of the Cut Plain landscape is the orientation of trunk (consequent?) drainage (Fig. 2-2). Two prominent stream orientations exist. Major trunk streams, such as the Brazos River, North Bosque River, Leon River, Cowhouse Creek, the Lampasas River, and a number of lesser tributaries flow southeastward, at a marked angle to the dip. Hog Creek, the Middle Bosque River, Neils Creek, Meridian Creek, and a number of other tributaries of trunk drainage flow generally eastward, down dip. This pattern of drainage is repeated throughout Central Texas.

An imaginary surface contoured on the highest divides of the Cut Plain clearly reflects the dip direction, and the direction of the tributary streams, but fails to explain the direction of the trunk streams (Fig. 2-2).

If trunk drainage was initially consequent drainage, then the initial slope was to the southeast. This consequent drainage may have resulted from (1) different geologic structure at the inception of modern drainage, (2) from trunk drainage established on higher units, now removed, in which a northward thickening wedge of overlying sediment provided the gradient along which consequent drainage developed (Montgomery, 1986), or (3) effects of Tertiary reactivation of deep-seated Paleozoic structural lineaments (Amsbury, 1990, oral communication.).

The Miocene (Oakville) strike is essentially normal to this trunk drainage direction (Fig. 4-9) as are the strikes of later units, leading to the speculation that perhaps initial slope was to the

1-47

southeast, and that this general drainage direction was established after eastward Eocene drainage, and before Miocene Oakville deposition (Sam mel, 1978).

o I

\ Miles

5 I

Structural contours on Edwards

Figure 4-8 Topographically controlled groundwater flow in the Washita Prairie. Notice that the groundwater flow-vectors generally follow the regional dip, but they are greatly modified by topography. Typically, flow-vectors follow topographic divides, eventually to feed into entrenched streams. Structural contours are on the Edwards Limestone over the area of Hog Creek and Middle Bosque basins. The Cretaceous limestones of the aquifer system (Comanche Peak, Edwards, and Georgetown) dip gently east-southeast at 20 to 40 ftlmi. DissectIon of the landscape alters the groundwater flow from dip-controlled to topographic-controlled (Cannata, 1988, p. 94-95). Note particularly the marked modification of flow lines along the divide between Hog Creek and the Middle Bosque, and along the divide between Coryell Creek and the Middle Bosque River. This modification is a product of dissection through the aquifer section by stream entrenchment. The spaCing of tributary valleys is related to the competition for groundwater discharge, which is, In turn, related to the degree of dissection of the landscape, and to the potential groundwater drainage area up-gradient from the site (simplified from Cannata, 1988).

1-48

I I I

Marked angle to early Tertiary contact

__ Creta ceous-Eocene contact

_ Normal to mid-Tertiary contact

(Early Miocene) contact

Figure 4-9 Relationship of (consequent 7) trunk drainage direction to strike of early and middle Tertiary rocks, Texas. While there is evidence for ancestral Brazos drainage as early as Eocene (Fisher and McGowen, 1969; Bammel, 1976), the dip-orlentatlon of main trunk drainage normal to Oakville strike suggests (admittedly very tentatively) that present trunk stream orientation dates from early Miocene Time and major subsidence in the Gulf Coast. While other factors were also involved, slope was ultimately the dominant control on stream dIrection (from Hayward and Allen, 1987, 1988).

STOP 5-TURKEY CREEK DIVIDE--Turning back the clock-(Sugarloaf Mountain Quadrangle [1979). 1/24,000; 31 04S'27"N; 97° 39'22''W) (Fig. 5-1)

We are on the Comanche Pediplain, at the headwaters of Turkey Creek, a southeastward flowing tributary of Neils creek. An unnamed tributary also flows northward from this divide Into Meridian Creek. This Stop, has fou r pOints of emphasis: (1) the anomalous location and configuration of the drainage divide; (2) the unusual cross-valley profile; (3) the equally unusual soils of the divide; and (4) the relationship of these features to problems of Cut Plain history.

Within this part of the Cut Plain most divides (particularly between small drainages) are defined by Edwards-capped highlands into which intermittent streams have incised narrowing, well defined valleys. The divide here occurs within a valley, a windgap, on a broad gentle Intermediate level well below the Edwards-capped uplands to east and west, but substantially above present dissection to north and south (Fig. 5-2). The cross-valley profile here at Turkey Creek Divide is in marked

1-49

IA\ Figure 5-1 (Sugarloaf Mountain Quadrangle, 1979, 1124,000;) Stop 5, Turkey Creek headwaters divide is in the bol/om of the valley in which we stand. The surface around us, veneered in Krum alluvial soil, is the original "intermediate surface", now called the Comanche Pediplain. Here is a Type II broad valley with narrow Edwards-capped divides, almost unmodified by modern drainage. We see the Pediplain graded from the divides, but clearly not to present drainage. Here it is an almost unaltered remnant of a distant geomorphic past.

1-50

contrast to most divide notches in the Cut Plain. The breadth of this divide-valley cannot be explained by present processes, for only a very few other such landforms exist in the region. It cannot be attributed to lithologic control, for no unusually resistant units are known to exist in the proper stratigraphic-topographic position. It is not developed in Walnut Clay, but in Comanche Peak Umestone, an unusual circumstance in this region of the Cut Plain where most such surfaces have been explained by the erosional properties of Walnut Clay. The profile of the upper slopes to the crests of Norse Hill to the west and Bee Rock Flats to the east are markedly more gentle and rounded than are equivalent slopes descending to Meridian Creek and the North Bosque River (Fig. 5-3), both more clearly products of modern dissection.

Also of note here are the soils of the Krum series, derived from old stream terraces, and formed originally "In valley areas over thick beds of unconsolidated clayey sediments" (Stringer, 1980, p. 48). Here the soils are in agriculture, an unusual land-use for Cut Plain divide lands, but very common for the pediplain. Perhaps most unusual is the presence of abundant sand and silt in the soil. Sands are not known from the Edwards and Comanche Peak formations, above us in the section (Amsbury, 1989, oral communication), so this soil-component was transported from another, more distant source.

There is coarse silt in the lower Walnut Texigryphaea banks (Amsbury, 1995), but here these beds are substantially below our present elevation of 860'. The Paluxy Sand (which is below the Walnut Clay, Fig. 0-2, 0-9), has abundant sand similar to much of the sand of this soil, but the nearest locality where Paluxy Sand occurs at an elevation above our present locality and high enough to provide for an adequate delivery gradient, is almost thirty miles westward along Meridian Creek valley from our present location, and across the divide into the Leon River Basin (Stop 7).

While we recognize the possibility of a distant source for quartzose sand from the Paluxy Formation, there yet remains a question. A characteristic of the soils of the pedlplain everywhere is a surface component high in quartz silt, even in locations where there is no possible access to Paluxy Sand. The silt so common to these soils may have been derived from loess. for over much of the Cut Plain they appear more loess-like than Paluxy-like. Slit Is also present in many of the soils of the Edwards uplands, and over most of the landscape identified as "old". Since silts and sands do not occur in the Edwards and Comanche Peak limestones (Amsbury, 1995), they are apparently not residual, yet to date, efforts to compare the silts of the Comanche Pediplain with loess from the Mid-Continent, and from the High Plains have been inconclusive (Bradley, 1988).

All of these factors combine to suggest that here at Turkey Creek Divide we are on an ancient landform, unrelated to present topography. The state of preservation suggests that processes active In this locality since the Initial isolation of this surface have done little to alter this relict landscape. It has been suggested (Moore, 1970) that this was a former pathway of Meridian Creek (Fig. 5-2), and that Turkey Creek is the beheaded remnant of that ancestral stream. This Is supported to a degree by the Krum soil at this locality (Stringer, 1980), derived from ancient alluvium If so, Meridian Creek then jOined Neils Creek, but at present that interpretation is based largely on stream alignments and soil series. Testing this hypothesis for absolute confirmation would require drilling a series of holes across this surface, but to date that has not been attempted.

The elevated surface here at Turkey Creek Divide, the Comanche Pediplain, represents an episode in landscape evolution in which entrenchment had almost ceased, and in which the dominant landform modification was valley widening over an ex1ended time-span.

1·51

Bee Rock Flats

Farmlands on Comanche Pediplain

Figure 5-2 Diagram showing the relationships between the old high-divide surface of Norse Hill/Bee Rock Flats and Turkey Creek headwaters divide; and between Turkey Creek Divide and the recent entrenchment of Meridian Creek. Notice that Turkey Creek and the northward­flowing tributary of Meridian Creek head within a valley, not on an Edwards-capped divide, and that Turkey Creek Divide is apparently unaffected by the recent entrenchment of Meridian Creek Valley (from Hayward, Allen, Amsbury, (990).

SOUTH NORTH

aee Rock Flat

d:=-~-,._~/ .......... ;;::::"""'!~~ - - t - -_. . --. - T i Conllnuous slopa to Pedlplaln, /"., " O. II lJ

. /; -/--~i:'~~' 2·L·fI~ ~::::-":\'/ Pedlplaln ven~.red with elluvlal (Krum) soli ---_. • ...

Meridian Creek tributary cutting Into Pedlplaln

Turkey Creek extending Into Pedlplaln

Figure 5-3 Diagrammatic section through Turkey Creek headwaters divide, showing relationships of the Comanche Pediplain to the Edwards-capped uplands, and to modern drainage. The Pediplain is clearly graded from the divide, but not to modern drainage. Here the pediplain is veneered by Krum soil, an old alluvial soil, suggesting that at one time the divide was the a pathway to through-flowing drainage (from Hayward, Allen, Amsbury, (990).

1-52

STOP 6··MERIDIAN CREEK VALLEY··Dissection and preservation of the Comanche Pedipiain (Cranfiis Gap Quadrangle [1979], 1/24,000; 31 0 47'36" N; 970 47'25" W) (Fig. 6·1)

Here at Stop 6 we are again on the Comanche Pedipiain first identified in Neils Creek Valley (Stop 4), then at Turkey Creek Divide (Stop 5). The view to the north Is of Meridian Creek Valley and the northern divide that separates Meridian Creek from Spring Creek drainage. To the south is Jenson Mountain, which marks the divide between Meridian and Neils Creek drainage.

At this locality there are four points of emphasis, (1) the very wide cross·valley profile, with a well preserved pediplain; (2) the obvious grading of the divide lands to the pediplain, not to present Meridian Creek; (3) the relationship of this pediplain to that of Turkey Creek Divide, and (4) again the presence of what appear to be anomalous soils of the pediplain.

The width of the cross-valley profile is clearly evident in the view north from Stop 6 (Fig. 6-2). The most conspicuous landform elements in the view are the flat Edwards-capped divides to the north and south. Second only to these is the well preserved pediplain, now in farmland. The lowest point on this surface is about 940 feet, 70 feet above present Meridian Creek, 80 feet above Stop 4 at Turkey Creek Divide. The straight-line distance to Turkey Creek Divide is about nine miles, giving a pediplain gradient to the east-southeast along Meridian Creek of about 9 ft.lmi.. West of this point to the pediplain at the head of Meridian Creek (11 miles west­northwest) the gradient is 25 fUmi" The present gradient of Meridian Creek over that same reach is 31 fUml, a difference of 6 fVmi Recent entrenchment has sharply increased tributary gradients, suggesting that the dissection of the Comanche Pedlplain is a product of baselevel lowering.

A second feature, visible in this view (Fig. 6-2), is that of uplands graded to the pediplain, not to present dissection, a point emphasized at Stops 4 and 5. The consistent curve of the profile from the base of the Edwards-armored divide to the low point on the projected profile is that of a pediplain, graded to a former drainage nearer the south valley wall. A gentie southward dip component of 13 fVmi may explain this valley asymmetry. While Meridian Creek has cut below the pediplain, that remnant surface is still clearly visible, as Is the relationship of this surface to the Edwards-capped divide.

At an elevation of 960' on the northern "pediplain" surface are soils of the Slidell Series, which ·".formed in valley areas over thick beds of unconsolidated, calcareous clayey sediments" (Stringer, 1980, p. 52). Slidell soils are not unusual in the Cut Plain, but they commonly occur as valley-fills, principally in first-order drainage on the pediplain (Bingham, 1989). Here they occur as soil variants on the near-flat pediplain where there is no low point to explain their presence. These therefore appear to mark former drainways no longer parts of the present networks.

One problem with "alien" soils, such as the Slidell Series, Is that they are often not greatiy different from other Cut Plain soils which we do not consider "alien". Here, clay soils of the Slidell series are surrounded by more-calcareous clay soils of the Purvis, Tarrant, and Denton series. Slidell soils have been identified as soils developed on unconsolidated clayey sediments (Stringer, 1980), but they lack the clear isolation from surrounding soils that would derive from a peculiar mineralogy which clearly separates the two. They appear out of place. They seem to relate to the history of the pediplain, but they do not provide the deeper satisfaction that would be provided by more convincing evidence (Brown, 1988).

\-53

... ····r·· ,.' I~'

, ."

I s n <

(" '/ f ~ ...... -

Figure 6-1 (Cranfils Gap Quadrangle, 1124,000; 1979) Stop 6, on the dissected toe of the Comanche Pediplain, looking northward across Meridian Creek Basin to the Pediplain and divide on the north side. The view to the north is of a Type " broad valley with narrow Edwards­capped divides. The valley is now being actively entrenched by modern drainage. Again the Pediplain is graded from the divides, but cfearly not to present drainage.

1-54

-' u. u.

SOUTH

MerIdIan Creek entrenched 60 " below Comanche Pedlplaln

, .. ~ ,~~ "'. STOP 6

~ -:;:~\h~e Pedlplaln

~ .. ~ "

- ------.l--:::~-~:.<--. ..... "l...: ----~, -.... ~-~---

Slidell soli \r.;>

, / - - - ~ - - ~ _, -...: \,...,rlP!!"'"".:"

-' .

Profile of recent dlssecllon - - . ............. Rock defended terrace

NORTH

Figure 6-2 Diagrammatic cross-valley profile, showing the Comanche Pediplain graded to the divides, and now being dissected by recent drainage of Meridian Creek and its tributaries. When the Pediplain was formed, the bed of Meridian Creek was 60 ft. higher than it is today. Here Meridian Creek flows on Paluxy Sand. At the time of formation of the Pediplain, Meridian Creek flowed on upper Walnut beds, and the Pediplain extended upward into the lower Comanche Peak formation at the base of the divide. Much of the view from this stop still consists of the relict earlier landscape (from Hayward, Allen, Amsbury, 1990)

When we first recognized a widespread "intermediate surface", we were uncertain of its significance. It was, however, truly a surface midway below the divides and above present drainage--and so the name. Later mapping in the Leon Basin (Brown, 1988) clearly demonstrated that the Leon Basin had its own "Intermediate surface", graded to an ancestral Leon River which flowed at a level several tens to more than a hundred feet above present drainage (Brown, 1988). Reconnaissance studies of several other basins indicate that most Cut Plain basins of today's section of the trip have intermediate surfaces. All are graded to a common high terrace level on the Brazos River.

Because streams of the Comanche Pediplain event in the Cut Plain were controlled by an ancestral Brazos, all such surfaces should have lowest elevations graded to this common base level. More importantly, all such surfaces reflect the same episode of landscape formation, for pediplain surfaces are preserved far up even minor tributaries, where again are found thick accumulations of colluvial-alluvial soils in protected pockets far above present drainage.

This latter again emphasizes that basin area was not a consideration. Tributaries that had cut through the Edwards before valley widening, were served by lesser tributaries that also participated in the episode of valley widening (Brown, 1988).

Thick, caliche-cemented, colluvial wedges occur at the toes of slopes of almost all uplands in the Cut Plain (Brotherton, 1978), clearly indicating that slope retreat is at present extremely slow or inactive (Fig. 5-3). These colluvial wedges--called Cranfils "soils"--vary from thin veneers to ten feet or more in thickness, and they join slope bases to the pediplain. Since their emplacement the toe of the slope has remained fixed in position. The only alteration of slopes has been "rounding-over" of the upper slopes, which supplied colluvium to Cranfils soils. Thus the Cranfils soil represents the most recent episode of slope modification. While it is caliche­cemented, it lacks the thick petrocalcic caliche characteristic of some of the ancient Cut Plain soils, and thus may represent a fairly recent addition, though the actual age is as yet unknown. (Flowers, 1987; Brown, 1988)

The evidence at Stop 6 therefore suggests that valley formation followed the pattern established at Stops 1 through 5; (1) initial downcuttlng extended through the Edwards limestone, and into the Comanche Peak (2) entrenchment ended as ancestral Meridian Creek reached grade here at an elevation of about 940 feet; (3) the ancestral streams remained at that level while slope retreat led to the formation of pediplains and attendant divides; and (4) far more recently, base-level lowering on the BrazOS-Bosque system, in a number of episodes of entrenchment and alluviation, created the present terraced inner valley of Meridian Creek (Fig. 6-2).

STOP 7--LANHAM ROAD CUT AND OVERLOOK--Fredericksburg stratigraphy, Fairy Gap, and the Bosque-Leon Divide (31 0 45' 25 "N; 970 55' 55" W; Fairy and Cutoff Mountain quadrangles, 1956, 1124,000) (Fig. 7-1)

At this Stop there are two points of emphasis, (1) the roadcut, the stratigraphy which has shaped Cut Plain evolution; and (2) the view, the geomorphic significance of the valley configuration of this very large basin.

THE ROAD CUT

Fredericksburg Rocks and the Cut Plain We have referred to the Fredericksburg rocks simply as Paluxy, Walnut, Comanche Peak, and Edwards Formations (Fig 0-9). Those terms are sufficient to understand the main features of the Cut Plain landscape, but variations in rock type within each formation, and lateral gradations

I-56

between formations, affect the second order of landscape features (Fig. 7-2, 7-3, 7-4, 7-5). Rocks are important to landscape development because of their relative resistance to erosion, permeability, and susceptibility to various soil-forming processes. Lateral and vertical changes in rock type within the Fredericksburg Division determine their properties as grist for the weathering and erosion mill.

The Lampasas Cut Plain also was formed where Edwards Limestone forms less than half of the Fredericksburg thickness. This is significant, because soft beds under the resistant Edwards Limestone are just as important to the physiographic process as is the limestone cap itself. We saw at the first few stops that streams that had incised into the Edwards, but not far below, either did not participate in the valley widening process, or if they did, the widening occurred in soft material above the Edwards and the diagnostic landscape has not been recognized.

Paleotopography Figure 7-2 (compare to Fig. 0-9) shows a slice through a very broad depostional basin (vertical exaggeration; 1000:1), a shallow extension of the East Texas Basin, overlying and expanding the earlier Trinity Shelf (Fig. 0-7). Fredericksburg rocks record the episodic northeastward spread of shallow water carbonate sediment from the San Marcos Arch. Rudistid debris, oolitic and skeletal grainstones, and peritidal dolomites are collectively named "Edwards Formation". Figure 7-3 illustrates dominant lithofacies near the end of Edwards Deposition. Earlier, terrigenous sand (Paluxy Formation) spread into the shallow basin from the northeast and northwest (Atlee, 1962; Owen, 1978). and terrigenous clay (Walnut Formation) was introduced from the northeast. The terrigenous material also came in episodically so that the rocks record a complex interplay of exotic terrigenous sediment with /n situ carbonate sediment. Fine grained carbonate muds became the nodular, white limestone beds of the Walnut and Comanche Peak formations (Fig. 7-4).

Where Edwards Umestone is 200-300 feet thick, the physiographic expression is the Edwards Plateau. Where the basal terrigenous section is thick, the result is the Western Cross Timbers (Fig. 0-4). In between, where 150-30 feet of resistant Edwards rocks are preserved, we find the Lampasas Cut Plain. Edwards lithologies thin northward; beyond the pinchout edge, the Fredericksburg and Washita rocks together form the Fort Worth Prairie.

Not all Edwards rock types are resistant to erosion. Soft, pure, almost uncemented dolomite was formed on or near tidal flats southwest of the late Fredericksburg grainstone barrier (Fig. 7-3). We suspect that this material formerly occurred widely under Washita strata across the San Marcos Arch and far to the west. But we do not know, because if it existed it has been eroded away completely. Clearly recognizable beds and surfaces can be traced northward through the middle part of the Tredericksburg (Fig. 7-5a). by tracing these surfaces we do know that the top of the Edwards to the west is older than the top of the Edwards to the east, thus we surmise that 50 to 100 feet of Fredericksburg material has been eroded away, and that the material was soft, peritidal dolomite.

Edwards/Comanche Peak Lithologies Stop 7 provides an opportunity to examine coarse rUdistid-fragment rock of the Edwards, overlain by deep soil of the Washita Prairie formed in Klamichi and Duck Creek (lower Georgetown) shale and nodular limestone (Fig. 7-5). The road cut below the Edwards is in material mapped as Comanche Peak Limestone. Nearly half of the unit exposed is shale and marl, but the convention is to use the dominant lithology found in normal outcrops. The unit exposed was termed "Pancake Member" by Amsbury (1988). When the correlative stratigraphic position can be seen again to the south across Leon Valley at Evant, most of the unit is represented by Edwards reef rock. A series of detailed measured sections down the north side of the Leon Valley demonstrated that the shale, marl, and nodular limestone go to Edwards rock bed-by-bed, that is, deposition was episodic and limestone formation was punctuated by innumerable interruptions that left shale partings and beds.

I-57

\

X//.36 \ ... ••

+ 0"45

o

Figure 7-7 (Fairy Quadrangle [7956]7124,000) Stop 7, on the eastern, Edwards-capped divide of the Leon River Basin, looking northwestward along the divide to Fairy Gap, headwaters divide of Meridian Creek Basin. The Pediplain is especially well preserved here, and Meridian Creek headwaters divide is in a valley, as it was at Stop 5 (Turkey Creek Divide), though alluvial soil is not present Here again the Pediplain is graded from the divides, but clearly not to present drainage,

1-58

/

&~ .......

"' . " .

•

--\ r'\ ;s-- <.,

~ )

~ '-" ~

-..1', '" --<-~

V 'VI

i'> v 0

e "-"-

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r, >-, V>

~

-0 ~ ~ ~

-< .,,)

~

~ ~

<:'\ ~

~

"-

----0 ~

'i' ~,

:';.... "-

\ "-

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T ~ ---- GEORGETOWN FMN PERSON FMNbCDUCK CREEK MBA!.. (EDWARDS A) t

a. EDWARDS (!) II: :J FMN :J m If)

MOFFATT LENTIL GEORGETOWN FMN

CDUCK CREEK MBR)

')10 KIAMICHHII~F~M~N!....:=;;j;::: I" , EDWARDS FMN - 7f -r T __ COMANCHE PEAK

GOODLAND (!)

o KAINER FMN ffi (EDWARDS B) ::,::a. O:J If)

a:~ @

r I FMN FREDERICKSBURG DIVISION

COMANCHE PEAK ,FMN

~;a.ffi ~WALNUTJJ J 5:J tu

~(!) c( ___ _ UPPER MARL MBR

FMNJJ _0 ::;: ... ..;.... 0:0: 0 .; ... " ... .., ........................... _ ... - W(!)O

M! ::: KEYSVALLE'I'---j- __ _ u. ftl MARL WALNUT

11 CEDAR PARK LS FMN

BEE CAVE MAR

~ PALUXY fil ~l ~ --- FMN II: U.

1-LOWER PALUXY

.......... _w ...........

---------UPPER pAL

-= ___ -!B~U~L~L~C~R!E!E!K~L!S--~~~ GLEN ROSE FMN ,. 100 KILOMETERS-I GLEN ROSE FMN

Figure 7-2 South to north cross-section along the eastern edge of the central Texas Fredericksburg outcrop from San Marcos to Fort Worth. The region of the field trip lies between the Moffatt Lentil and the right-hand gap in the section. Stop 7 is on the thin Edwards Limestone plus most of the Comanche Peak Formation. near that gap. Note that the Fredericksburg Group on the San Marcos Arch, where the Person Group built up above the Kiamichi Shale. is not equivalent to the Fredericksburg Group west of Fort Worth. where the tOJr Glen Rose bounding surface is lost in Paluxy Sand. Throughout most of the Cut Plain. the Fredericksburg Group (rock-stratigraphic) and Division (allostratigraphic) are coincident. [Diagram based on detailed surface studies by Rose (1972). Moore (1961). 1964). Young (1974a.b). Amsbury (1995). and Perkins (1961). Modified from Lozo (1959. Rg. 1) and Rose (1972. Figs. 19.33).

~ 7 ---~ ~l I,

' . •

'.

.~

... •

.- 'I . \'V ' \.;

• o o ~

'(j · · · -" •

--'

" .-.. '..

:: Q

-'

.~

~'i ,,, i.~ \~

... ...... · .. r::. i'i \

\.-

. -ID .' ~~'

/

1-60

'. •

'0

~ :

~

'" ~

Figure 7-2 Generalized Facies Map of the Edwards Formation. The Edwards . Formation within the field trip area represents 1/3 or less of Fredericksburg deposition. Growth of rudist reefs and grainstone bodies began along the northeastern flank of the San Marcos Arch, far to the south, and progressed episodically into the trip area. A linear body of oolitic, skeletal, and pelletal grainstone formed at and northwestward from Lake Belton, growing to 130 feet or so thick. Peritidal dolomite formed southwestward from the graInstone, while northeastward from it, open· marine deposition of lime mudstone, marl, and calcareous clay conlinued until near the end of Fredericksburg deposllion. The final stage was the extensive - and presumably rapid -spread of rudist patch reels, associated lagoonal deposits, and grainstone-filled channels throughout the area.

Klamlchl Shale of the overlying Washita Division pinches out southward onto the grainstone body. Note that Impressive Incised meanders are preserved in the grainstone body now hosting Lake Belton. Similar IncIsed meanders along the Leon River are Incised In Glen Rose rocks between Gatesville and Hamilton, and are preserved In floodplain deposits that probably date to latest Wisconsin/earliest Holocene throughout the Leon Valley (Amsbury, 1984).

.

§, ·~.I;'brvWI~ IIUC ..... ItIlU .. PS(.WS~ .-

........ _1Id1111_1U. UI&Il _ CIMI l1li111.1 'tPJ,...,. Ir--.&. ~ &I... '

"ttlh"'~, .. Ita 11_"'5 IfId cI-Wllllle "5.

~: .•.

12.8 I I i

~ i I ! rNhlta. ""~~', _\111111'"1",1111_ WS',

'.'Lo;'F • IT"'. ".17El.

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jW"'-, "' ........ , ..... _ ... 1' ... 5 .. '" r ........

~ -

I .ss

ILn' ...... 1

, - ,

us ~ ~':"'" MS,." , . s. INII';-

,,' .. !

,

--I-

c.-... lo.~ ._--

~ ' .... _'1-' ..... !"OI WS. I

- II_PS.

!~PS.~ ~WHall~ • ~'I"'-I'IS. \

Ii"~!; =;:,':.::::f ~~ -t-·1·":"I-; i I U_WS/PS .. . -i 15:8 !"-" '''r T"'· us 1"..:.;;-, ."',. - ----;.~ T I , .. .! I --If--+- .. -. .----.. -.----.1

• .~ _II ..... , .. , ......

. ... , .... , , , f-'--~ 'IS. ',.. i

! -. i I, ."-.J.. .;u/"

I ---- :

-z., I'I~.II""S. ;

- .. WI" ~ ... t.-Ilus.II~- tMll...,.lllla. ·Z..j~~;1 ._--, - 'j-- ---:

Figure 7-4 Measured section of the Edwards Formation and part of the Comanche Peak formation (Pancake member of Amsbury, 1995) along a gravel road about five miles southeastward from Stop 7, The top of the fairly resistant, white, nodular limestone (Cold Spring Limestone of Amsbury, 1995) at the base of the section is about 60 feet above the top of the Keys Valley Marl member of the Walnut Formation, and about 200 feet above the top of the Glen Rose Formation, In the section, ms=mudstone, ps=packstone, ws=wackestone, gs=grainstone, bs=boundstone (from Amsbury field notes, 1963),

1-62

'5.

GRAINSTONE

- - - - -._--------_ .. _--:------ ------ --- -- --- ---... ---- ~OIEPEJUt------~ - ... -- - - -- ---~ - -- - -~ - - ---:.. ~ -:--------------- -- -------~--

Figure 7-5a Northwest-Southeast Cross Section Along the Colorado/Brazos, Lampasas/Leon, and Cowhouse Creek/Leon Divides. The cross-section cuts obliquely through the Edwards grainstone body near Lake Belton, extending north westwards into the peritidal dolomite facies. Scattered patch reefs, particularly at or near the local base of the Edwards Formation, occur along this line. Correlations based on recognizable surfaces and key beds underneath Edwards rocks are shown by light dashed lines. The actual amount of peritidal dolomite that once capped the divide is impossible to determine, but by projection of the stratal top from the southeast it is evident that a significant amount of Fredericksburg-equivalent material has been removed by erosion (from Amsbury, 1995).

SW S"rRATAL = EIlHl\RDS G __

___ _ -,..;:'00 1.. ... ~.1

.B..-s::: _______ _ - - -- - -- - -CXIlOOI~~=§~EIl~RARD~~S~ __ ~o~",,~tE~~~ 'N~ ~s REEF

--.-- ---- -~-----~~: .. ;,..'-=='---=~-~ =:-.-== - ---- ----'- - - - -- --- - -",'"

Figure 7-5b Southwest-Northeast Cross Section Across the Lampasas/Leon and Leon/Bosque Divides. The stratal position of basal Edwards rocks to the southwest compared to that to the northeast is shown by light, dashed correlation lines. There are no upper Fredericksburg outcrops within the Leon Valley, of course, but the interpretation Is controlled by detailed outcrop and core data near Lake Belton. The cross-sections have a common point at Shive, in extreme northwestern Coryell County (from Amsbury, 1995)

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Distinctive thin beds, surfaces scoured by submarine{?) action and then bored by rock-boring clams, and clayey beds are most welcome to the stratigrapherl

Evidently conditions were favorable for molluscan growth during deposition of the Pancake Member near Lanham. Ammonites, clams, marine snails, echinoids, marine oysters, calcareous algae, and burrowing organisms are extremely abundant in many beds.

The relatively resistant white, nodular limestone bed at the base of the section was termed "Cold Spring Limestone" by Amsbury (1988). This unit is a persistently traceable tongue of Comanche Peak lithology that underlies the muddier Pancake Member, and overlies the Upper Shale Member of the Walnut Formation (probably the Marysville Marl of Perkins [1961)).

Fredericksburg Stratigraphy and the Comanche Pedlplain • Along the lower slope of the Leon Valley, but northeast of Highway 36, we see a persistent line of knolls at about the same elevation along the valley. Slopes cut into the Upper Walnut Shale are graded either to the tops of the knolls, or to lower elevations between them. Slopes below the crest of the knolls are graded to a second, lower base level near high terraces along the Leon River. The base level of the knolls defines a valley surface about 100 feet above the present flood plain of the Leon River; in this reach of the Leon, the high base level defines an intermediate surface, the Comanche Pedlplain.

The knolls along Leon Valley are formed in a relatively resistant, massive accumulation of gryphaeid oyster shells at the top of the Keys Valley Marl Member of the Walnut Formation (Moore, 1964; Flatt, 1976). Lower slopes are cut in Keys Valley marl and shale, and in the Paluxy Sand. Here, just as in the Upper WalnuVComanche Peak/Edwards triplet, a resistant bed over less-resistant beds provide the material for a distinctive landform.

The fact that the Comanche Pediplain in this part of the Cut Plain is coincident with a resistant unit at mid-level within the valley caused us to wonder if in fact the "Intermediate Surface" was a figment of our imagination. Perhaps the "Surface" is merely a set of rock-defended benches at approximately the same position within the valley, and does not record a discrete period of erosional history. Further work, though, led to the recognition of the Comanche Pediplain is on lower and lower stratigraphic units westward, and higher ones eastward. Brown 1988) mapped the pediplain throughout the Leon Valley. Flowers (1987) extended recognition along the upper Colorado River and into the upper Concho basin. And Amsbury recognized the pediplain in Blum's (1989) Terraces A and B in the upper Pedernales River during the 1989 Friends of the Pleistocene trip. We think now that correlative landscapes occur along major streams draining the southern Edwards Plateau, marked by Hill's "Uvalde Gravel" (Hill, 1900, p. 5) along Median River, Seco Creek, Sabinal River, Nueces and West Nueces rivers, and the Rio Grande at Del Rio. Similar surfaces perhaps formed in the same way and at the same time occur in the Pecos Valley (Mescalero Plain, and including the surface that extends southwestward through Fort Stockton Into the Davis Mountains), and the Rio Grande Valley above EI Paso (the La Mesa Surface) (Amsbury and Hayward, 1996).

THE LANDSCAPE

The view northwest from the overlook at the top of the road cut (Stop 7) is of the divides and pedlplain which separate Meridian Creek drainage (North Bosque River) from Egg Creek (Leon River) drainage. The crossroads about one mile west of our viewpoint is the small community of Lanham (a whole community of Joneses, descendants of the original Jones whose wagon broke down here in the late 1870's). Visible farther to the west is the recent inner valley of the Leon River that has greatly dissected the Comanche Pediplain, (though the pediplain is still visible in the accordant summits southwest of the River). The far Edwards-capped divide of the Leon Basin is 50 miles west, and not visible from this point (Fig. 7-6).

\-64

Probably the most impressive aspect here is the immensity of the view to the northwest. The view is a product of two factors; (1) the major dissection caused by the Leon River, and (2) the effect of structural dip. Because the dip is slightly greater than the mean gradient of the Leon River system the Edwards-capped divides rise to the west- northwest, and the total relief increases. Thus, entrenchment first encountered Edwards Limestone far to the northwest, where it created the first entrenched valley. As general landscape lowering exposed Edwards Limestone down dip to the east-southeast, entrenchment also progressed to the east southeast, while upstream the older valley widened by slope retreat, eventually creating the landforms we see here (Fig. 7-6).

The most recent entrenchment into the Edwards Limestone is at Belton, Bell County, 50 miles to the south-southeast. The northernmost Edwards-capped western divide is Stop 9, in Brown County, 50 miles to the west, at the end of today's traverse. The northernmost Edwards­capped eastern divide is Long Point, visible on the skyline about eight miles northwest of Stop 7. Thus in map view, the Edwards-capped divides form a somewhat distorted V, 100 miles "high," 50 miles wide, converging downstream to the southeast (Fig. 7-6).

Probably the greatest significance of this view, and of the broader picture which this view emphasizes, is that ancestral Leon River entrenchment may have started in the northwest before dissection first encountered the Edwards Limestone. Since that time, entrenchment has lowered the course of the Leon River 840' below the northernmost divide elevation.

During that same period of entrenchment, slope retreat created a basin 50 miles wide at the northern part of Edwards capped divide. Downstream at Belton, general surface lowering has just now exposed Edwards Limestone to active erosion.

Throughout much of the length of both western and eastern divides, a pediplain graded to an ancestral Leon River is yet preserved. The divide profiles suggest that almost all the slope retreat, which now accounts for this very wide basin, was the process of formation of the pediplain. This again suggests that the episode of slope retreat, introduced at Stops 4 to 6, was the product of a major event (or events) of slope retreat, and that little has happened since the time of that event (events?) in these old divide lands.

Along major, through-flowing, trunk streams, such as the Brazos and Leon Rivers the Time Zero Surface first began to be destroyed (and the Cut Plain formed) in the far northwest of Fredericksburg presence. With continued erosion, Cut Plain formation progressed southeastward. In contrast. along tributaries such as the North Bosque. Hog Creek. and the Middle Bosque the direction was reversed. as nick-point migration proceeded northwestward from the incising trunk streams. Over most of the Cut Plain. entrenching streams had achieved grade before the pediplaination phase. It was during their time-at-grade that the Comanche Pediplain formed.

A point of frequent emphasis is the contrast between the widths of the ancient valleys and the widths of inner entrenchments. Slope retreat in the inner valley of the Leon River rarely exceeds one mile. The older. pediplain valley is as much as fifty miles wide where the Edwards capped divides are yet preserved. The width of the inner entrenchment represents the effect of slope retreat since entrenchment began in mid-Pleistocene time (Tharp. 1988). How much time. or what rate of slope retreat. is represented by the very wide valleys of the ancient Cut Plain? The answers which come first to mind are (1) that these wide valleys represent an amount of time perhaps far greater than all of early Pleistocene. or (2) that the process of valley widening in the "old" Cut Plain was radically different in mechanisms and rate than that 0 the recent entrenchments (Tharp, 1988).

1-65

Margin of Cretaceous rocks I

'" \'..-

""''''''/r'\.. \ \..- - a Miles V t....-, " ' .... -------' 50 ,- _./ \

''"'' I --, \ ~

STOP 11 /( "­\1'

\ , "­" " '\

~E:.-_-STOP 7

\: L.ampasas Cut Plain

Figure 7-6 Sketch map showing the downstream 'V' of the Leon Basin Edwards-capped divides. Stop 7, where we now stand, is near the northern limit of the eastern divide. The point marked Stop 11 (from an earlier trip) is at the end of the western divide. At Stop 17, the last stop of today's trip, the Edwards Limestone plunges into the subsurface. Far the 'oldest" landscape of the Leon Basin is that near the point shown as Stop 11 (from Hayward, Allen, Amsbury, 1990), where total relief from divide to Colorado and Leon Rivers exceeds 800 feet.

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~

~ :.>'" i:')o ...--

'- ~ • Comenche

'1fQ) ~

• Hamilton

Edwards-capped dIvIdes

~~Jl!~'ltt~

cr· ." ~ l:'

mc~

~ o 10 20 30 40 50 ,

Miles

. Figure 7-7 The Comanche Pediplain in the Leon River Basin at the final stage of its formation (after Brown, 1988). The clearly pediment-like surface shown here was reconstructed from extensive Pediplain remnants preserved throughout the Basin. Adjacent basins, even small ones, also preserve such remnants, each graded to an ancestral tributary in that individual basin. The common base level for all Pediplain streams was the ancestral Brazos River. Using methods similar to those for the Leon Basin, it should now be possible to reconstruct the entire Pediplain surface of the Lampasas Cut Plain as of the time of its formation (from Brown, 1988).

Initial Cut Plain

f'(~~---~--'"'-= ...... -- -"-'.

y W"h'" P~""

=---~-

.-- ------­---'~--=

Expanding Cut Plain

~ ';,~..--_ R. W .. h'"

-";.., ---. - ~"- - ?-< __ • .-..." ,''1

STAGE 1

Prairie

----=-' - -~~ .:::.----~"ry,.'-!.b ~ - - - ~ -----

:/-~ - '--~;:::?~~3£~~ ~.)..l' ----~ '.-l.-). __ ~

~~

STAGE 2

STOP 11--=~

II.

:;~~1:~~STOP 17

"I.co" II.

STAGE 3

Colorado R.

Figure 7-8 A series of block diagrams showing stages in the evolution of the Lampasas Cut Plain in the Leon Basin. Initial dissection began in the north (see "Stop II", Fig. 7·6), and over time extended southward. Following entrenchment, the river flowed near grade for an extended period while slope retreat continued unabated, forming the vel}' wide Comanche Pediplain. Renewed downcutting by the Leon River in response to climatic variations, or to base level changes in the Blacklands to the east (near Belton, Stop 17) led to the formation of a "new" inner valley. Renewed downcutting by the Leon River and its tributaries in Late Pleistocene­Recent time has modified the landscape, leaving the Pediplain as an abandoned high surface (from Hayward, Allen, Amsbul}', 1990)

1-68

Clearly the Leon River is an old system, from earliest Cut Plain formation established generally in its present position. Near the northern limit of the Cut Plain at the time of first encounter with the Edwards Limestone, it flowed at an elevation at least 840 feet above its present grade. Since that time, the river entrenched first to the level of the pediplain, and apparently flowed at grade at that elevation for a period of time adequate to form pediplain slopes as much as 30 miles wide. Then in more recent times, it entrenched again, dissecting the older surface in an involved recent history that included a number of cycles of entrenchment and aggradation, eventually to form the terraced inner valley of the present Leon River.

The northwesternmost Edwards-capped divide of the Leon River Basin is the southeastern­most terminus of the Callahan Divide, the mesa-like divide which separates Colorado and Brazos river drainage in the upper reaches of both stream systems. It also marks the boundary between two structural provinces; (1) to the west a Callahan Divide province where the dip in

Cretaceous rock ranges from two-to-five ftlmi to the east, and (2) from Brownwood eastward, the westernmost influence of the East Texas Basin, where the eastward dip increases to about ten ftlmi, and continues to increase to the eastward, where Comanchean rocks disappear into the East Texas Basin (Fig. 0-2).

There is a remnant pediplain along the Callahan Divide as well, equivalent to that of the Cut Plain, indicating that the episode of pediplaination was late, involved a very wide region, and that it affected both structural provinces at the same time.

This view therefore introduces a series of questions. Is the landscape we see a product of slow evolution over a very long period, under processes not greatly different from those now active? Or is it a product of rapid evolution over a much· shorter time-span, and under circumstances radically different from those now active? The discussion of groundwater effects, and of Fredericksburg stratigraphy suggest that it may have been much more rapid than present processes might indicate. Only very recently have we began to appreciate the enormous potential of groundwater as a geomorphic agent In the Cut Plain (Allen, 1990). Of equal Importance, the similar landscapes cited in this guide (In Texas, the Callahan Divide, parts of the Caprock Escarpment; in New Mexico, the Canadian River Canyon, Ute Creek; in western Oklahoma; in southern and western Kansas; and perhaps in northern Mexico) have groundwater and landform relationships similar to those postulated for the Cut Plain.

STOP So-CORYELL CREEK-Scarp retreat and pediplain formation (31 0 2S' 37" N; 930 37' 50" W; Gatesville East and Oglesby quadrangle [1979], 1/24,000) (FlgS-1)

The view west from this point is of the valley of Coryell Creek, a minor tributary of the Leon River. A much smaller, unnamed tributary enters Coryell Creek from our right (north) in the Immediate foreground. It is separated from the main stem of Coryell Creek by an Edwards capped divide, and on the east facing margin of this divide are a number of scarp face drain ways, in various stages of dissection.

This view encompasses the two points for our discussion; (1) the Comanche Pediplain in tributary valleys as a key to pediplain evolution; and (2) the role of groundwater In scarp recession, as a model for the formation of the Cut Plain.

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-.:... o

\

- -- --... '.- '. Figure 8-1 (Gatesville East Quadrangle [1979] 1124,000) Stop 8, overlooking Coryell Creek Valley, and a small

C tributary valley, both of which show well developed Comanche Pediplain remnants. That in the tributary valley is almost undissected, indicating that during the pedimentation phase, even the smallest tributaries participated. This suggests that that the baselevel remained remarkably stable for an extended period, or that during the pedimentation episode slope retreat was unusually rapid

( 06J) /. .~ .. ;;.~ \ I( ~ v )"\ ~ 2~\j "( ~0~\:l \,\%~

lJ

I

I I

I

Pedlplain in tributary valleys The cross-valley profile of Coryell Creek shows clearly the Edwards/Georgetown armored uplands, the rounded slope to the pediplain, the pediplain, and the recent entrenchment of Coryell Creek into the pediplain. More surprisingly, the unnamed tributary just to our right also shows the same features, though the basin area is only about 0.25 sq mi, clearly indicating that basin area was not a factor in pediplain formation (Fig. 2-4).

Pedimentation along Coryell Creek valley and into its most minor tributary valleys proceeded in response to entrenchment and valley widening by pedimentation along the Leon. That in turn was in response to entrenchment and valley widening by pedimentation along the Brazos. The ultimate cause for entrenchment of the major trunk streams is not yet known, nor is the actual time of this earliest dissection. But it took place over a sufficient period of time, and under such circumstances of scarp retreat that most of even the minor tributary valleys took on the "traditional" Cut Plain profile. Here again, landform configuration indicates that groundwater availability was a major control (Table II, Stop 4), a point to be considered in some detail here at this Stop 8.

To belabor a point, through-flowing trunk streams were local baselevels for the Comanche Pediplain, as they are for much of the present Cut Plain. Because of their volumes of flow, during initial phases of cut plain formation they rapidly cut downward. As nick points migrated upstream, each upstream tributary in turn felt the effect of trunk stream entrenchment, and responded by downcutting. As grade was reached and maintained for an extended period in the main stem, a steeper graded profile was established for each of the tributaries, and along all channels pedimentation proceeded by slope retreat, the rates apparently determined largely by groundwater availability.

Again we emphasize, the pedimentation phase persisted long enough that it eventually involved all of the network that had achieved grade in Fredericksburg rocks, even the smallest tributaries. The undulating, leaf-like landform which includes the combined pediments of all the Cut Plain basins is the Comanche Pediplain. It consists of a vast number of Individual basin pediments, joined at common elevations only at stream junctions, and all ultimately graded to a distant base level in younger rocks far east of the Cut Plain.

Groundwater and scarp recession The role of groundwater in tributary formation, scarp recession, and formation of individual basin-pediments has been discussed (Stops 2, 3, 4). Here at Stop 8 the effects can actually be seen. In the Washita Prairie (the uplands to the north of the road) the Georgetown and Edwards limestones form an unconfined aquifer with a water table 20 It below the surface. Two types of porosity are present in the aquifer, (1) matrix, and (2) fracture. Fracture porosity is far the largest contributor to effective porosity (Cannata, 1988, p. 64), with values ranging from .51% to 1.49%, and an average value of 1 %. Solutioning has greatly increased the effective porosity in some areas, and zones of highly effective vertical solutioning along fractures have been noted (Cannata, 1988, p. 64). Where fracture porosity has been increased by solUtion, permeabilities of as much as 22 Darcys occur, equivalent to hydraulic conductivity of .0007 fVsec, or an actual flow rate of as much as 60 fVday at existing head differences.

This is of critical importance, for the values cited here are (for the long history of Cut Plain formation) minimum values. Any increase in precipitation, reduction in evapotranspiration, or increase in permeability (through original presence of more permeable beds, since removed by erosion) would increase groundwater flow, and hence scarp retreat. At the flow now existing, little is happening in the Cut Plain. But once a critical threshold value of groundwater flow is achieved, scarp retreat rate should increase markedly.

1-71

WEST

t::::> EdwarGs..,.. --=~--:-.....~ .. ~ . ..,... ..... __ - - '...... ....-.~.""

~" ' . , ~

~ ....

Comanche Pediplain I Recent entrenchment and aliuviation

,.. .! a; >

~ co -; Spring line ..,

EAST

~ :;r-t:'- _ ...

~ ~.~~~s::Jijjr-= Camanche Peak \

Comanche Pediplaln

Figure 8-2 Diagrammatic section and profile of Coryell Creek Valley, showing the Edwards­capped divides; the Comanche Pediplain in both Coryell Creek Valley and the small tributary valley to the east; the inner entrenchment of Coryell Creek, and the spring line that marks the slope break in the tributary valley. The presence of the Comanche Pediplain in the tributary valley indicates that basin area was not a limiting condition in pedimentation (from Hayward, Allen, Amsbury, 1990). .

WEST o ,

Local flow system

Miles

EAST 2 ,

Figure 8-3 Section showing local, intermediate and regional flow systems, Middle Bosque River basin. Notice that regional flow systems most responsible for stream incision are confined to the Washita Prairie east of the entrenched valley of the North Bosque River (from Cannata, 1988).

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--W-i

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

¥ pip.

Gully A

W= wldlh of area contributing flow to pipe

Flaw lines

Gully B

I valley deposits pip.

Gully C

Figure 8·4 Diagram showing relation between width of the contributing area of groundwater discharge to a stream, and stream length (after Selby, 1982). Notice that there is a cascade effect. As the stream lengthens, the contributing basin increases in area. As the basin increases in area, the rate of elongation increases. This sequence, continuing to the divide, appears to explain the evolution of dip·controlled drainage in the Cut Plain (from Hayward, Allen, Amsbury, 1990).

3~----------~------------~------~~--r /'

.//

•

o 1 2 3

Aquifer length

Figure 8·5 AnaJysis of a small sample (12) of first·order streams In the Cut Plain indicates that stream length (SL= stream head to junction with next higher-order stream) is directly related to aquifer length (AL= stream head to divide) above the stream head. The effect is to form first order tributaries which head some distance (AL) from the divides. The 'undissected" divide area is, none-the-less, a major contributing basin, accounting for headward migration of tributaries (from Hayward, Allen, Amsbury, 1990).

1-73

Flow systems in the Washita Prairie and Cut Plain are controlled by structural dip and local topography. For example, local, intermediate, and deep flow systems occur within the shallow aquifer system of the Middle Bosque Basin (Fig. 8-3) (Cannata, 1988), and have been recognized in diverse shallow Cretaceous aquifer systems throughout Central Texas (Barrett, 1989; Barquest 1989).

Based on the Middle Bosque Basin, reconstruction of the long term changes in the groundwater system at a given locality in the Cut Plain can be reconstructed by comparing groundwater behavior in less-dissected sections of the basin (Fig. 8-3), to its behavior in increasingly dissected areas of the same basin, and assuming that these represent the sequence of phases in drainage development as the landscape evolved. The result of these comparisons is simply that with increasing dissection flow systems become ever more localized.

The dominant dip controls groundwater flow lines along major divides, but topography becomes the dominant control in dissected areas (Fig. 8-4). Dissection of the Georgetown Limestone results in localized reorientation of the groundwater flow net. The increased flow, directed by lithology and structure, enhances local weathering and initiates the sapping process. This in turn initiates headward elongation of the valley. While the tendency for the main stem of the stream network to be directed down dip, many of the tributaries are oriented with local fracture sets (N 200 to 400 E). As the main stem tributary continues to lengthen headward, its size and length are continuously related to the increasing basin area of groundwater flow that it has captured (Fig. 8-S). Within an individual basin, groundwater flow volume is related to (I) the degree of dissection, (2) the saturated thickness of the aquifer system through which the stream has cut, (3) the effective porosity and permeability of the aquifer, and (4) climate. Within a given area these four factors remain relatively the same for all first order basins, and dissection should reach an equilibrium dependent upon the local structure and stratigraphy.

Dissection acts to concentrate both groundwater and overland flow. Surface water acts to aid in the removal of sapped debris, and the reduction of fall blocks by both mechanical breakdown and chemical solution. Level of entrenchment is controlled by local base level, and the volume of groundwater redirected into overland flow is controlled by degree of topographic dissection.

Assuming that this is a valid model for the systems that formed the Cut Plain, there should also be a relationship between stream length and the area of captured aquifer (also expressed as length of captured basin above the channel head). As a test of this hypothesis, a small sample of small tributary basins in the larger Coryell Creek Basin were measured and compared (Fig. 8-S), indicating a consistent relationship between channel length below the Edwards free face, and contributing basin area above the Edwards free face. Of particular interest is the difference between north and south flowing tributaries. North flowing (up dip) tributaries have shorter lengths (and hence areally less extensive areas) of contributing aquifer than the south flowing (down dip) tributaries, tending to confirm the greater water availability from such dip-controlled aquifer systems

Note: Here we leap from stop 8 to stop 16, but there Is an explanation_ These stops were origInally part of a two-day field trIp (Hayward, Allen, Amsbury, 1990). For the present trip that fIeld guide has here been reduced to a one­day trip, and the Stops missing from the one-day trip (those Stops that took us to the far western margin of the Cut Plain) have been deleted in favor of tIme, wIth the convIction that one day in the Cut Plain is vastly more satisfying than no days In the Cut Plain. In order to equate the stops of the earlier trip to those of the present one we have chosen to maintain the original stop numbers_ We hope this is not too confusing_

1-74

STOP 16--LEON VALLEY OVERLOOK-High terraces of the river (310 18' 47" N; 970 29' 15" W; Leon Junction and Eagle Springs quadrangles [1979], 1/24,000) (Fig. 16·1)

Here at Stop 16, near the eastern margin of the Lampasas Cut Plain, the Leon River Is deeply entrenched through Edwards and Comanche Peak rocks. The valley Is narrow and clearly defined, and obviously a product of the stream which now occupies It (Fig. 16-1).

At this stop we consider only one topic, the history of the Leon River as we now Interpret It.

Here where we stand Is a bedload terrace of an ancestral Leon River. It rests on Edwards Limestone 100 feet above the floodplain. The narrow, entrenched valley, so conspicuous here, begins upstream near the junction with Coryell Creek about 9 miles northwest of our present location. The valley remains entrenched almost to Belton, our last stop on this trip.

The entrenched valley Is widely meandering, in loops far larger than those of the present River. It has long been believed that these valley meanders reflect on older, larger River (Lewand, 1969; Tharp, 1988), which once conducted much of the flow now conducted by the modern Brazos. This ancestral Leon River, then conducting both the Clear Fork and Double Mountain Fork, was beheaded above Eastland (Eastland County) leaving the beheaded river about as we see it now.

The time of this diversion (Tharp, 1988) may have been Wlsconslnlan. The terrace gravels of the Leon are composed almost entirely of quartzose components, not available from the present basin, and similar to Ogallala deposits. l.:.imestones, so widely available In the modern basin, are almost absent in the terraces, but they become dominant In the modern floodplain. Thus the diversion must have taken place between the time of emplacement of the lowest terrace, and the formation of the modern floodplain (Tharp, 1988).

Several other episodes of river history are also reflected in the landscape. On the highest divide about four miles southwest of our present position (and on major divides of the Leon Basin throughout the Cut Plain) are lags of large, rounded, exotic gravels, of varicolored quartzite, quartz and chert, some to six inches in diameter. Two suites are present; (1) an exotic suite of quartz, quartzite, and varicolored cherts (some containing fusilinids); and (2) a "local" suite of Edwards derived chert. The exotic suite has petrology similar to Ogallala gravels far to the northwest. The local suite, commonly more angular and abundant, was clearly derived from Edwards limestone, which Is increasingly cherty to the south and to the west of our present location .•

Below the high divides just south of our present position are other terrace lags similar In composition, but at lower elevations. These also show the great abundance of siliceous gravels, and do not show the dilution by limestone which might be expected if they represent reworking of higher gravels (Fig. 16-2).

The distribution of these gravels between the highest divides and the third terrace suggests that there may be several levels in that interval. There are few areas where these occur, and to date information Is inadequate to define additional terrace levels. The area of their occurrence Is similar in topography to much of the divide region downstream from Gatesville, and it is possible that other localities will eventually help to provide answers to such questions (Tharp, 1988).Thls overlook Is on the third terrace level, and it is 200 feet below the nearest gravel lags of the highest divide. A little less than five miles west of our present position an

1-75

_/ - -C,'"'' \ ~--~~~~--'" ",\~ (}ff'~ ,: ' l .;

" , " I " ,­

, , " rEI' ,;

s;; ,/ " ~" ,.,-

(/-2~;';' -,?-~~~

'. "

" " " "

Figure 16-1 (Eagle Springs Quadrangle, 1124,000; 1978) Stop 16, overlook of the Leon Valley at Mother Neff Park, with emphasis on the third terrace (100' terrace) and its relation to the Comanche Pediplain. The terraced inner valley of the Leon River is clearly visible, and the relation of terraces to drainage history is evident. The third terrace, Stop 16, is the age of the Comanche Pediplain, and antedates the incision of the valley before us, and of the Pediplain proper.

l-76

\

-.:., -..l

Coarse siliceous gravel on hIghest dIvIdes

other terraces

.~ --I ) ; i • J'! I I I I i ;:;; Edward~';' I I I I I I I FQ

' l ( I 1 , 1 i i I~ , ---

c~7:e Pedlplaln Georgetown 100 " terrace

60 " terrace

Comanche Peak 30 ")erraCe

'le ., £dS

FloodplaIn

/ " : '-', . ,"

• . . .:....

Figure 16-2 Diagrammatic profile and section, showing the relationships of terraces to landform and stratigraphy. Terraces are best preserved where they are rock defended, and higher terraces are almost limited to rock defended localities. Cross profiles of the Leon Valley here, and particularly in the Lake Belton area just downstream from our present locality, strongly suggest that the entrenched valley was the product of rapid (?) knickpoint migration initiated by baselevel lowering along the Brazos River (from Hayward, Allen, Amsbury, 1990).

abandoned meander, 100 feet above the present floodplain as is our present position, also exposes similar gravel. Upstream for many miles, at an elevation of about 100 feet above the present floodplain are found siliceous gravels and old alluvial soils of the Krum series (McCaleb, 1985) ..

It is to this third terrace level that the Comanche Pediplain of the Leon Basin was graded. The Leon River controlled the evolution of the early Cut Plain with its extensive pediplain. Even then the Leon River was a stream, at grade, in about its present position, widely meandering on an alluvial veneer (Fig. 17·7).

The inner entrenched valley into which we look was apparently the product of knickpoint migration along a meandering valley extending northwest from near Belton. Knickpoints were apparently generated by base level changes in the Brazos River system in the Blacklands to the east. Even this last incision was a product of a number of events reflected in a sequence of terraces in the inner valley.

Dissection by knickpoint migration appears to have been the mechanism of valley deepening below the third terrace, because the entrenched meanders of the Leon, particularly in the reach between Gatesville and Belton, are generally symmetrical in cross section. While meander necks in softer rocks upstream, such as Twin Mountains Sand, give the clear impression of Slip off slopes, in the lower river where the valley is walled in harder rock most meander necks exhibit near vertical faces and cross valley profiles are generally symmetrical. Furthermore, valley meander geometry is that of free meanders of a much larger river, and none of the preserved meanders exhibit the exaggerated meander shape so often encountered in deeply ingrown meanders. The mechanism of earliest entrenchment (that which initiated the development of the Cut Plain) is of course unknown. The presence of possible terraces between the divide gravels and the third terrace suggests that this entrenchment may have been the product of staged incision as well (Tharp, 1988).

STOP H--BEL TON DAMSITE--The eastern margin of the Cut Plaln--(31° OS' 43" N; 970 28' 17" W; Moffat and Belton quadrangles [1974], 1/24,000) (Fig. 17-1)

Here at stop 17, we stand at the eastern margin of the Cut Plain, overlooking the Texas Blacklands to the East. The Edwards Limestone plunges steeply eastward into the subsurface at the western margin of the East Texas Basin (marked by the Balcones Fault Zone). At Stop 17 the top of the Edwards Limestone is at an elevation of 640 feet where it forms the top of the bluffs along the river valley. Four miles south, where Nolan Creek crosses 1-35, Edwards Limestone forms the floor of the creek, at an elevation of about 470', giving a dip component of about 40 ftJmi.. Upstream to Stop 15, a distance of 15 miles, the elevation difference is 80 feet, and the dip component is a little more than 5 ft.lmi.. Thus, between here and 1-35 at Belton, dip increases by eight times, and the rollover is clearly visible. East southeast of our present position, on the skyline about seven miles away, the most prominent feature is Scott and White Hospital. It is at an elevation of 740 feet, on the crest of the Whiterock Escarpment, near the top of the Austin Chalk. At that point, the Edwards limestone is at an elevation of about 200 feet, about 540 feet below our present position (Fig. 17-2).

Therefore, at Stop 17 we are on the eastern margin of the Central Texas Platform, and on the western margin of the East Texas Basin proper. Both are structural provinces with great significance in the evolution of the Lampasas Cut Plain. Here we consider (1) the relationship between the Cut Plain and the Blacklands to the East, and (2) what we have seen on this trip, and what it all may mean in the long history of the Lampasas Cut Plain.

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/,:..--- /l Figure 17-1 (Belton Quadrangle, 1124,000; 1974) Stop 17, overlook of the Leon Valley at Belton Dam. This Is the last stop of the trip--a view of the eastern edge of the Cut Plain, the western margin of the East Texas Basin, and of the Texas Blacklands which bound the Grand Prairies to the east. This is also the end of the deeply entrenched inner valley of the Leon River

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-~.

WEST

Comanche Pedlpleln .. Approximate prollle of Comanche Pedlplaln ~ <i ~ o

'" !!! :;;

EAST

STOP 15 STO\ 16

STOP 17 ~

" C

-'- ---

Comanchean

- --- --~ : --- -- --....... -- ---..................... ..... , ~

........

Central TaX8S Platform

Balcones Faull Zone

G

o u Ul

- -- AIJStJl) C'" c::::.::; ... / k

- GUlf/an

----ComaOCh.ao'GU",.o cOo/,c/

EdMlard3

Comanchean

T xas Basin East e

Figure 17-2 West-east diagrammatic profile and section from Payne Gap (Stop 15, Hayward, AI/en, Amsbury, 1990) to the Whiterock Cuesta, showing topographic-structural relationships of the Grand Prairie (of which the Cut Plain is a part) to the Black Prairie; and from the Central Texas Platform to the western margin of the East Texas Basin. Notice that the stratigraphic position of the Comanche Pediplain climbs section from Walnut Clay in the west, through Comanche Peak limestone, to the top of the Edwards Limestone at Stop 16. We are uncertain of its position at Stop 17. Eastward in the Blacklands there are "Uvalde gravels· which undoubtedly correlate with one or another of the high gravels of the Cut Plain. They occur at elevations much above the third terrace and Comanche Pediplain (Hayward, AI/en, Amsbury, 1990), though earlier we suggested such a correlation (Hayward and AI/en, 1987)

The crest of the valley wall of the Leon River which we saw at Stop 16 we interpreted as the base level to which Leon Valley pediments were originally graded. The inner valley we attributed to knickpoint migration, controlled by base level change in the Blacklands to the east.

The history of the Cut Plain has involved several episodes:

First was the formation of a "Time Zero" surface, a stripped structural plain on Edwards Limestone, extending northward from the present Edwards Plateau to the area of the Callahan Divide and an unknown distance farther to the north. This was a northern extension of the Edwards Plateau. This surface was locally or regionally veneered with alluvium containing abundant coarse quartzose gravel, derived from far to the west. The age of this surface antedates the Ogallala, for Ogallala deposits onlap only the lower slopes of the Callahan Divide at Big Spring, Sweetwater, and Roscoe. Farther to the northwest, near Post, the Callahan Divide is buried by High Plains deposits. The divide beneath the High Plains appears to be a mature landscape (Cronin, 1969; Seni, 1980; Gustavson and Finley, 1985; Fallin, 1989), which

existed in its present form at the time of burial. Thus the age of the ancient surface exceeded that of the oldest Ogallala, and by a considerable time.

By mid-Miocene Time there was already a mature Cut Plain-like landscape along the Callahan Divide. The western Cut Plain is continuous with the eastern limit of the Callahan Divide, and appears to be of the same age. Thus, there may have been a long period in which the older Cut Plain landscape existed before the Yarmouthian terraces were finally emplaced on the pediplain along the axes of various basins (Dahl, 1987).

Second, a pattern similar to modern drainage was established on this Time Zero surface. This included an ancestral Brazos and an ancestral Colorado in the region west of the western limits of this trip. These streams entrenched through the old Edwards Plateau to the level of the pediplain, where they achieved grade, a profile they maintained for an extended period. This was apparently controlled by events in the Blacklands to the east, for entrenchment on the upper rivers was dependent on local base levels established by trunk drainage. During the period at which these streams flowed at grade siliceous gravels and sands were principal components of trunk stream loads, and they remain as the third terrace, tentatively dated as Yarmouthian (Stop 16) (Tharp, 1988). So the old Cut Plain with its pediplain still existed in Yarmouth Time.

Inner valleys in that ancient surface were probably products of erosional stripping of the Blacklands to the east (Montgomery, 1986) beginning in mid-Pleistocene time. This too was episodic. The inner valley, post-Yarmouth entrenchment was intermittent. The upper terrace represents the first alluviation event, followed by dissection and alluviation through three rock defended terraces, and a floodplain.

The terrace chronologies of the Trinity, Brazos, Leon and Colorado Rivers have been compared (Fig. 17-3) (Tharp. 1988). Most of these age assignments are based on assumptions that are plausible to many, though not all, students of the problem, so here we consider some of the evidence on which they are based. These major streams appear to have similar terrace geometrys; in that each stream system has an alluvial floodplain of Wisconsin to Recent age; a first terrace system at about 20 to 30 feet above the floodplain (the 30 foot terrace), of Wisconsin age; a second terrace system at about 50 to 60 feet above the floodplain (the 60 foot terrace), of Sangamon age; and a third terrace system at 90 to 110 feet above the floodplain (the 100 foot terrace), of Yarmouth age. Above the third terrace are numerous gravel lags, and a few actual terrace sections, but generally too few to permit mapping of yet higher terraces above the third terrace and below the lags (and occasional

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RIVER • J.CT1JAL DATED EVEIlTS

T'RrNrN RIVER BRAZosANeR l..,Mlp A$AS R lYE R CXl.CP.AlXJ RNeR w:NIWER Shul ... 19~ BroN.U9" ISiSO O'tM!um anct .sa....;hI.., SId'\IPtotI and Co&. 1S137 Adluna and NICIl

S10vIII AIId UcAnuity F". and L«II'IW lR5.2 , ... ....Ul ... 1R42 'IIlD 10., Slncldin '00' u ... _ , .. , L .. ."d , .. ,

Crool!. and Haml 1157 B.mlld. 1'1c. 1M2 Loo"" IOn ThaI'D IU7 SLaughll' IHO, M. Wi\lan ,M2 and a.u.r ISl77

NJE n, 17 anct RKm-.. s.nw et. aI. 1110 au. anct PtnlMd~

"., Lund.aus ,nt Or~.'Sln a.,natd and lt8Llnc EDI:III 1 Sl1:I , ... Nonn 1817 Wlllimo" tOn Collins 1012 Farn"; 11111

ftcx:::oPLAIN •

LATE VaLI::ISro and 0 ..... FLCXX>Pl,AIN •

FLOCOP\.AIH • FLDODP LA>< • P..IX:CP\.AI< 1870 D. &33 L .... .",d l!i116S1 D. 18

IClJXa.E FIlM; 1R811 p. 80t Blml&l'd ... .,. lV70 Eddy 1S11:1 ~ 2D

S1lugl'lfI' 1017 p. 11 P. SotlS Pr..-t 1S17. p. ,.to

EARlY Nann 1017 pel'W)l'\&J SaUl and Plntl.oo. ............... er,ILana 1077

p. ""

TERRACE 1 PCTJ . LATe TERRACE 1 P1l . TE R RACE , (3CIl • Urt)anec lH3 p. eo

8rcnIU;t'I lV~ P. 22 OI..rum.-.:t sz.u;h\et BaNI, and Plntl.oo. TERRACE 1 QCTJ

WIS:ONSII EDI:III II n p. '.20 11e1 P. "1$ Orl'lllna tSin TERRACE 1 ~ • ..""

EARlY Willi,"," IBM Po 4U Feml'l9 Hlee Po 801 TERRAC£ 2' (6CJ'1 SlaugMW lU7 D. SI

TERRACE 2' (110'1 . Shw., lQ35 p. '2

~ StCHall &lid IoIcAII'Il1ty TERRACE 2' (601 • TERRACE 2 (.., TERRACe :3 (loa) TERRACE 2' (80') •

1;., p. 2" Lund ...... 11" P. 241 CtOOll; II'Id H.",.

Adkins and Aller.

I1S7 p. Zl 1;30 g. 87·SI

SlaU;h1w 1117 g. I

(WNDISAN

YJIR,OOUlH teRRACE 3 (100'1 TERRACE 3 (100")

_SAN TERRACE 3 (100'1

.IFlt:N1OH

NEIPAS<AN

Figure 17-3 Table showing terrace correlation for Central Texas streams. The uniformity from basin to basin strongly suggests that terrace events were products of regional climate that effected aI/ Central Texas streams equal/y. Since climate is regional, this further suggests that the terrace history of Central Texas may extend far beyond the confines of the region of Central Texas, perhaps involving much of the south central United states (Tharp, 1988).

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terrace sections) probably erroneously designated as Uvalde (300 or more feet above the floodplain) (Tharp, 1988)

Valley widening since entrenchment has been almost insignificant in comparison to the great valley width of the pediplain, leading to the conclusion that this earlier episode of scarp retreat was probably more rapid, and probably of far greater duration than the episodes that created the inner valleys ..

CONCLUSIONS

And now a return to the original assumptions which were stated in the introduction to this trip.

On casual glance the earlier assumptions appear valid, yet recent work directed specifically at Cut Plain evolution has cast doubt on most or all of them. The present revision of these earlier conclusions might read as follows:

First, the Cut Plain is indeed an intermediate stage in the evolution of the Grand Prairie, but it is a complex Intermediate stage. Evidence suggests that several episodes of Cut Plain modification, some of very long duration, were interposed between the initial stage of Cut Plain formation and the Cut Plain we see today. Evidence of at least five of those stages is yet widely visible and was seen on this trip (Figs. 17-4 to 17-8).

Second: no one will deny the overwhelming landscape control exerted by Edwards Limestone, but within the Cut Plain almost of equal importance are the effects of lithologic facies, groundwater, stream gradient, and time. Of particular importance are thresholds of process initiation, for there were apparently times of rapid slope retreat, almost province wide, followed by periods when the landscape remained essentially static, apparently for long periods. Even when thresholds were exceeded, not all Cut Plain valleys participated in valley widening, for groundwater fed baseflow and stream gradient appear to have had major influence (Fig. 2-2, 2-3).

Third: the mechanism of slope retreat originally described appears to be more Imagined than real. The slump blocks which should everywhere mark retreating slopes are rare. They occur on only a very small fraction of upland margins. They should be most abundant where spring lines are most conspicuous, but instead they often appear to have no readily explained pattern. The profiles of Cut Plain divides, which should show the slope breaks at the base of the Edward, show instead a continuous slope from the top of the Edwards. Edwards blocky rubble, which should be a major component of colluvial veneer, is commonly a minor fraction. While groundwater appears to have been perhaps the major control on slope retreat, the actual mechanism appears to have involved the entire slope, from the top of the Edwards down. If slump blocks were a common product of retreat, they have since been weathered away over most of the Cut Plain.

On the larger scale, slope retreat appears to have involved processes of tributary formation, elaboration, abstraction, and finally divide destruction by processes that are as much chemical as physical and which originate on the slopes (Stops 2, 3, 4, 8). Most models of slope retreat predict outliers in abundance. Most result in highly crenulated divide margins. Yet the common characteristic of much of the Cut Plain is the near absence of outliers, and the existence of very long straight divides, particularly straight on up dip margins. The formation of such straight segments is not well understood, but it appears to have involved mechanisms universally active on the slopes themselves, and almost certainly involved groundwater effects. The sequence of drainage net initiation, elongation, and abstraction (Fig. 4-5) clearly applies to stream network evolution, but there were apparently other highly significant aspects not confined to channels, and

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active over the whole surface of the slope. Of these latter, we have well-grounded speculations, but they are yet unproven (Stops 2, 3, 4, 8).

Fourth; the "migrating" Cut Plain appears to have remained remarkably stationary for a very long while. It exists now over much of the area which it originally covered. Parts of it may have existed since late Miocene time. Even the earliest remnants of the Callahan Divide, buried under Ogallala deposits, appear to have been of mature landscapes. Modification since that time has changed the Cut Plain, but far less than intuition suggests that it should.

Fifth; in an overall sense, the pattern of stages in Cut Plain evolution is as earlier outlined, but a more detailed assessment introduces complexity. Stages in Cut Plain evolution apparently began with a "time zero" surface upon which topography and drainage were radically different than those of today. Into this surface initial dissection began the process of Cut Plain formation. Eventually streams approached grade, and those which had achieved grade maintained graded profiles for long periods during which slopes retreated leaving the broad Comanche Pediplain. Later, streams again entrenched in a history of dissection and alluvlation to the present time, and left a record in valleys and terraces below the old pediplain surface.

Sixth--in this discussion of the Cut Plain, we have only briefly considered the terrace histories of the rivers which drain the region. This constitutes another problem of considerable complexity. Our justification for this omission is that clearly defined terraces occur almost entirely within the inner valleys which have cut below the Comanche Pediplain, and they record "minor" events in a largely modern drainage history. Our discussion, ranging in time from Eocene(?) to Recent, has concentrated on the major chronology of the larger elements of this landscape.

Seventh--the evolutionary sequence In the Cut Plain has implications far beyond the limits of the Cut Plain. Major elements of Cut Plain landscapes appear to reflect long duration climatic events, which also left their impress on landscapes of much of the South Central United StatesFeatures similar to the Cut Plain pediplain have been recognized over much of south central North America, along the eastern margin of the High Plains, along the valley of the Canadian River and its tributaries in northeastern New Mexico, in the Edwards Plateau of southwest Texas, and in southern Kansas, and over much of the southwest. They may not be related, but they appear to reflect similar histories, which suggests the possibility of equivalence.

Dating of the older events is yet highly speCUlative, but the sequence of events appears to be decipherable.

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Figure 17-4 The next five illustrations constitute a recapitulation. Figure 7-4 pictures the oldest geomorphic event recorded in the landscape, the Time Zero surface. On the west this was an Edwards Plateau-like landscape, on the east a Washita Prairie-like landscape, perhaps with an alluvial veneer, watered by west-to-east flowing drainage, originating far to the west .

. The age of this surface Is uncertain, but some evidence suggests that it is older than Late Miocene, for a Cut Plain-like landscape is apparently buried under late Miocene deposits of the Southern High Plains. Relict upland gravels of the ancient Washita Prairie streams apparently represent a minor fraction of the total bedload of mixed-load streams. The gravels of this high . surface came from far west or northwest of the present High Plains. Other than that, little is known about these earliest of systems (from Hayward, Allen, Amsbury, 1990).

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Figure 17-5 While the Time Zero surface was stili in existence, and during most active subsidence of the Gulf Coast, drainage was reoriented from the apparent west-to-east direction on the Time Zero surface, to the northwest-to-southeast direction of the present pal/ern. The time of this reorlentalion Is uncertain, but it is suggested that it took place in early Miocene, since an apparently mature Cut Plain landscape is buried beneath the late Miocene deposits of the High Plains. Initial entrenchment into an Edwards Plateau-like surface, shown here, began in the northwest, beyond the limits of our trip. By late Miocene, both the Callahan Divide and the Lampasas Cut Plain had been formed, at least to a substantial degree. Thick Ogallala deposits (Late Miocene-Pliocene) bury a Cut Plain-like landscape near Post, and the feather edge of the thinning Ogallala onlaps the foot of the Callahan Divide west of Abilene (from Hayward, Allen, Amsbury, 1990) .

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Figure 17-6 Entrenchment of reoriented drainage into Fredericksburg rocks to form the Cut Plain began in the northwest and progressed southeastward, somewhat down dip, along major trunk streams and their tributaries. The cause of this initial incision Is uncertain. We have long considered local mechanisms, but since similar landscapes, of similar age apparently exist over much of the south central United States, the mechanism was probably climatic. The absence of clear evidence of intermediate stillstands in the episode of dissection suggests that this initial incision to the level of the Comanche Pediplain may have been a single event of entrenchment, apparently followed by a stillstand of long duration, in which valley widening was the dominant activity (from Hayward, Allen, Amsbury, 1990).

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Figure 17-7 The time of slope retreat was apparently a long one, for the expanding pediplain entered even the smallest of tributary valleys along those trunk streams which had participated in the valley-widening event. Before the end of this episode, valley-widening along trunk streams was measured in tens of miles. The ultimate landform was the Lampasas Cut Plain of very wide valleys and narrow divides, in which more than nine tenths of the original cover of Edwards Limestone was stripped away. The processes and rates of slope retreat of the valley­widening phase are unknown, but groundwater appears to have played a major role. The presence of a loess·like silt veneer over much of the older Cut Plain, and the preservation of petrocalcic soils suggests that climates varied from humid to arid, In a cycle common to much of the unglaciated south central United States. In mid-Pleistocene time, toward the end of the pedlmentation phase, major trunk streams flowed in wide meanders over valley alluvial plains. Alluvial soils preserved as high terraces are remnants of that once extensive depositional surface. Very large entrenched meanders mimic the original free meanders of the ancestral streams. Meander geometry suggests that through-flowing trunk streams were greatly augmented, probably by contributions from other sources since lost (from Hayward. Allen. Amsbury. 1990).

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Figure 17-8 During mid-Pleistocene time, renewed incision began on all trunk streams transecting the Cut Plain, creating deeply entrenched inner valleys within the old Comanche Pedlplaln. Here again the action was widespread, Involving the whole Cut Plain, adjacent geomorphic provinces, and perhaps much of the south central United States. This last major Incision, however, was a complex one, involving at least four episodes of entrenchment and alluviation. This Included (1) the last alluvial phase of the Comanche Ped/plain, preseNed as the "100 foot" terrace of mid-Pleistocene age, (2) entrenchment to the level of the "60 foot" terrace, of early late-Pleistocene age: (3) entrenchment to the "3~ foot" terrace of Wisconsin age; and (4) incision to the present floodplain, of late Wisconsin to Recent age. During the period of inner entrenchment there have been widespread and significant fluctuations In climate, but slope retreat within the Inner valleys has been modest. By implication, the same has been true of the slopes of the older Comanche Pedlplain, suggesting that either there was a distinct difference in the duration of slope retreat episodes in the era of the Comanche Pediplain, and the post-pediplaln era of entrenchment and slope retreat, or there there was marked difference in the mechanisms and rates of slope modification between the time of formation of the Comanche Pediplain and the more recent inner valleys (from Hayward, A/len, Amsbury, 1990).

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SOME OBSERVATIONS AND SUGGESTIONS FOR THE FUTURE

In the five years since the body of this guide was written, reconnaissance in other areas has continued the search for landscapes having similar histories and similar chronologies. Our purpose is to test the thesis that what we see in the Lampasas Cut Plain is truly the geomorphic response to a regional climatic history, that is, a sequence of different climates. If it is, landscapes over a wide area should have participated in that climatic history. Landforms in other areas might be different depending on the effects of the initial landscape from which today's descendent was formed, details of stratigraphy, structure, erosional agencies, and climatic variations within the overall climatic zone. but landforms in all the areas under a similar climatic sequence should have gone through similar histories of climate-induced change.

For example, the major events in the history of the Lampasas Cut Plain that should be reflected in some form in time-equivalent landscapes beyond the Cut Plain inclUde:

I) --the existence of an earlier regional landscape from which the present landscape has been derived. In the case of the Cut Plain, that ancient surface was apparently a stripped structural plain on rocks of varying resistance to erosion, forming a surface later veneered with coarse, supermature, siliceous gravels. The age of this surface is uncertain, but evidence suggests that it could be as old as early-to-middle Miocene.

2) --the dissection of that ancient surface through the incision of trunk streams and their major tributary networks. In the case of the Cut Plain, that incision (pOSSibly staged) cut through a substantial thickness of rock, as much as 700 feet, ultimately to establish a graded profile that became the base-level control for pediplaination on a major regional scale.

3) --an episode (or episodes) of valley widening on a grand scale by processes of slope retreat and pedimentation, and the establishment of widely meandering trunk streams in alluvial bottoms, at grade, within these broad pediment valleys. In the case of the Cut Plain there are valleys of this event that exceed fifty miles in width and 700 feet in depth, that are even now in a remarkable state of geomorphic preservation. The time of beginning of this pedimentation episode is uncertain, though there is evidence suggestive of late Miocene landforms appropriate to this event. There is far firmer evidence that the pediment-phase of landscape development was still the active landscape until mid-Pleistocene time.

4) --the penultimate stage, and the last major event in the active developmental history of this old pediplain landscape was a period of massive caliche formation largely along the pediplain valley-centers. This was followed by a period, perhaps short, when pulverulent caliche was altered to a highly-distinctive petrocalcic caliche, a distinctive marker still existing over thousands of acres of pediplain surface.. In the case of the Cut Plain, this petro calcic caliche is unique to the pediplain surface. It occurs nowhere else in the landscape.

5) --the final stage in landscape modification has been episodic entrenchment of trunk stream and tributary networks into the ancient pediplain surface. In smaller tributary valleys it is a process that yet continues. In the case of the Cut Plain, there appears to have been three major episodes of alluviation and entrenchment, evidence of each alluvial event now preserved in a distinctive terrace level. Comparison of terrace histories of major streams in Central Texas indicates that trunk streams share a common or near-common terrace sequence. In the Cut Plain the oldest such terrace level is that of the pediplain toe, the 100 foot terrace, late Kansan or Yarmouth in age. Below this is the "second terrace", the 60 foot terrace, of Sangamon age. Yet lower, at an elevation

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I I

I

I

of about 30 feet above the floodplain, lies the third terrace, of Wisconsin age. Lowest of all are the alluvial floors of the rivers, of Wisconsin to Recent age. And in this incision of major trunk stream valleys across the Cut Plain, the wide meander loops of the ancient pediplain streams have been remarkably preserved as great valley meanders.

In projecting these episodes to other regions, a wide variety of landforms might be involved, but a general chronology should relate them as common participants in a history of regional landscape development. Still, in our search for common landscape histories, our first need was to identify landscapes, the form of which seemed to offer strong evidence of similar origins.

We found them.

First, we extended the search westward along the Brazos and Colorado Rivers to the Caprock Escarpment and beyond the High Plains to the Pecos River. At the same time reconnaissance was extended to the east into the Black Prairie, and on eastward to the Trinity River. Additional search extended northward, first into the Fort Worth Prairie and then to the Red River. Additional landscape-hunting trips sought 'familiar" landforms in western Oklahoma, southern Kansas, south Texas, and northern Mexico.

As an aside, this was not unpleasant 'work". The nice thing about research in regional geomorphic history is that it Is probably among the last of the 'golden age" reconnaissance level geological problems still open to today's investigator. And these problems can be done while on family vacation trips, public relations speaking tours, and bedrock geology field trips; or while having coffee in a country cafe, enjoying the view from a high point, and while alone, with companions, or with wife and kids ..

Out of this most enjoyable of investigations there has come to us a strong indication that Cut Plain history may be representative of much of the southern midcontinent, and may well extend Into west Texas, Coahuila, and Chihuahua. And we feel that great region Is probably a minimum.

We now realize that we have bitten off far more than we can effectively chew.

In each area we have found landscapes that appear to us to justify further investigation, but we also recognize the need for Independent eyes, not already committed to a pre-approved outcome. An effective effort will require the combined input of far-better Informed colleagues In each area. Therefore we propose a multi-authored regional study of the evolution of late Tertiary to Recent landscapes of the southern midcontinent. Our experience has led to the generalization that in this case the mystical "scientific method' simply boils down to attempts to answer the questions: ·What was going on across this great region for all of that time, and how did it function?"

We need your help.

We appreciate any suggestion you might offer, we welcome you enthusiastically to this regional effort, and we remind you that its greatest appeal is not only In the grand dimensions of the effort in space and time, but that it takes place In the great open expanses of the most beautiful parts of southern North America, In a region where memorable coffee, mouhwatering pie, and barbecue of superior quality are the norm.

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BIBLIOGRAPHY

Adkins, W. S., and Arick, W. P. (1930) The Geology of Bell County, Texas; University of Texas, Bureau of Economic Geology, Bull. 3016, p. 92

Allen, P. M. (1990) oral communication

-=-_:_-;----:-;---;-;-,(1989) oral communication, also in Soil Survey of Limestone County, U S Department of Agriculture, Soil Conservation Service, in press.

Amsbury, David L, (1995) Stratigraphy of Fredericksburg Rocks (Middle/upper Albion Cretaceous) of north-central Texas; unpublisheahanuscript, Ferdinand Roemer Geological Ubrary, Waco, TX.; p. 70.

Amsbury, D. L., and Hayward, O. T. (1996) Comanche Pediplain, a major element of the southwestern landscape (Abs.) Geological Society of America, South Central Section, Abstracts with programs, March 11-12, 1996, p. 2.

Allee, W. A., 1962, The Lower Cretaceous Paluxy Sand in Central Texas: Baylor Geological Studies, Bulletin no. 2, p. 26.

Bain, J. S. (1973) The nature of the Cretaceous-Pre Cretaceous Contact, North-Central Texas; Baylor Geological Studies, Bull. 25; Baylor Univ., Waco, TX.; p. 44.

Baker, V. (1977) Stream channel response to floods with examples from Central Texas; Geol. Soc. Am., Bull., vol. 88, p.1057-1171. .

Bammel, Bobby (1979) Stratigraphy of the Simmsboro Formation, east·central Texas; Baylor Geological Studies, Bull., No. 37p. 40.

Barquest, Bradley (1989) A hydrologic assessment of the Austin Chalk outcrop belt, Central Texas, Unpublished Master's Thesis, Baylor University, Waco, TX., p. 174 ..

Barrett, Daniel (1989) A hydrologic assessment of the Ozan Formation, Central, Texas; Unpublished Master's Thesis, Baylor University, Waco, TX., p. 141.

Beede, J. W., and Bentley, W. P. (1918) The Geology of Coke County, Texas; University of Texas, Bureau of Economic Geology; Bull. 1850; p.81.

Bingham, N. L. (1989) Anomalous soils of the Lampasas Cut Plain, nature, distribution, and significance; unpublished student paper, Geology 5346, Baylor University, p. 69.

Bradley, Robert G. (1988) Anomalous soils of the Lampasas Cut Plain--nature, distribution, and significance; Unpublished student paper, Geology 5346, Baylor University, p. 69.

Brown, Thomas E. (1988) Relationships between basin parameters and landform, Lampasas Cut Plain, Central Texas; Unpublished master's thesis, Baylor UniverSity; p. 166.

Brotherton, M. A. (1978) The geomorphic evolution of the North Bosque River; Unpublished bachelor's thesis, Baylor Univ.; Waco, TX.; p.131.

Bunting, B. T. (1961) The role of seepage moisture in soil formation, slope development, and stream initiation; Am. Journ ... Sci., vol. 259, p. 503-518.

1-92

Byrd, C. L. (1971) Origin and history of the Uvalde Gravel, Central Texas; Baylor Geological Studies, Bull. 20; Baylor Univ.; Waco, TX.; p.48.

Cannata, Stan L. (1988) Hydrogeology of a portion of the Washita Prairie Edwards Aquifer: Central Texas; Unpublished M. S. thesis, Baylor University, Waco, TX.; p. 205.

Collins, Andrew D. (1989) Geochemistry and flow characteristics of springs discharging from the Edwards Aquifer: Washita Prairie, Central Texas; Unpublished M.S. thesis, Baylor University, Waco, TX.; p.137.

Cronin, J. G. (1969) Ground Water In the Ogallala Formation of Texas and New Mexico; U. S. Geological Survey, Hydrologic Atlas 330, p.9.

Dahl, Suzanne (1987) The Intermediate surface across the Brazos River, I\Iertlral Texas, and its relationship to Lampasas Cut Plain evolution; Unpublished student paper, Geology 5346, Baylor University, p. 50 (approx.)

Epps, L. W. (1973) The geologic history of the Brazos River; Baylor Geological Studies, Bull. 24, Baylor Univ., Waco, TX.; p.44.

Fallin, J. A. (1989) Hydrogeology of Lower Cretaceous strata under the Southern High Plains of Texas and New Mexico; Tex. Water Development. Board, Report 314, Austin, TX.; p.39.

Faulkner, H. (1974) An allometric growth model for competitive gullies; Zeitschrlft for geomorphologie; Supplement 21, p. 76-87.

Fisher, W. L. and McGowen, J. H. (1969) DepOSitional systems in the Wilcox Group of Texas and their relationship to the occurrence of oil and gas; American Association of Petroleum Geologists Bull. vol. 53, p. 30-54.

Flatt, C. D., 1976, Origin and significance of the oyster banks in the Walnut Clay Formation, Central Texas: Baylor University Geological Studies, Bulletin no. 30,47 p.

Flowers, Beth A. (1987) Geomorphic evolution of the upper Colorado River basin; student paper, Geology 5346; Baylor University; p. 66.

(1989) Sediment budget analysis of the Coryell Creek drainage Basin, Coryell County, TX., Unpublished M. S. thesis, Baylor University, Waco, TX., p. 106.

Glock, W. S. (1931) The development of drainage systems: a synoptic view; Geographical Review, Vol. 21, p. 475-82.

Gupta, A. (1983) High magnitude floods and stream channel response, In Spec. Pubs. International. Assoc. Sedimentology; No.6, p. 219-227.

Gustavson, T. C., and Finley, R. J. (1985) Late Cenozoic geomorphic evolution of the Texas Panhandle and northeastern New Mexico; Univ. of Texas, Bureau of Economic Geology, Report of Investigations No. 148; p. 42.

Hayward, O. T. (1988) The Comanchean section of the Trinity Shelf, central Texas, In Southcentral Section of the Geological Society of America, Centennial field guide, vol. 4 (0. T. Hayward, ed.) Geological Society of America; p. 323-328.

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Hayward, O. T., and Allen, P. M. (1987) The Lampasas Cut Plain; Form, Process, Evolution: Guidebook, South Central Section, Geological Society of America; Baylor University, Waco, TX., p.84.

Hayward, O. T., Allen, P. M. (1988) The Lampasas Cut Plain; Form, Process, Evolution (2nd ed.): Guidebook, Southwestern Association of Student Geological Societies, Baylor University, Waco, TX., p. 119.

Hayward, O. T. Allen, P. M. and Amsbury, D. L. (1990) The Lampasas Cut Plain-Evidence for the cyclic evolution of a regional landscape, Central Texas, Field Trip #19. Geological Society of America 1990 National Meeting, Dallas, TX. p.128.

Hayward, O. T.; Dolliver, P. N.;, Amsbury, D. L.; and Yelderman, J. C. (1992) The Grand Prairie of Texas: land, history, culture; Program for Regional Studies, Baylor University, Waco TX .• p. 98.

Hill, R. T., (1899) A preliminary annotated check list of the Cretaceous invertebrate fossils of Texas. accompanied by a short description of the lithology and stratigraphy of the system: Texas Geological Survey Bulletin 4, 57 p.

Hill. R. T. (1900) Physical geography of the Texas Region; United States Geological Survey, Topographic Atlas of the United States, Folio 3; p. 22.

=-__ (1901) Geography and Geology of the Grand and Black Prairies, Texas; U.S.G.S., Twenty First Annual Report, Part VII; p.666.

Huckabee, J.; Thompson, D.; Wyrick, J.; Pavlat, E. '(1977) Soil Survey of Bell County Texas; U.S. Department of Agriculture. Soil Conservation Service; p. 75.

Ikin. Arthur (1841) Texas: its history. topography. agriculture, commerce and general statistics; reprint, Texian Press, Waco, TX, 1964, p.100.

Irving, Washington (1835) A tour of the prairies; reprint, University of Oklahoma Press. Norman OK. p. 216.

Kendall, George W. (1844) Narrative of the Texas Santa Fe expedition ........... ; In two volumes, reprint of the first edition, Steck Company, Austin, TX. (1936) Vol. 1, p. 405. Vol. 2, p. 406.

King, L. C. (1962) Morphology of the Earth; Haffner. N. Y.; p. 699.

Knapp, M. (1990) oral communications -- work In progress, nature, origin. and significance of the high gravels. Callahan Divide, Texas).

Knighton, D. (1984) Fluvial forms and processes; Arnold, London; p.218.

Laity, Julie E. and Malin, Michael C. (1985) Sapping processes and the development of theater­headed valley networks on the Colorado Plateau. Geological St).oiaYca:>fBulietin, v. 96, p. 203-217.

LeFevre. Stephen B. (1989) Geologic controls on the main channel of the Middle Bosque River, Central Texas-a bedrock controlled stream system; Unpublished master's thesis; Baylor Univer­sity, Waco. Tx.; 117 p.

Lemons, D. R. (1987) Structural evolution of the Lower Cretaceous (Comanchean) Trinity Shelf; Unpublished master's thesis. Baylor Univ.; Waco. TX.; p.300.

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Lewand, Raymond L. (1969) The geomorphic evolution of the Leon River system; Baylor Geological Studies, Bull. 17; Baylor Univ.; Waco, TX.; p.27.

Lozo, F. E., 1959, Stratigraphic relations of the Edwards Limestone and associated formations in North-central Texas: in Lozo, F. E., Nelson, H. F., Young, Keith, Shelburne, O. B., and Sandidge, J. R., Symposium on Edwards Limestone in Cehblillersi:ljfarof Texas Bureau of Economic Geology Pub. No. 5905, p. 1-19.

-=-_" and Stricklin, F. L., Jr., 1956, Stratigraphic notes on the outcrop basal Cretaceous, Central Texas: Transactions, Gulf Coast Association of Geological Societies, vol. VI, p. 67-78.

Marcy, Randolph B. (1854) Exploration of the Red River of Louisiana in the year 1852; Executive Document, 33d Congress, 1st Session, House of Representatives; A. O. P. Nicholson, Public Printer, Washington D. C.; p. 286, 20 plates.

McCaleb, N. (1985) Soli survey of Coryell County, Texas; U.S. Department of Agriculture, Soil Conservation Service; p. 129.

Mikels, J. K. (1977) Geomorphology of the Lampasas Cut Plain, Central Texas; Unpublished bachelor's thesis, Baylor University, Waco, TX.; p.42.

Montgomery, James A. (1986) The geomorphic evolution of the Taylor Black Prairie between the Trinity and Colorado Rivers, Central Texas; Unpublished master's thesis, Baylor University, p. 148.

Moore, C. H., Jr., 1961, Stratigraphy of the Walnut Formation, South-central Texas: Texas Journal of Science, vol. 13, p. 17-40.

-;-::;-:-" 1964, Stratigraphy of the Fredericksburg Division, South-central TexaShe University of Texas at Austin Bureau of Economic Geology Report of Investigations no. 52, 48 p.

Moore, T. H. (1970) Water geochemistry, Hog Creek Basin, Central Texas: Baylor Geological Studies, Bull. 18; Baylor Univ.; Waco, TX.; p.44 .

.,,-___ (1989) Aquifer-stream interactions and their hydrologic implications in nonkarstic limestones; Washita Prairie, Central Texas; Unpublished M. S. thesis, Baylor University, Waco, Tx., p.180.

North American Comm. on Stratigraphic Nomenclature (1983) North American Stratigraphic Code: American Association of Petroleum Geologists Bulletin, v. 67, no. 5, p. 841-875.

Owen, M. T., 1979, The Paluxy Sand in North-central Texas: Baylor University Geological Studies, Bulletin no. 36, 36 p.

Parish, B. R. (1989) Geomorphic response to regional structure, Lampasas Cut Plain, Central Texas; Baylor Geological Studies, No. 55, Baylor University, Waco, TX., pp. 28.

Parker, R. S. (1976) Experimental study of drainage basin evolution and its hydrologic implications; Colo. State Univ., Ft. Collins, Colo.; Hydrologic Papers 90; p. 58.

Perkins, B. F. (1961) Biostratigraphic studies in the Comanche (Cretaceous) Series of Northern Mexico and Texas: Geological Society of America Memoir 83, 138 p.

1-95

Poole, W. C. (1964) Bosque Territory; Chaparral Press, Kyle, TX., p.206.

Proctor, C. V. (1969) The North Bosque Watershed, Inventory of a drainage basin; Baylor Geological Studies, Bull. 16; Baylor University; Waco, TX.; p. 40.

Roemer, Ferdinand (1848) Texas, with particular reference to German Immigration and the physical appearance of the country,; translated by Oswald Mueller, (1935); reprint, German Texan Heritage Society (1983); p. 308.

Rose, P. R., 1972, Edwards Group, surface and subsurface, CetltrBAerSiEyi:asxf The Texas at Austin Bureau of Economic Geology Report of Investigations no. 74, 198 p.

Segrest, Charles C. (1977) Geomorphic evolution of the Middle Bosque River, Central Unpublished student paper; Baylor University, Waco, Tx.; p. 107.

Selby, M. J. (1982) Hillslope materials and processes;Oxford University Press, p. 264.

Texas;

Seni, S. J. (1980) Sand body geometry and depositional systems, Ogallala Formation, Texas; University of Texas, Bureau of Economic Geology, Report of Investigations 105, p.36.

Slaughter, Bob H. (1966) The Moore Pit local fauna--Pleistocene of Texas; Journ. Paleon­tology, v. 40; p.78-91.

Smith, Scott E. (1984) Evidence of ancient cross·divide drainage, western Bosque Basin, Central Texas; Unpublished bachelor's thesis, Baylor University; p.65.

Spencer, Kevin (1991) Drainage evolution across the White Rock Cuesta, Central Texas; unpublished Master's Thesis, Baylor University, Waco, TX, p. 177.

Stringer, B. (1980) Soil survey of Bosque County, TX; U. S. Department of Agriculture, Soil Conservation Service; p. 102.

Tharp, Tommy Lee (1988) Aspects of Leon River drainage history with implications to other Central Texas streams; Unpublished master's thesis, Baylor University, p. 260.

Thuma, Jeffrey J. (1987) The original topography of the Edwards Limestone across the Llano Region, and its relationship to Colorado River History; Unpublished student paper, Geology 5346; Baylor University, p. 50 (approx).

Vaughan, T. W. (1900) Folio No. 64, Uvalde, Texas; United States Geological Survey, Washington D. C.; p. 7, 3 maps.

Walker, J. R. (1978) Geomorphic evolution of the Southern High Plains; Baylor Geological Studies, Bull. 35; Baylor Univ.; Waco, TX.; p.32.

Yelderman, J. (1987) oral communication, Baylor Univ., Waco, TX.

Young, Keith, 1967, Comanche Series (Cretaceous), south central Texas: in Leo Hendricks, ed., Comanchean (Lower Cretaceous) stratigraphy and paleontology of Texas: Permian Basin Section, Society of Economic Paleontologists and Mineralogists Publication 67-8, p. 9- 29.

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__ , 1974a, Lower Albian and Aptian (Cretaceous) ammonites of Texas: in B. F. Perkins, ed., Aspects of Trinity Geology, Geoscience and Man, vol. VIII, Louisiana State University Press, p. 175-228.

_, 1974b, Edwards Plateau ammonites: in P. R. Rose, ed., Stratigraphy of Edwards Group and equivalents, Eastern Edwards Plateau, Texas: South Texas Geological Society Guidebook for AAPG-SEPM Field Trip 2, March 30-31,1974, p. 59-75.

, 1986, Cretaceous, marine inundations of the San Marcos Platform, Texas: Cretaceous Research, vol. 7, p.117-140.

Friends of the Pleistocene April 19 - 21,1996

Trip Log

April 19, 1996,7:30 am. Lampasas Cut Plain.

0.0 Turn right (west) onto the frontage road beside La Quinta (next to Popeye's) and move into the far left lane. Turn left under Hwy 190 and left again onto the frontage road (east). Enter H wy 190 East

18.7 Hwy 190 curves left (north) and joins Interstate 35.

62.5 Exit #335 at Baylor (Fort Fisher) Exit. Meet in the parking lot to begin. We will follow the trip described in your guidebook stops 1-8 and 16-17.

At the end of the trip take loop 121 to Highway 190 west and return to La Quinta.

April 20, 1996, 8:00 am. Fort Hood, Cowhouse Creek.

0.0 Turn right (north) onto Fort Hood SI. at the front of La Quinta.

1.3 Turn left onto base (Tank Destroyer Blvd) at T-intersection.

2.2 Turn right on 31st (stay in the left lane). Cross Central Ave and Park Ave.

2.9 Turn left (west) on North Ave. North Ave is a divided road but both sides have two way traffic.

3.8 Turn right cross the division between the North Aves and then turn left. You are now on the north side of North.

3.9 Turn right on Rod and Gun Club road. At the back of the parking lot, park near "Area Acess. "

4.1 After checking in with area access turn left (south) on 53rd.

4.2 Turn right (west) on North Ave.

7.4 Curve right (north) onto West Range Road.

8.8 Stay right on West Range Road (to "North Fort Hood")

10.5 Cross Cowhouse Bridge. We will turn left after crossing Cowhouse Creek. Watch the lead vehicle since our exact route may change depending on off road conditions. Stop I. Cutbank exposure.

10.8 Turn right (south) on West Range Road. Cross Cowhouse Creek and watch lead vehicle. Stops 2 and 3 are on the right after crossing the creek.

12.0 (approximate mileage). Turn right (south) on West Range Road.

13.8 Turn right (west) on Elijah Road.

16.4 Turn right on Old Georgetown Road.

20.3 Cross Cowhouse Creek. Turn right off Old Georgetown Road and follow the lead vehicle to stop 4. Lunch.

24.3 (approximate mileage). Turn left (south) on Old Georgetown Road, cross Cow house Creek and turn right following the lead vehicle.

25.2 (approximate mileage). Turn left (south) on Old Georgetown Road.

28.6 Turn left on Elijah road.

31.2 Fork right onto West Range Road (south).

34.5 Curve onto (heading east) North Ave.

35.6 Turn right, cross median, turn left onto North Ave.

36.6 Turn right (south) on 31st Ave.

37.5 Turn left (east) on Tank Destroyer.

38.5 Turn right (south) on Fort Hood.

39.8 Turn left into La Quinta

April 20, 1996, 6:30 pm. Trip to Frank's Lakeview and Anchor Club.

0.0 Turn right (west) onto the frontage road beside La Quinta (next to Popeye's) and move into the far left lane. Turn left under Hwy 190 and left again onto the frontage road (east). Enter Hwy 190 East

15.0 (mileage approximate). Exit at Loop 121. Turn left and head north. At 439 turn left (west) and follow the signs to Frank's.

2

April 21, 1996, 8:00 am. Eastern Training Area archaeological sites: rockshelters, burned rock mounds, and lithic procurement areas.

0.0 Turn right (north) onto Fort Hood St. at the front of La Quinta.

1.3 Turn right (east) on 439 (Rancier).

2.0 (Second traffic light). Turn left next to the Dollar Store. Proceed I block to Yield sign, turn right and then immediately head straight/left onto Garth. You wiII be heading north. Continue straight at the four-way stop. After this point continue straight (the road name wiII change to Martin) until the road stops at Central.

2.7 At Central turn right and then left (back onto Martin).

3.4 At stop sign next to heliport, continue straight. Note: this is not a four-way stop. You are now on East Range Road.

11.3 Cross Cow house Creek.

11.9 Turn right (east) on gravel road. Note results of recent fire on your left.

17.0 The road wiII curve to the left (north) and climb the escarpment.

17.5 Turn right onto dirt/gravel road (east).

IS.5 Turn right to lithic procurement site (watch lead vehicle). Stop I. 4IBL60S.

IS.7 Return to dirt/gravel road and turn right (east).

19.9 Turn left onto pipeline road (north).

22.3 Turn right onto dirt road (east).

22.4 Turn right. Follow lead vehicle. Stop 2, 41BL233. After Stop 2, turn around. At the dirt road turn right.

22.7 Stop to walk down to the rockshelters. Stop 3, 4lBL671 and 41BL670. Final stop. Turn vehicles around and return to the pipeline road.

23. I Turn left (south) on the pipeline road.

25.5 Turn right (west) on dirt/gravel road.

27.9 Turn left (sonth) and head down the escarpment. Notice view of Belton Reservoir.

33.5 Turn left (south) on East Range Road.

3

42.0 At stop sign proceed straight. You are now on Martin Road.

42.7 At Central curve right and make an immediate left turn to stay on Martin Road. Continue straight on Martin/Garth until you reach Rancier.

43.4 Turn right on Rancier.

44.1 Turn left on 439 (Fort Hood)

45.4 La Quinta/Highway 190.

4

SOILS AND GEOMORPHOLOGY OF COWHOUSE CREEK, FORT HOOD, TEXAS

by

Lee Nordt

2-1

Introduction

During the past decade geoarchaeological investigations in Texas have provided a unique opportunity for reconstructing alluvial stream histories, studying rates of pedogenesis, and making paleoenvironmental interpretations. Since 1989, with monies appropriated from the environmental staff at the Fort Hood Military Reservation, the alluvial histories of eight streams at Fort Hood, Texas have been reconstructed with the support of nearly 70 radiocarbon ages, and over 100 cutbank and 92 trench descriptions (Nordt, 1992). Cowhouse Creek, the largest stream within the Fort Hood Military Reservation, has a rich archaeological record that has yielded nnmerous radiocarbon datable samples. The focus of this report is the late Quaternary alluvial history of Cowhouse Creek. Specific objectives were to: 1) identify the major stratigraphic units, interpret their environments of deposition and establish a nnmerical stratigraphic chronology, 2) study soil chronosequences and chronofunctions within the stratigraphic framework, and 3) assess presentation potentials for the archaeological record.

Study Area

Geology of Fort Hood

Fort Hood is situated on the dissected eastern margin of the Edwards Plateau in the Grand Prairie Land Resource Area of Texas CD.S.D.A., 1981). Hill (1901) named the southern portion of the Grand Prairie the "Lampasas Cut Plain." Differential erosion of resistant and nonresistant Cretaceous rocks has produced a distinctive landscape configuration whereby nnmerous prominent flat-topped ridges are dissected by narrow and deeply entrenched stream valleys. These landscape features bear a close relationship with mapped geomorphic surfaces and soil distributions.

Major rock units that crop out in the Fort Hood area are Lower Cretaceous, which parallel ancient north-south trending marine shorelines (Bames, 1979) (Fig. 1). Sediments from Lower Cretaceous formations were deposited as part of a series of marine transgressions and regressions within an overall transgressive trend (Adkins, 1978; Rose, 1979). Marine transgression proceeded from the east along a broad shallow shelf as a result of subsidence of the East Texas Basin. Deposition of sediments in this low energy environment resulted in formation of limestones, marls, shales, and clays (Davis, 1974; Moore, 1969; Stricklin, 1971; and Nelson, 1973). Prograding fluvial and strandline sands, although not as abundant, were also deposited (Owen, 1979; Anderson, 1989) during relatively brief periods of terrigenous sediment influx, primarily from North Texas, Oklahoma, and Arkansas. Couplets of resistant limestones and nonresistant limestones, sands, sandstones, and shales produce a micro-stepped landscape on backwearing upland slopes in the Fort Hood area.

2-2

Figure I.

REESE CREE)(

L.EON RIVER

t

I ~ EDWARDS, DUCX CREEK, AND COMANCHE PEAK LiMESTONES

E3 WAL.NUT CLA.Y

EZEJ GL.EN ROSE FORMATION

_.- FORT HOOD MlL.lTARY RESERVATION BOUNDARY

Ok.

Major Rock units and stream patterns of Fort Hood Military Reservation.

2-3

The Cretaceous fonnations, from lower to upper, that are principal contributors of Quaternary alluvium in the Fort Hood area are summarized below (Barnes, 1979) (Figs. 1 and 2):

(1) Glen Rose Fonnation - fme-grained, chalky to hard limestone; interbedded dark gray clay and marl; partly arenaceous.

(2) Paluxy Sand - light gray to red, very fme grained quartz sands; locally interbedded shales and limestones.

(3) Walnut Clay - nodular and chalky clay, limestone, and shale. (4) Comanche Peak Limestone - hard and nodular, gray to white limestone; numerous

shale partings. (5) Edwards Limestone/Kiarnichi Clay undifferentiated - massive, rudistid limestone,

abundant chert; nodular clay, limestone, and shale.

The Glen Rose Formation forms the basement rock on which Cowhouse Creek flows, and along with the overlying Walnut, is the main contributor of sediment for fluvial transport and deposition. Therefore, alluvial sediments are abundant in sand-sized carbonates, large carbonate clasts derived from local limestones, and chert nodules from the Edwards Limestone.

Landscape Evolution of Fort Hood Area

Scattered water-worn siliceous channel lag on flat-topped ridges provide some of the earliest evidence of post-marine landscape history in the Lampasas Cut Plain (Hayward et al., 1990). These gravels were deposited by ancestral Central Texas rivers as they migrated across the Edwards Formation, thus forming what are now remnants of the "High" surface of Hayward et al. and the Manning surface of this study (Fig. 2). Hayward et al. (1990) correlate this surface with the Callahan Divide of North-Central Texas, which is stratigraphically 60 to 70 m above the southeastward projection of the Southern High Plains surface of Texas. This indicates that the Manning surface pre-dates deposition of the Ogallala Formation and development of the High Plains. By inference, because the Ogallala was deposited sometime between late Miocene and late Pliocene (Walker, 1978; Byrd, 1971), the Manning surface may be at least Miocene-age (Hayward et al., 1990).

Approximately 25 to 35 m below the Manning surface, resting primarily on the Walnut Clay and partly on the Comanche Peak Limestone, lies the broad and gently rolling "Intermediate" surface of Hayward et al. (1990) and the Killeen surface of this study (Fig. 2). Sometime during the late Tertiary or early Quaternary, widespread valley entrenchment resulted in abandonment of the Manning surface. The Killeen surface then formed as a result of long-term landscape stability and pedimentation as the Manning surface and Edwards rocks retreated laterally. Tributaries that sculptured the Killeen surface graded down to large base­level controlling trunk streams, such as the Brazos and Leon Rivers, when they flowed some 25 to 30 m above their modem counterparts. The Killeen surface is now left as a relict drainage network containing erosional and depositional counterparts.

2-4

t;' ...

Figure 2.

Manning Surface

'" Killeen Surface

"" Cretaceous Units

ITJJ ~ E-m--I ~

Kiamichi Clay IEdwards limestone Comanche Peak limestone Walnut Clay Glen Rose Formation

Modem Drainage

T2 ! T1 TO ---1---

Quaternary Un~

[::=:::J Alluvium

Schematic geologic and landscape cross-section of the Lampasas Cut Plain, Central Texas (from Nordt, 1992)(no scale).

Colluvial soils with petrocalcic horizons on footslopes connecting the Manning and Killeen surfaces, provide strong evidence that lateral retreat of the Manning surface has been inactive for some time. The Killeen surface lies just above the highest terraces, of the Brazos and Leon Rivers, and as a result may be as old as early to middle Pleistocene (Hayward et al., 1990).

The Killeen surface was probably abandoned sometime after the middle Pleistocene during the last major episode of valley entrenclunent (Hayward et al., 1990). Since its initiation several cut and fill episodes have followed fonning prominent terraces, particularly in the larger stream valleys (Fig. 2). These terraces and the modem stream valleys are Pleistocene to Holocene in age. Modem channels are commonly entrenched 25 to 30 m below the Killeen surface.

Soils of Fort Hood

Soils at Fort Hood can be grouped into six associations that reflect the influence of surface geology and landscape age (Fig. 3) (McCaleb, 1985). Family classification of the soils are given in Table 1. The Eckrant -Real rock outcrop association consists of shallow rocky soils mapped on the Edwards Formation of the Manning surface. Eckrant and Real soils are thin Haplustolls and Calciustolls, respectively, and have silty clay to clay loam surfaces that are sometimes gravelly. Given that the Manning surface is pre-Quaternary in age, the soils mapped on this surface are poorly developed even considering that they may have formed from hard limestone. Locally however, petrocalcic horizons do occur and have also been identified on similar surfaces and formations in the western Edwards Plateau Land Resource Area (Rabenhorst, 1983).

Several soil associations are mapped on the broad and gently rolling Killeen surface. The Nuff-Cho association is found mainly on the Walnut Clay portion of the Killeen surface. They have thin clay loam to silty clay loam surfaces that are locally gravelly to very stony. Both the Nuff and Cho are Calciustolls and exhibit more pedogenesis than many soils on the older Manning surface probably a result of fonning in clays and marls rather than in the more resistant limestones. Cho soils are on more stable remnants of the Killeen surface and exhibit Stage 4 (Oile et al., 1966) carbonate morphology in the form of well developed petroca1cic horizons.

The soils of the Slidell-Topsey-Brackett association are Pellusterts, Calciustolls, and Ustochrepts, respectively, and are mapped on the Walnut Clay Formation of the Killeen surface. Slidell soils are mapped in thick clayey relict valley fills, while the Topsey and Brackett soils formed in convex positions and steeper side slopes, respectively. Slidell soils have appreciable carbonate accumulation deep in many profiles, which suggests that some carbonates may have been present in the solum and now have been partially leached.

Soils of the Doss-Real-Krum association are mapped principally where the Olen Rose Formation outcrops on erosional margins separating the Killeen surface and younger stream terraces. This portion of the landscape is younger than the Killeen surface and in places grades to high terraces of modem streams. Doss soils (shallow Calciustolls) form on

2-6

Figure 3.

1 Eckrant-Reai Rock 2 Nuff-Cho

EXPLANATION

3 S Ilde II-T<.:lpse!:l-Bracket t 4 Ooss-Real-Krum

3 km

5 Bosque-FrIO-Lewisvi I Ie 6 Bastl!-Mlnwells

General soils map of Fort Hood, Texas. (McCaleb, 1985).

2-7

Table 1.

Series

Bolar

Bosque

Brackett

Cho

Crawford

Denton

Doss

Eckrant

Frio

Krum

Lewisville

Nuff

Purves

Real

Slidell

Topsey

Common Soils of the Southern Portion of the Grand Prairie and their U.S.D.A. Classification.

Classification

Typic Calciustolls; fme-loamy, carbonatic, thermic

Cumulic Haplustolls; fine-loamy, mixed, thermic

Typic Ustochrepts; fine-loamy, carbonatic, thermic, shallow

Petrocalcic Calciustolls; loamy, carbonatic, thermic, shallow

Udic Chrornusterts; fme, rnontrnorillonitic, thermic

Typic Calciustolls; fme-silty, carbonatic, thermic

Typic Calciustolls; loamy, carbonatic, thermic

Lithic Haplustolls; clayey-skeletal, rnontrnorillonitic, thermic

Cumulic Haplustolls; fme, rnontrnorillonitic, thermic

Vertic Haplustolls; fme, rnontrnorillonitic, thermic

Typic Calciustolls; fme-silty, mixed, thermic

Typic Calciustolls; fme-silty, carbonatic, thermic

Lithic Calciustolls; clayey, rnontrnorillonitic, thermic

Typic Calciustolls; loamy-skeletal, carbonatic, thermic, shallow

Udic Pellustert; fme, rnontrnorillonitic, thermic

Typic Calciustolls; fme-loamy, carbonatic, thermic

2-8

erosional hilIslopes while Krum soils (Vertic Haplustolls) develop in a variety of alluvial and colluvial landscape positions such as relict valley fills associated with the Killeen surface, or on high strath terraces in upper drainage basins where the Walnut Clay was beveled by lateral stream migration.

The Bastil-Minwells association formed from siliceous and mixed mineralogy alluvial sediments deposited by the Leon River. These soils, unlike the carbonatic parent materials derived from local sources, are acid PaleustaIfs with reddish Bt horizons and form mainly on terraces.

The Bosque-Frio-Lewisville association consists of soils formed from alluvial sediments deposited by modern streams in the form of terraces and floodplains. Bosque and Frio are deep Curnulic Haplustolls mapped in rarely to frequently flooded floodplain positions. Lewisville, a Typic Calciustoll, is mapped on higher stream terraces. The Fort Hood portion of this field trip focuses on a soil chronosequence developed from soils occurring on these alluvial landforms, specifically, those within Cowhouse Creek.

Methods

This investigation began with a review of literature pertinent to alluvial stream histories and paleoclimates of Central Texas. Analyses of topographic, soil survey, and aerial photographic maps of Cowhouse Creek were undertaken to assess geomorphic and soil relations. Field reconnaissance proceeded with three principal goals in mind: 1) to identify the major alluvial landforms for assistance in construction of the geomorphic maps, 2) to locate cutbank exposures for identifying major stratigraphic units and establishing their relative temporal order, 3) to locate trenching sites for testing the accuracy of landform maps and lateral extensiveness and outcrop nature of the stratigraphic units, and 4) to collect radiocarbon samples from critical stratigraphic localities.

Construction of the geomorphic map of Cowhouse Creek relied on 1:24,000 U.S. Geological Survey and 1:4,800 U.S. Corps of Engineer topographic maps, 1:24,000 U.S. Department of Agriculture aerial photographs, and field observations. Landform designations were adapted from Brackenridge (1988) where TO represents the most frequently flooded surfaces and n, T2 and so on, represent higher terraces that are infrequently flooded or not flooded.

Soil-stratigraphic descriptions, including texture, color, horizonation, reaction to HCl, structure, consistence, boundary, and identification of special features, were written from over 100 cutbank exposures and 92 trench sites following standard procedures outlined by the Soil Survey Staff (1981). Soils were classified according to Soil Survey Staff (1990).

Subsurface sediments were grouped informally into stratigraphic units and designated from oldest to youngest beginning with A and progressing through the alphabet toward younger sediments. These stratigraphic units are similar to Allostratigraphic units in that they represent packages of sediment that mayor may not have similar internal character and that

2-9

are bound by laterally traceable disconformities (North American Commission on Stratigraphic Nomenclature, 1983). The upper boundary may bea geomorphic surface or buried soil.

Complete soil characterization (Hallmark et ai., 1986) was performed on samples collected from four pedons on Cowhouse Creek and are the focus of this field trip. Analyses considered important for determining chronosequences included determination of particle size distribution, organic carbon, calcium carbonate equivalent, dithionite-citrate extractable iron (FeJ content, pH, bulk density, clay mineralogy, and thin section analysis. These properties can be used as chronological indicators in the absence of numerical dating techniques and for the detection of stratigraphic discontinuities and reconstruction analysis.

Standard radiocarbon assays were performed by Beta Analytic Laboratories and Balcones Laboratory (University of Texas), and accelerator assays by Geochron Laboratories. All ages were determined from charcoal, wood, or bulk humate samples, presented in years before present (B.P.), and corrected for variations in carbon-13.

Stable carbon and oxygen isotope analyses were performed on bulk carbonates and organic carbon of alluvial sediments and soils by Dr. Thomas Boutton, Department of Rangeland Ecology and Management, Texas A&M University.

Stable Carbon Isotope Theory

An important indicator of soil age is the accumulation of secondary calcium carbonate. Carbonate accumulations can be described quantitatively (Birkeland, 1984; Machette, 1985; McFadden and Tinsley, 1985) or descriptively as stages (Gile et al., 1966). In soils containing large quantities of lithogenic carbonate, morphological distinction between pedogenic and lithogenic carbonate forms is not always possible (Rabenhorst, 1983; West, 1986). In recent years, however, stable carbon isotope analysis has been used to successfully differentiate between pedogenic and lithogenic carbonate forms in soils formed in calcareous sediments (Salomons and Mook, 1976; Magaritz and Arniel, 1980; Rabenhorst et al., 1984; Pendall and Amundson, 1990) and to identify the kind and density of vegetation biomass growing during pedogenesis (Ceding, 1984; Amundson et ai., 1988; Quade et al., 1989; Pendall and Amundson, 1990). The following equation, derived by Salomons and Mook (1976), expresses the relationship of the isotopic composition among bulk soil, pedogenic, and parent material carbonate. This equation can then be used to calculate the percentage of pedogenic carbonate present in a soil horizon that contains lithogenic components:

% pedogenic carbonate = 1113C (bulk soil) - 1113C (P.m.) X 100 (1) 1113C (pedogenic) ~ 1113C (p.m.)

where: 1113C (bulk soil) = bulk soil carbonate 1113C (pedogenic) = soil organic matter plus a fractionation factor of 14%0 11 13C (p.m.) = parent material carbonate

2-10

Delta 13C values are ratios of 13C/12C relative to the Pee Dee belemnite standard expressed in parts per thousand (%0 or per mil) (Craig, 1957).

Marine limestones commonly have /l13C values near zero (Hoefs, 1987) while that of bulk soil carbonate, which typically contains both pedogenic and lithogenic carbonate forms, has values that are somewhat less. For Equation (1), both the parent carbonate and bulk carbonate values can be determined directly from the soil profile in question. The most difficult estimation in Equation (1) is that of the /ll3C of pedogenic carbonate because it is dependent on the isotopic composition of soil organic carbon, molecular diffusion of soil respired CO2, and isotopic fractionation during chemical phase changes in the CO2 (g)­CaC03(s) system (Deines, 1980). The derivation of the pedogenic carbonate end member value involves stable carbon isotope theory and is discussed below.

Dissolution and reprecipitation of carbonates in soil systems can be expressed as:

CO2(g) + H,D + Caco3 = Ca·2 + 2HCOf (2)

In humid regions where soil respiration is high, and there is little mixing of atmospheric CO2, the source for soil respired CO2 at depth is derived mainly from decaying organic matter and root respiration (Ceding, 1984; Amundson et aI., 1988; Quade et aI., 1989). The isotopic composition of soil organic matter is determined by the proportion of C3 and C4 species growing on the soil. C3 plants, which includes all tree species and grasses that experience a cool growing season, and C4 plants, which includes mostly warm season grasses, have different photosynthetic pathways resulting in different 13C/12C fractionations (Deines, 1980). Both plant types discriminate against the heavier 13C isotope, but by differing amounts, so that their /I 13C values are always negative. Average /I 13C values for C3 plants and C4 plants are -27%0 and -13%0 (Deines, 1980), respectively, and approximates that of the soil respired CO2, Additionally, because the diffusional coefficient of 12C02 is greater than 13C02, CO2 that remains in the soil and occupies pore space is about 40/00 heavier than that respiring from the soil (Ceding, 1984). Therefore, a pure C3 and C4 plant community should yield soil CO2 /ll3C values about -23%0 and -9°/00, respectively.

An "open system" is assumed during equilibrium-based CO2(g)-C02(aq) -HC03" -Caco3 carbon phase changes. In this system there is complete isotopic exchange between the CO2 and HC03" carbon species in the presence of an unlimited reservoir of soil CO2 before carbonate precipitation (Magaritz and Amiel, 1980; Rabenhorst, 1983; Ceding, 1984; Quade et al., 1989). Equilibrium is maintained during the phase changes of Esuation (2) because carbonate precipitation proceeds slowly relative to the flux of soil respired CO2 (Ceding, 1984). As a result, carbon in the precipitated carbonate species is derived from soil CO2 with little, if any, contribution from lithogenic carbonate sources. Due to isotopic discrimination during physical chemical fractionation of the CO2(g)-C02(aq)- HC03"-CaC03 carbon phases, precipitated carbonate will be enriched in 13C by about 10.2%0 relative to the gaseous phase at 20· C (Emrich et al., 1970). Because virtually all CO2 consumed during the dissolution and reprecipitation of carbonate is derived from soil CO2, the isotopic composition of pedogenic carbonate will be equal to that of soil organic matter and the sum of the diffusional

2"1l

and chemical equilibria fractionations. Knowing this combined value and that of the parent material (lithogenic) and bulk soil carbonate, the percentage of pedogenically derived carbonate in calcareous soil systems can be ascertained using Equation (1).

For this study c3 H C values were determined on organic carbon. Considering diffusional fractionation, a factor of 4% was added to this value to arrive at an approximation for the c3 H C value of soil CO2, To this value was added the sum of carbonate equilibria fractionation factors equalling about 10%0. The total value added to the original c3 H C value of organic carbon is therefore 14°Alo, which should approximate the c3 H C value for pedogenic carbonate.

The next section is a discussion of the alluvial stratigraphy of Cowhouse Creek taken from Nordt (1992).

Alluvial Stratigraphy of Cowhouse Creek

Terrace 2 (TI)

The oldest terrace bordering Cowhouse Creek, TI, is situated approximately 10 to 15 m below the Killeen surface, 16 to 18 m above the modern channel, and 5 to 7 m above T1 (Figs. 2, 4 and 5). Depositional remnants of TI are preserved on both sides of the modem stream and suggest a relict valley width of up to 2 km. This relatively flat and featureless surface supports soils mapped mainly as part of the Lewisville series (Table 1)(McCaleb, 1985). Pit and cutbank exposures show that the alluvial fill beneath TI, named the Jackson alluvium (unit A), rests unconformably on the Glen Rose Formation. Thickness of the Jackson alluvium ranges from 2.5 to 4 m. Trenches and several partial cutbank and pit exposures reveal two alluvial facies. The lower facies consists of laterally accreted channel gravels and sands (sometimes cemented) that exhibit trough cross stratification and minor channel fills. The upper facies consists 1 to 2.5 m of laterally and vertically accreted, yellow to gray, bioturbated loam from which the Lewisville soils formed. A bulk humate radiocarbon age of 15,230 B.P. was obtained from a fine-grained channel fill about 2.8 m below the modern TI surface (Fig. 4, cutbank 10; Fig. 5). Charcoal-humate radiocarbon pairs in Holocene sediments demonstrate that humate ages are consistently 300 to 1700 years older than charcoal ages. The 15,000 year age for the Jackson alluvium is therefore an approximation of the time at which soil formation began for Pedon 4. Blum and Valastro (1989) recognize several Pleistocene terraces from the Pedemales River of the Central Edwards Plateau, the lowest terrace of which yielded a bulk humate radiocarbon age from fine-grained sediments of about 18,000 B.P. This may suggest synchroneity of latest Pleistocene deposition across the Plateau.

Terrace 1 (T1)

Terrace 1 (Tl) constitutes the broad, featureless paired surface immediately bordering TO and the channel of Cowhouse Creek (Figs. 4 and 5). Three major stratigraphic units, the Georgetown alluvium (B), Fort Hood alluvium (C), and West Range alluvium (0), are

2·12

Figure 4.

~a-a

+

K"

+

EXPLANATION

c:::=J TO - MODERN CHANNEL A~D FLOODPLAINS

C3 TI - HOLOCENE TERRACE

~ T2 - PLEISTOCENE TERRACE

TRENCH LOCALITIES • CUTBACK LOCALITIES

-t- UTM COORDINATES

OEOFlOETO'lll'N FlO

CO.'HO'JSE CREEK

~1-13

+

Geomorphic surface map of upper Cowhouse Creek.

2-13

W(IT II"-NIIIIO.

\" Z

~

~ 15,270±26o. _)

190±90 (8oIa371lOO)

~ T2 300±100 [IX"'7)

<:>.. 370±18o. [IX .... )

390±6o. 1"""""'77) <

C

15o.0±6o. (801838"4)

1690±90 _,<50)

6o.0±16o. (TX87OO)

,650±15o. (TX8701) }

2720±11o. (451) 30.1 0±11 0. [lX1J703) .

Ped~n 1

" ,'------..--....:,-"1

", , ./' 01

TO

E/ 02,/ C B !

5210±23o. 1""""452)

T1 Pedon3

Glen Rosa limeston,

~I?\ /./{ '\. A 6:::: :::: :546O±701 ... .,.,IltI

8830±7o.l .... 3:lOn 6850±90\BOi03'618) 395D:t290:/T'x~~bl)1 .:

23B0±15o. ~6~02tgg:4170±10o.~ 2860±5Q<1 .. ;.,i"h)

EXPLANATION

Alluvial Units A -Jackson B - Georgetown C - Fort Hood 01 -lower West Range 02 - upper West Range E - Ford

• Radiocarbon age (B.P.)

TO - Landform

TTTi['Tl' Royalty Paleosol

rrrn Other soil

57 40±300-(GX15602

I&n

&,

Figure 5. Generalized composite geologic cross section of Cowhouse Creek showing the recognized stratigraphic units and trench localities from which the soil chronosequence was developed. Charcoal ages are shown in bold and bulk humate ages in plain text (from Nordl, 1992). Valley width is approximately 1 km. .

associated with this terrace. In most areas the Fort Hood and West Range alluvium crop out to the same surface elevation and therefore cannot be differentiated by geomorphic mapping, thus making Tl a diachronous terrace. The Georgetown alluvium is the oldest unit beneath Tl. It is always deeply buried, rests unconformably on a scoured bedrock valley floor just above the thalweg of the modern channel, and is unconformably overlain by the Fort Hood alluvium. The Georgetown alluvium consists of 1 to 2 m of fme, well sorted, channel gravels overlain by 1 to 3 m of massive and bioturbated, yellow to gray, very fme sands to sandy clay loarns. The upper portion of the Georgetown is recognized as a truncated paleosol named the Royalty. This soil is represented by a brown Bk horizon having encrusted stage I carbonate morphology. Modern analogues show that encrusted filaments and threads of calcium carbonate form by the downward gravitational flow of water carrying Ca+2 and HCO', ions and the subsequent precipitation of Caco, on ped faces with desiccation (Sobecki and Wilding, 1983). Groundwater carbonates tend to be disseminated throughout the fme-earth matrix because of the preferential upward flow of water by capillary forces. It is possible, however, that the Royalty paleosol contains Caco, formed by both processes.

The bulk of sediment associated with Tl is the Fort Hood and West Range alluvium (Figs. 4 and 5). The Fort Hood alluvium overlies the Georgetown alluvium unconformably and comprises about one-half of the Holocene valley fill. Cutbank exposures show that the Fort Hood alluvium channel regime completely removed the Georgetown alluvium in numerous localities, in which case the total alluvial fill is up to 10 m thick. Well sorted, fine to medium basal gravels, commonly contained in beds 1 to 3 m thick, are typically overlain by thick, fme-grained, yellowish brown floodplain deposits, which in some cases enclose fme­grained channel fills. Radiocarbon ages obtained from a basal channel fill and from upper flood basin deposits, respectively, show that deposition of the Fort Hood alluvium had begun by 6850 B.P. and was still ongoing at 5200 B.P., but to no later than 4170 B.P. (Fig. 5). Blum and Valastro (1989) recognize early Holocene stratigraphic units similar to the Fort Hood alluvium on the Pedernales and Colorado/Concho Rivers that range in age from about 10,000 to 5000 B.P.

The West Range alluvium crops out immediately adjacent to the modern, entrenched Cowhouse Creek floodplain (TO) (Figs. 4 and 5). It is laterally inset to the Fort Hood alluvium on the inside of modern meander bends, and in some places partly truncates and overlies the Fort Hood alluvium on the outside of modern meander bends. The West Range channel system trenched down to nearly the same depth as the Georgetown and Fort Hood alluvium before aggrading. Aggradation occurred in two episodes between 4200 and 600 B.P., which was punctuated by a brief erosional period 2000 to 3000 B.P. The lower member spans the early part of this episode and is called the lower West Range alluvium (D1), while the upper member spans the latter part and is called the upper West Range alluvium (D2). Both the Fort Hood and West Range alluvium accreted to the same surface elevation in much of the valley. However, the upper West Range alluvium in the upper Cowhouse Creek basin is laterally inset to the lower member, but at a slightly lower elevation (1m). Downstream, the upper member overlaps and buries the lower member and the truncated Fort Hood alluvium. Because both members of the West Range alluvium crop out beneath Tl, soil age can vary considerably. Sedimentological character is similar for both

2·15

West Range units. Basal deposits typically consist of several meters of relatively coarse gravels and sands contained in large sets of laterally accreted point bars. Brown and dark brown loamy overbank deposits either directly overlie channel gravels and sands, or laterally truncate and partly overlap, the Fort Hood alluvium.

Degree of soil development in the Fort Hood and West Range alluvium reflects differences in soil age. However, both these soils have Stage I carbonate morphology of encrusted fIlaments and threads in Bk horizons that meet the criteria for a calcic horiwn. Soils associated with both alluvial units are mapped as part of the Lewisville and Bosque series.

Modem Floodplain (TO)

The modern Cowhouse Creek is a meandering stream that is entrenched by about 9 to 10 m relative to the T1 surface (Figs. 4 and 5). Acute channel bends suggest that there may be partial structural control from the underlying limestone bedrock. Cowhouse Creek is an intermittent perermial stream with numerous periods of no flow. However, the surrounding limestone uplands produce steep slopes, sparse vegetative cover, and shallow soils. Coupled with high intensity convectional rainstorms, peak-flood discharges of Central Texas streams are extremely large and episodic. Baker (1975) has shown by magnitude-duration flood curves that Central Texas has rainfall runoff events that rank among the worlds greatest.

Modern Cow house Creek sediments are named the Ford alluvium (unit E) and consist of several facies: channel lag, laterally accreted point bar sands and gravels, and overbank sands and clays. Modern point bars have accreted to several meters above the low-flow channel as a result of wide variations in stream discharge. Most overbank deposits are located on the innermost portions of large meanders and are inset to, and lower than T1 by about 2 to 3 m (Figs. 4 and 5). Along straight channel segments vertically accreted deposits as much as 7 m thick may occur on both sides of the channel. Overbank sediments of the Ford alluvium are highly stratified with differences in texture indicating coarse-grained deposition during initial overbank flooding and fine-grained deposition during flood recession. A suite of radiocarbon ages, all determined from dispersed and culturally-derived charcoal, demonstrate that modern channel trenching occurred sometime between 600 to 400 B.P. (Fig. 5) and that deposition has been ongoing since. Numerous alluvial investigations in Central and North-Central Texas demonstrate that a widespread episode of valley trenching occurred sometime around 1000 B.P. (Hall, 1988, 1990; Mandel, n.d.; Blum and Valastro, 1989, Ferring, 1990) and again, events at Cowhouse Creek appear to generally concur with the regional alluvial record. The soils developed in Ford alluvium are also mapped as part of the Bosque series.

Stratigraphic Summary and Paleoclimatic Inferences

Radiocarbon ages from around the Military Reservation suggest that widespread channel trenching was proceeding sometime between 15,000 and 10,000 B.P. This event abandoned the 1'2 floodplain and initiated scouring of the Holocene valley. Bryant and

2-16

Holloway (1985) have shown by pollen analysis that climatic conditions changed from relatively cool and wet to wann and dry in Central Texas between 14,000 and 10,000 B.P. Nordt et al. (1994) also show that cool season C3 species constituted as much as 60% of the biomass production in the region approximately 15,000 B.P .. Blum and Valastro (1989) believe that this change to drier conditions near the end of the Holocene induced widespread valley trenching of the Pederna1es River. These climatic conditions may also account for channel degradation of Cowhouse Creek at this time.

Widespread valley alluviation of the Georgetown alluvium began sometime after valley entrenchment between 15,000 and 10,000 B.P. and had ended by 8500 to 8000 B.P. This channel regime apparently carried relatively uniform, nonepisodic base flow with local high water tables, as reflected in the fme-grained and well sorted sedimentological character and mottled sediment colors. In addition, the vertical relief between the inferred low water channel and the overbank sediments of the Georgetown alluvium is several meters less than units deposited later in the Holocene. This suggests that the younger Holocene units were deposited under more flashy stream discharge conditions than the Georgetown. Given that climatic conditions were sufficiently moist to support an expansive upland vegetative cover during the early Holocene, rapid upland knickpoint migration, in response to the lowered base level of the trunk drainage network, may not have occurred. This would have limited the source of Georgetown alluvium to reworking of Jackson sediment. The Royalty paleosol contains carbonatic filaments and threads on ped faces that indicates some period of landscape stability or slow cumulic deposition. Because this soil was truncated at about 7000 B.P., perhaps as much as 500 to 1000 years of soil formation occurred cumulicly.

Between 7000 and 4500 B.P., much of Central to Northwest Texas was undergoing a warming trend (Hall, 1988; Holliday, 1989, Nordt et al. (1994) that was accompanied by slow fluvial deposition and soil formation (Ferring, 1990), eolian sedimentation (Holliday, 1989; Caran and Baumgardner, 1990), and fan deposition (Blum, 1989; Mandel, n.d.). This climatic shift to warmer and drier conditions may have been the catalyst for brief channel trenching or stream metamorphosis as low frequency, high intensity rainstorms increased. Following this model, deposition of the Fort Hood alluvium proceeded as the surrounding uplands became progressively depleted of vegetation and resulted in increased runoff and sediment yields. This sediment was most likely derived from thick, fme-grained upland soils that formed during the Pleistocene as the newly entrenched tributary network migrated into the Killeen surface. The reddish-yellow hue of the Fort Hood sediments suggested that the eroded upland soils were well oxidized.

Between 4800 and 4200 B.P. the channel network throughout Fort Hood adjusted to an . increase in coarse-grained sediment load. This shift to coarse-grained sedimentation, which resulted in deposition of the West Range alluvium between 4200 and 600 B.P., may be explained by one of two working models. One postulates that as Holocene drying continued, and many of the deep fme-grained soils of entrenched sections of the Killeen surface were eroded, larger proportions of parent rock were eroded and delivered to the trunk streams. The increased abundance of bedload sediment and subsequent imbalance between sediment load and discharge may have induced channel metamorphosis by decreasing sinuosities and

2-17

increasing width-depth ratios, even in the absence of a significant shift in stream discharge. Previous investigators have demonstrated this important relationship, which mayor may not be climatically induced, between drainage basin soil sediment supply and fluvial erosional and depositional cycles (Bull, 1991; Tonkin and Basher, 1990).

An altemative mechanism for delivering coarse-grained sediment to the trunk channel network after 4200 B.P. is an increase in discharge in response to wetter and possibly cooler climatic condition. Such a shift in climate at this time has been interpreted by Nordt et al. (1994) and Holliday (1985a,b). Wetter conditions could have stabilized upland hillslopes by increasing vegetative cover, reducing sheet erosion, and effectively slowing the rate of knickpoint migration into the Killeen surface. Continued downcutting within the previously entrenched tributary network would have provided significant quantities of parent rock for delivery into trunk streams. The relatively higher stream discharges at this time would presumably have had the competency to carry the newly introduced coarse-grained sediment. In support of this model, Blum and Valastro (1989) attribute coarse-grained sediment transport by the Pedemales River between 4000 and 1000 B.P. to moist climatic conditions that produced higher effective discharges and competency levels.

Nordt et al. (1994) show that climate conditions because dryer for a brief period between 3000 and 2000 B.P. This also coincides with the age of an erosional conformity separating the lower and upper West Range alluvium. Radiocarbon ages from Cowhouse Creek also demonstrate that stream metamorphosis, and brief channel trenching, occurred between 600 and 400 B.P. However, the channel facies of both the West Range and Ford alluvium are at nearly the same bedrock basal level indicating minimal channel trenching. Rapid fluvial adjustments in response to a continued coarsening of the sediment load or shift in discharge, without significant channel incision, must be considered as a possible cause of stream metamorphosis at this time.

Soil Chronosequences

Development of the alluvial stratigraphy of Cowhouse Creek is nearly complete. Soil interpretations, however, are in their preliminary stages. The following is an attempt to express our current ideas and indicate approaches we are taking in efforts to establish relationships between the observed soil properties and soil age.

The traditionally recognized factors influencing soil formation are climate, biota, parent material, topography, and time (Jenny, 1941). If the first four factors are held constant, with only time varying, a soil chronosequence, and possibly chronofunctions, can be established. The chronological alluvial sequence of Cowhouse Creek was established in the previous section. Based on the stratigraphy, assumed soil ages of the four soil pedons under consideration on this field trip, are listed in Table 2.

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Table 2. Stratigraphic unit ages and estimated soil ages of the study pedons.

Unit No. (Terrace) Unit Age Range (B.P.) Pedon Soil Age (B.P.)

Ford (E) TO 600 to present 1 modern Upper West Range (D2) T1 4200 to 600 2 2000 Fort Hood (C) T1 7000 to 5000 3 5000 Jackson (A) T2 15,000 4 15000

The age of Pedon 1 is essentially modern (TO) because of active flooding and deposition. The minimum age of Pedon 2 is 600 B. P. because channel trenching, abandonment of T1, and termination of deposition of the West Range alluvium (D2) occurred at about that time. The actual age of Pedon 2 can be deciphered from Figure 6, which displays a schematic cross section of an alluvial exposure near Pedon 2 (Fig. 4; Trench 1). A channel fill within the upper West Range alluvium (02) is shown beveling and overlying the lower West Range (D1), Fort Hood alluvium (C), and Georgetown alluvium (B). This demonstrates that active lateral channel migration was ensuing around 2380 B.P. Pedon 2 parent materials can be traced in beds dripping toward the buried channel that dates to 2380 B.P. This suggests a soil age of about 2000 years for Pedon 2.

Radiocarbon ages from around Fort Hood suggest that where the Fort Hood alluvium was not truncated by West Range erosion, deposition terminated between 4800 and 5200 B. P. (Fig. 5) .. As a result, the soil age of Pedon 3 (associated with TI) is assumed to be 5000 years. The soil age of Pedon 4 (associated with T2) is more uncertain because it is based on a single humate age. However, an age of 15,000 years provides an approximation until further ages are obtained (Fig. 5).

Pedon 1 - Trench 5

Pedon I-Trench 5 is excavated in the Ford alluvium associated with TO, which is inset to T1 by about 2 m. This area represents the modern floodplain that is actively aggrading (Figs. 4, 5). Terrace 0 is mapped as part of the Bosque rarely flooded map unit (McCabeb, 1985). A radiocarbon age of 650 B.P. was determined on hearth charcoal from a depth of 383 cm beneath this surface and demonstrates that Pedon 1 is less than 650 years old. In other localities, ages as young as 190 B.P. show the youthfulness of the Ford alluvium. A second age of 2100 B.P. was obtained on hurnates from a depth of about 7 m at the cutbank near pedon 1 and indicates that the upper West Range alluvium partly underlies the Ford alluvium and TO in this position. Pedon 1 formed in stratified sediments typical of the Ford alluvium as can be seen by depth distributions of the percentages of clay-free sand and silt, and organic carbon content (see pedon 1, Appendix). Carbonate values show that at least 50% of the matrix is comprised of calcite, virtually all of which is lithogenic. Two faintly expressed buried A horizons are evident within this unit indicating that deposition was occasionally puntuated by brief periods of landscape stability and soil formation. Few to

2-l9

'" ~

"-t T1 j;

02 (upper)

• 238D±150

Glen Rose limestone.

-v (\of

, \

""'" Surface soil in upper West Range alluvium (02) (Pachic Calciustolls)

TTT Truncated and buried soil in lower West Range alluvium (01)

iii i Truncated and buried soil in Fort Hood alluvium (C)

o Stratigraphic units • Radiocarbon ages (charcoal)

B •

883D±70

c

Figure 6. Schematic cross section of alluvial and soil stratigraphic units beneath T1 of Cowhouse Creek 1 and 2 near West Range Road in Figure 4) (no scale-total alluvial thickness approximately 9 to 10m).

,\ <, , . J

(cutbanks

'7 rDf1

many, fme patchy carbonate pseudomycelia are on ped surfaces of loamy Bk horizons. Because of the presence of an ochric epipedon, absence of a cambic endopedon, and an irregular distribution of organic carbon with depth, this pedon classifies as a Typic Ustifluvent; coarse-loamy, carbona tic, thermic.

Pedon 2 - Trench 1

Pedon 2-Trench 1 was excavated in sediments of the upper West Range alluvium (02) associated with Tl, and like TO, is mapped as part of the rarely flooded Bosque map unit (Figs. 4, 5, 6). Depth distributions of clay-free sand and silt percentages and organic matter content (pedon 2, Appendix) show slight stratification in the original soil parent material, although it is not as great as in Pedon 1 of the Ford alluvium. The bulk soil of Pedon 2 contains slightly fewer total carbonates than Pedon 1 as values range between 40 and 50%. The Bk horizons of Pedon 2 described below a depth of 85 cm, contains between 5 and 15% carbonate filaments and threads (Stage n in ped interiors and on exteriors. These Bk horizons meet the requirements of a calcic horizon because of greater than 15% calcium carbonate equivalent and more than 5% by volume of identifiable secondary forms (disseminated films and threads). Because of the calcic horizon and "overthickened" mollic epipedon, this pedon classifies as a Pachic Calciustoll; fme-Ioamy, carbonatic, thermic.

Dithionite-extractable Fe (Fe.J values in the solum of Pedon 2 are slightly higher than those of the parent materials for the Ford alluvium. Assuming that Ford and West Range alluvium parent material values approximate each other, some accumulation of iron oxides has occurred in Pedon 2 during 2000 years of pedogenesis.

Pedon 3 - Trench 19

Pedon 3-Trench 19 is located in the Fort Hood alluvium on the outer margin of Tl near the Cretaceous valley wall (Figs. 4 and 5). The parent material that Pedon 3 developed in is somewhat different than Pedons 1 and 2, in that Pedon 3 has 1) greater clay and silt contents, 2) less total carbonate, and 3) higher Fed contents. As a result, caution must be taken in differentiating pedogenic differences that are related to parent material and those related to in situ weathering and the passage of time. The increase in Fed content in the mid­profile position of Pedon 3 does suggest, however, some increase in the release of pedogenic Fed with time. Significant clay translocation has not been initiated because of the absence of clay films and the absence of an increase in the fme clay-total clay ratio with depth.

Carbonate morphology of Pedon 3 is also Stage I; however, filaments and threads are thicker and more deeply encrusted into ped surfaces than in Bk horizons of Pedon 2. Greater than 5% by volume of carbonate forms, coupled with more than 15% total carbonate in the Bk horizons, substantiate the presence of a calcic horizon. As with Pedon 2, this soil has a "overthickened" mollic epipedon. This soil has sufficient COLE values and cracking when dry to be vertic; therefore, because vertic character has precedence over pachic, this soil is classified in the Vertic Calciustolls; fme, montmorillonitic, thermic family.

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Pedon 4 - Trench 13

Pedon 4-Trench 13 was opened in the Jackson alluvium associated with T2 (Figs. 4 and 5). Reconstruction and rnicromorphic techniques were performed on this pedon to make inferences about the amount and kind of changes that soil properties have undergone since pedogenesis began (Brewer, 1976). Applying this methodology assumes unifonnity in parent material; however, particle size distribution expressed on a carbonate-free and clay-free basis for Pedon 4, along with Zr and Ti contents, indicates that there is some stratification within the solum (see Pedon 4, Appendix). Nevertheless, this analysis proceeds under the assumption that parent material is sufficiently uniform to estimate volume changes of marker constituents with time.

Reconstruction analysis shows a net loss of 69 and 50 g per cm2 of sand and silt-sized carbonates in Pedon 4, respectively, relative to the assumed parent material (see Pedon 4, Appendix). These losses are occurring from dissolution of carbonates in the sand and silt fractions, which make up about 50% of the parent material. Most of this loss is in the upper 98 cm where carbonates have either been dissolved and moved to lower portions of the pedon or completely leached from the solum. Although there is a net increase in sand, silt, and total carbonate in the 150-250 cm section (Bk3 through Bk5 horizons), there is substantial loss of lithogenic carbonate and substantial gain in pedogenic carbonate (see next section). Overall, these data indicate a total loss of carbonates from Pedon 4 of 133 gjcm', or a rate loss of 8.9 gjcm'll000 years. Furthermore, as weathering proceeds, occluded clays are released from the carbonates and left as residue, thus accounting for a higher proportion of the total mass of the soils than in the parent material. Further support for net carbonate removal is exhibited in the pitted nature of lithogenic carbonate clasts contained in the upper BAk and Bkss horizons. Thin section analysis shows that these clasts are cretaceous renmants and not of pedogenic origin because they contain Cretaceous fossils, do not engulf matrix particles, do not show stress features around the nodules that would indicate incremental growth, do not have iron manganese stains, and do have quartz rinds resulting from dissolution. Delta 13C values on three of these nodules from different depths were all near + 1.0, which is in range of values from Glen Rose limestone. These data indicate that the pits in the nodules are a result of dissolution and leaching of carbonate. Dissolved carbonates may subsequently reprecipitate deeper in the solum, or be removed from the solum via percolating waters.

As a result of carbonate dissolution, reconstruction analysis also shows a net gain of 19 g of fme clay per cm' colunm for Pedon 4. Only the Al horizon shows a fme clay loss while the zone from 14 to 120 cm exhibits substantial fme clay gain. Patchy clay films in the lower solum and fme clay-total clay depth distributions indicate that clay translocation has occurred. However, because clay films constitute less than 1 percent of the surface area in thin section and the amount of clay increase is inadequate, the minimum requirements for an argillic horizon are not met. Because of high clay content and COLE values, and evidence of shrink-swell in the form of slickensides in Bkss horizons, this pedon classifies as a Udic Chromustert; fme, montrnorillonitic, thermic. Also note, from a chronological viewpoint, that relative to Pedons 1, 2, and 3, Pedon 4 has redder hues and lower pH values in the upper solum.

2-22

Pedogenic Carbonate Chronosequences

Bulk carbonate and organic carbon samples from selected A and Bk horizons of Pedons 2, 3, and 4 were subjected to stable carbon isotopic analysis in an attempt to differentiate pedogenic from lithogenic carbonates and to quantify pedogenic carbonate accumulations (for theory see Methods section). Vertically superposed horizons containing similar field estimated quantities of disseminated filaments and threads were grouped together for analysis.

The &13C values for the organic carbon were all taken as -19.00Alo for all pedons based on reconstructed vegetation history of the area (Nordt et al., 1994). In other words, during pedogenesis for the last 15,000 years, -19.00/00 gives a reasonable representation of the proportion of C3 and C4 plants growing in the area (Table 3). Given a 13C enrichment factor of 14%0 in the organic matter-C02W-CaC03 system, pedogenic carbonate in the soils should have &BC values around -5%0.

Samples from the Glen Rose limestone, considered to be the predominant source of soil lithogenic carbonate in the alluvium, were analyzed to determine their BC composition. Values of +1.5 and +2.8%0 were obtained from two Samples (Table 3). The lower value (+ 1.5) was used as the parent material value in Equation (I) for calculating percent pedogenic carbonate because the higher value did not fall within the ranges published by West et al. (1988) for limestone-derived parent materials of Central Texas. All carbonate accumulation in this area is assumed to be derived from in situ carbonates with little contribution from airborne influx based upon monitoring of dust in the region (Rabenhorst, 1983; West, 1986).

Figure 7 displays total carbonate and calculated pedogenic carbonate contents of Pedons 2, 3, and 4. Note that the percentage of pedogenic carbonate does not increase systematically with time. Furthermore, pedogenic carbonates are somewhat uniformly distributed throughout the profiles indicating a wide range of effective wetting depths. Although a parent material Caco3 content is difficult to determine because of deep weathering, all pedons have probably lost carbonate, most noticeably in Pedon 4.

A more revealing indicator of secondary carbonate accumulation in soils is the whole-proftle index (Machette, 1985). This index is used to measure the cumulative total of grams of pedogenic carbonate in a soil column I cm2 in diameter. It is calculated from bulk density, percent pedogenic carbonate, and horizon thickness. We used moist bulk density values to eliminate volume changes associated with shrink-swell soils. In addition, as revealed by the adjacent cutbank exposure, the sola of Pedon 2 is 25 cm deeper than originally described. This thickness was added to the last horizon for calculating the index. We also extended the pedogenic carbonate content of the last horizon of Pedon 4 down to the same depth as Pedon 3. It is obvious from Figure 7 that pedogenic carbonate exists in horizons above and below those horizons from which we have data. Our calculations therefore slightly under estimate pedogenic carbonate accumulations.

The whole-profile index shows that Pedons 2, 3, and 4 have accumulated about 22, 69, and 41 grams of secondary carbonate per cm2 column (Fig. 8). For Pedon 2, pedogenic carbonate has

2-23

Table 3. Stable Carbon Isotope Analysis.

6 13C

Pedon No. (Unit) Depth Horizon Bulk Carbonate Organic Carbon

(cm)

2 Upper West Range

Unit 02 -19.0

85-155 Bk3&4 0.2

155-166 BkS 0.0

166-215 Bk6&7 0.1

215-268 Bk8 0.2

268-347 Bk9 0.2

3 Fort Hood

UnitC -19.0

59-94 BW . -1.6

94-122 Bkl -1.9

122-153 Bk2 -1.7

153-189 Bk3 -1.7

189-252 Bk4 -1.7

252-320 BkS -1.8

320-370 Bk6 -1.4

4 Jackson

Unit A -19.0

38-98 Bkss2 -2.1

98-120 Bkl -1.2

120-150 Bk2 -0.4

150-190 Bk3 0.2

190-250 Bk4&5 -0.1

250-270 BC 0.2

270-350 CB 0.4

Limestone Sample 1 1.5

Limestone Sample 2 2.8

2-24

'" N '-"

0 %CaC03

20 40 60 0 20%caC03

0

:1 I Peden 2 ~ 2000 B.P.

Peden 3

50

5000 B.P.

t- AB

BAk

Bk2

~ 100

E :2- 1-150 a. '" 0

200

250

300

350 C=I Total calcium carbonate equivalent

';';;::::«:::";'~~'J '%w!l'~. Total pedogenic carbonate

Figure 7 . Mean pedogenic carbonate and total calcium carbonate contents for Pedons 2, 3, and 4.

%CaC03

Peden 4 15,000 B.P.

been accumulating at a rate of about 11 g/cm2/1000 years and for Pedon 3 at a rate of about 14 g/cm2/1000 years. These rates are comparable and suggest a relatively uniform carbonate accumulation rate over the last 5000 years. If the rate of accumulation of Pedon 2 is projected over the last 15,000 years, Pedon 4 should contain about 150 grams of pedogenic carbonate. Because Pedon 4 has only 41 grams, as much as 11 0 grams or about 73 percent, of the previously accumulated pedogenic carbonate has been removed from the soil solum. Net carbonate loss by these calculations support our conclusions based on reconstruction analyses. Holocene alluvial soils have secondary carbonate accumulations rates much greater than those listed for the arid Southwest where rates are normally less than 1 g/cm2/1000 years (Machette, 1985). Even though the calculated pedogenic carbonate percentages approximate our field estimations, it is possible that parent materials contain small quantities of pedogenic carbonate inherited from erosion of soils from the surrounding uplands. Additional parent material samples will be analyzed in the future to make this determination.

Iron Oxide and Organic Carbon Chronosequences

Our data show that Fed increases linearly with time (Fig. 8). McFadden (1988) has shown that in semiarid to arid regions of the Southwestern U.S., late Pleistocene and middle Holocene soils have accumulated 0.5 to 2.5 percent Fed in the clay fraction. Furthermore, Miles (1981) calculated a Fed percentage of 1.5 percent from a late Pleistocene terrace soil of the Brazos River in North-Central Texas. Nordt (unpublished) calculated an Fed percentage of 1.2 from a late Pleistocene Leon River terrace soil in the Fort Hood area.

Organic carbon distributions in the upper 50 cm show that similar quantities have accumulated in the three pedons under study (Fig. 8). In addition, accumulation commonly reaches steady state within 2000 years and may decline slightly with increasing age up to 15,000 years. Birkeland (1984) shows data generally supporting these results.

Soil Order Chronosequences

Even though currently aggrading, the surface horizon of Pedon 1 of the Ford alluvium qualifies as a mollic epipedon except for light color values. TItis demonstrates that even though surface age is constantly being rejuvenated, minimal time is needed to form a mollic epipedon and Mollisol. The other three pedons have well expressed mollic epipedons with similar quantities of organic carbon in the upper 50 cm.

It has been shown in this study that calcic horizons form within 2000 years because rainfall is sufficient to rapidly dissolve and translocate lithogenic carbonates, but not so great as to remove all of the dissolved carbonate by-products from the solum before secondary carbonate precipitation occurs. Pedogenic carbonate accumulation appears to have been continuous through time. However, sometime between 5000 and 15,000 B.P. appreciable pedogenic carbonate depletion also becomes apparent. Petrocalcic and calcic horizons in soils of the Killeen surface are poorly expressed and seem to be confmed more to areas where limestone is situated in the near surface, which may contribute to petrocaicic

2-26

N , N -J

200 5000 B.P.

160 J n 1-2.0 c E ::J

~ 2000 B.P. 0 c ()

E 15,000 B.P. C\I ::J

1.5 E o 120 ~ ()

C\I ~

E c ~ 0

.0 ~ ~

CD 80 1.0 ~ -<ll () c 'c 0 <ll .0 0) ~ Modern <ll ~

0 0 40 • .5 '0

c <ll '0 Ql

U.

0 0 Pedon 1 Pedon 2 Pedon 3 Pedon 4

Carbonate lIB Fed - Organic carbon

Total

Pedogenic

Figure S. Pedogenic carbonate, dithionite-citrate extractable iron oxide (Fed» and organic carbon in g/cm2 column for Pedons 2, 3, and 4.

horizon fonnation by in situ carbonate dissolution and reprecipitation. On the other hand, these portions of the Killeen surface may have been eroded during the Holocene, thus removing upper soil horizons and bringing the petrocaicics into near-surface positions. In thick clayey soils on the Killeen surface, most of the carbonates have either been transported to deep portions of the profIle or completely removed from the sola. Both processes are consistent with the model of net carbonate loss coupled with apparent short -tenn pedogenic carbonate accumulation shown for the alluvial soils.

Argillic horizons have not been identified in Fort Hood except in areas on the Leon River terraces where mixed and siliceous parent material mineral assemblages occur. Clay fIlms in the solum of Pedon 4 of Cow house Creek indicate that clay movement does occur and becomes observable in 5000 to 15,000 years, but it is not the primary pedogenic process. Interestingly, clay quantities increase with time as occluded clays are released during sand and silt carbonate dissolution. As a result, vertic properties become more prevalent with time, thus destroying many illuviation features.

Holliday (l985a, 1985b) has evidence demonstrating rapid rates of pedogenesis in the Southern High Plains of Texas. He found that mollic, calcic, and argillic horizons could fonn withing 2000 years. Rapid fonnation of calcic and argillic horizons was attributed in large part to carbonate eolian influx. Within 450 years· all soils in his study developed into Inceptisols, Mollisols or Alfisols. In the Fort Hood area, Entisols (Ustifluvents) develop rapidly into Inceptisols and Mollisols (Hapustolls and Calciustolls), in part because of the preweathered nature of the soil parent materials. The vertic subgroup of Pedon 3 may arise from the fme-grained nature of the parent material; however, reconstruction analysis of Pedon 4 indicates that occluded clays are released from sand-sized and silt-sized carbonates and increase in quantity with time. The development of a Vertisol (Chromusterts) in Pedon 4 from loamy carbonatic sediments within 15,000 years supports this conclusion. These relationships are illustrated in Table 4.

Table 4. Summary of fonnation of diagnostic features with time for the alluvial soils of Cow house Creek, Fort Hood, Texas.

Soil Age (Years Before Present)

Modern 2,000 5,000 15,000

Diagnostic Ochric mollic mollic mollic Horizons (mollicl) calcic calcic calcic

Great Group Ustifluvents Calciustolls Calciustolls Chromusterts (Haplustollsl)

lMollic epipedon may fonn quickly, or in ideal situation, be inherited from parent material.

2-28

ACKNOWLEDGEMENTS

I thank Kimbal Smith and Dr. Jack Jackson, archaeologist and environmental scientists at the Fort Hood Military Reservation, for encouraging and obtaining permission for this excursion. They have done so on numerous other occasions as well. Bill Roberts was in charge of trenching operations. I also thank Glenda Kurten for patiently typing the manuscript, John Jacob for coordinating soil characterization analysis of the four pedons, and Drs. Larry Wilding and Richard Drees and their Agronomy 606 class for the reconstruction analysis of soil pit 4. Special thanks to Dr. Tom Hallmark and Dr. Michael Waters for contributing to various parts of this investigation.

2-29

· APPENDIX

Pedons 1-4

2-30

PEDON 1 SOIL SERIES: BOSQUE VARIANT PEDON: S90TX-099-OQ4 COUNTY: CORYELL

PEcaN CLASSIFICATION: TYPIC USTIFLUVENTS: COARSE-LOAMY. CARBONATIC. THERMIC

tOeA HON: FT. HOOD, TEXAS; CHST31TRf.

LANOFORM: FLOODPLAIN

PARENT ~ATERIALS: ALLUVIUM

ELEVATION (M): SLOPE: 0-1~ SLOPE ASPECT:

FORMATION: HOLOCENE ALLUVIUM (UNIT E)

TOPOGRAPHY: NEARLY LEVEL QRAINAGE: WELL DRAINED LANOUSE: NATIVE

COLLECTORS: JACOB ANO KORGEL DATE:

HORIZON

A

2BI<1

:lBK2

3C

4AKB1

SBWS1

6AKB2

7C1B2

aC2B2

9C3B2

IOC4B2

DEPTH (C~) SOIL DESCRIPTION (COLORS FOR MOIST SOIL UNLESS STATED)

0-25 BROWN (10YR 4/3) LOAM. GRAYISH BROWN (fOVR 5/2) DRY; MODERATE COARSE AND VERY COARSE ANGULAR BLOCKY STRUCTURE; HARD; FEW FINE PORES; COMMON FINE ANa MEDIUM ROOTS: MANY VERY FINE CARBONATE CLASTS; MOOE'RATELY ALKALINE: STRONGLY EFFERVESCENT; CLEAR SMOOTH BOUNDARY,

25-57 BROWN (fOYR 4/3) FINE SANOY LOAM, BROWN (10YR 5/3) ORv; WEAK COARSE SUBANGULAR BLOCK V STRUCTURE; VERY HARD; FEW MEDIUM AND COARSE ROOTS: FEW SHELL FRAGMENTS; ~ANY WORM CASTS: FEW MVCELIAL CARBONATES: MANY FINE CARBONATE CLASTS: MODERATELY ALKAL[NE; STRONGLV EFFERVESCENT: CLEAR SMOOTH BOUNDARY,

57-75 BROWN (fOYR 4/3) FINE SANOY LOAM, YELLOWISH BROWN (10YR 5/4) DRY: WEAK MEDIUM SUBANGULAR BLOCKY STRUCTURE; HARO: FEW MEOIUM AND COARSE ROOTS: FEw SHELL FRAGMENTS: COMMON TO MANY MVCELIAL CARBONATES: MANY FINE CARBONATE CLASTS; FEW LARGE LIMESTONE FRAGMENTS AND PEBBLES: MOOERATELV ALKALINE; STRONGLY EFFERVESCENT; CLEAR SMOOTH BOUNOARV,

75-100 YELLOWISH BROWN (fOYR 5/4) FINE SANOY LOAM, VERY PALE BROWN (IOVR 7/3) DRY; STRUCTURE LESS MASSIVE: SLIGHTLV HARO; FEW MEDIUM AND COARSE ROOTS: COMMON SANO LENSES: SANDS ARE OOMINANTLV CARBONATE; COMMON DISCONTINUOUS LOAMY STRATA WITHIN HORIZON: LOAMY STRATA HAS COMMON MYCELIA CARBONATES: MODERATELY ALKALINE: STRONGLV EFFERVESCENT: CLEAR SMOOTH BOUNOARY,

100-114 BROWN (10YR 4/3) LOA~, yELLOWISH BROWN (IOYR 5/4) ORY: MOOERATE MEOIUM ANO COARSE SUBANGULAR BLOCKV STRUCTURE: SLIGHTLY HARD: FEW MEDIUM AND COARSE ROOTS: COMMON SCATTER EO CHARCOAL FRAGMENTS: COMMON MYCELIAL CARBONATES: ~OOERATELV ALKALINE: STRONGLV EFFERVESCENT; CLEAR SMOOTH BOUNOARV,

114-122 VELLOWISH BROWN (fOYR 5/4) LOAM, VERY PALE BROWN (fOYR 7/3) DRY: WEAK MEDIUM SUBANGULAR BLOCKY STRUCTURE; SOFT; FEW MEDIUM AND COARSE ROOTS; SAND GRAINS ARE DOMINANTLY CARBONATES: NOTABLE SAND LENSES; MODERATELY ALKALINE: STRONGLY EFFERVESCENT: GRAOUAL SMOOTH BOUNOARY,

122-143 OARK BROWN (IOYR 3/3) LOAM, BROWN (IOYR 4/3) DRY: WEAK MEOIUM SUBANGULAR BLOCKY STRUCTURE: HARO: FEW MEDIUM AND COARSE ROOTS; FEW SHELL FRAGMENTS: COMMON MYCELIAL CARBONATES; MODERATELY ALKALINE: STRONGLY EFFERVESCENT; CLEAR SMOOTH BOUNOARY,

143-171 OARK BROWN (10VR 3/3) FINE SANOY LOAM, BROWN (tOYR 4/3) ORY: WEAK COARSE PRISMATIC STRUCTURE; SLIGHTLY HARO: FEW MEDIUM ROOTS: ~NO GRAINS ARE OOMINANTLY CARBONATES: COMMON PATCHY AREAS OF ~YCELIAL CARBONATES; COMMON SCATTEREO CHARCOAL FRAGMENTS: ~O~ OF HORIZON AS fOYR 5/3 (10YR 7/3. ORY) OISCONTINUDUS SAND LENSES: MOOERATELY ALKALINE: STRONGLY EFFERVESCENT: CLEAR SMOOTH BOUNOARY,

171-220 OARK BROWN (fOYR 3/3) LOAM, BROWN (fOYR 4/3) ORY: WEAK COARSE pRISMATIC PARTING TO WEAK MEOIU~ SUBANGULAR BLOCKY STRUCTURE: HARO; FEW MEOIUM ROOTS:

220-237

237-266

COMMON SANO LENSES; COMMON SCATTER EO CHARCOAL FRAGMENTS: COMMON TO MANY MYCELIAL CARBONATES: MOOERATELY ALKALINE; STRONGLY EFFERVESCENT; CLEAR SMOOTH BOUNOARY.

YELLOWISH BROWN (IOVR 5/4) VERY GRAVELLY SANOY LOAM, PALE BROWN (IOVR 6/3) ORV: STRUCTURELESS MASSIVE: FEW MEOIUM ROOTS: GRAVEL AND COARSE SAND LENSES; GRAVELS ANO SANOS ARE CARBONATES: MOOERATELY ALKALINE: STRONGLV EFFERVESCENT: CLEAR SMOOTH BOUNDARY,

YELLOWISH BROWN (IOYR 5/4) SANDV LOAM, PALE BROWN (IOVR 6/3) ORY; STRUCTURELESS MASSIVE; SLIGHTLV HARD: NO ROOTS: COARSE SANO LENSES: ON TOP OF HORIZON IS A LOAMY STRATA WITH MVCELIAL CARBONATES: MOOERATELY ALKALINE; STRONGLY EFFERVESCENT.

2-31

Pedon 1 SOIL CHAR~CTERIZ~TICN lABORATORY

SOIL ANO CROP SCIENCES OEPT .. THE TEXAS AGRICULTURAL EXPERIMENT STATION

SOIL SERIES: 80SQUE VARIANT SOIL FAMILY: TYPIC UST{FLUVENTS: COARSE-LOAMY, CARBONATIC. THERMIC LOCATION: CORYELL COUNTY

PARTICLE SIZE OISTRIBUTION (M".)

-----------------SANO---------------- -----SlLT-----

LAB NO DEPTH HORIZON

VC (2.0-

l.O)

C M ( 1 .0- (0.5-0.5) O.lS}

, v, TOTAL FINE TOTAL 10.2'5- (0.10- (.2 .0- {a.02- (0.05-0.10) 0.05) a.OS} 0.002) 0.002)

-----CLAY-----FINE TOTAL

( < ( < 0.0002) 0.00:2)

(eM) ----------------------------------~----------------------------------40:14 40:15 4036 .t037 4038 4039 4040 4041 4042 4043 4044

0- 25 2'- '7 .7 - 7' 75-100

100-114 114- 122 122-143 143- 17 f 171-220 220-237 237-266

·A 26KI 2BK2 ,c 4AKBt 5BWB1 6AKB2 7CIB2 aC2B2 9C3B2 IOC4B2

0.5 a .• 5 • 0.2 L, 12. 7 0.0 0.' '.G o. , ,. , lS.5 o. , '.0 16.0 O.G 11.3 37.0 o. , a .• '.0 o. , 1.2 13.6 0.' 1.7 10.1 '.7 15.7 25.9 LA '.5 24.5

20.9 19.9 48.0 19.:3 32.6 12.1 19.4 29.6 20.5 64.8 12.2 20.1 .. , 14.5 29.2 27 .5 GO.G 12.3 2:1.4 to.2 16.0 :14.5 21.4 74.G B.7 12.6 7.G 1.2 . a 17.1 14.0 50.2 18.0 30.6 11.2 19 . .2 22.8 7.' 79.G B. , 11.1 G. , .. , 15.9 18.3 .43.2 20.0 32.2 14.9 24.7 33.5 14.9 63.3 14.0 21.1 ••• 15.6 19.5 16.5 48. t 20.1 31.0 I 1.6 20.9 11. f .. , 72.7 11.0 14.0 7.' 13.3 19.3 10.5 64.2 13.3 21.2 B.8 14.6

TEXTURE CLASS

L FSL FSL 'SL L LS L 'SL L SL SL

LAB NO

ORGN PH -------NH40AC EXTR BASES------ KCL EXTR NAOAC BASE CAL- OoLO- CAC03 C (H2O) CA MG NA K TOTAL AL CEC Ecec SAT ESP SAR CITE "'ITe EO

COARSE FRAG-IoIENTS

% a a a a a 0 a a a

A' a

GYP SUM

i! 1:1 ----------------------MEQ/lOOG--------------------- ----%--- -----------%-----------4034 4035 4036 4037 4038 4039 4040 4041 4042 4043 4044

1.09 0.63 0.54 0.34 0.58 0.30 0.64 0.32 0.66 0.40 0.57

.. ~ 43.1 B.' :16.3 B.5 37.1 B.' 33.6 .. , 36.6

••• 31.2 B.2 51.0 8.A 43.8 B.A 49.3 8.2 42.9 '.2 44.3

2. , o. , 1.0 413.3 113.2 L6 o. , 0.7 :18.7 10.1 2.0 0.1 0.7 40.0 11.0 L6 o. , A.' 35.8 .. , 2.' o. , 0.6 39.6 14 .2 LA 0.1 0.2 32.9 ••• 2.7 0.1 O.A 54.2 16.6 1.7 0.1 0.2 45.9. '.0 2.' 0.2 O.A 52.4 12.6 1.8 0.2 0.2 45.1 7.7 1.. 0.2 0.' 46.7 B.'

SATURATEO PASTE EXTRACT LAB ELEC H20 NO CDNO CONT

"'MHOS/CM X Col MG NA K C03 He03 CL S04

4034 4035 4036 4037 4038 4039 4040 4041 4042

A"" 4044

LAB NO

---------------------MEQ/L-·------------------

PARTICLE Sl2E DISTRIBUTION (CLAY-FREE BASIS) - - -.- - - -.--- -SANO- - - - - - - - - - - -- -----SlLT-----

'00 '00 '00 '00 100 '00 '00 '00 (00 '00 '00

BULK OEN 0.33 AIR BAR ORY ---G/cc--

, , , , , 2 , , 2 , 2

COLE CIot/CM

1.29 1.580.070

I. 16 1.38 0.060

1.01 1. 23 0.068

43.3 0.0 43.3 49.4 0.0 49.4 42.4 0.0 42.4 55.0 0.0 5S.0 58.7 0.0 58.7 76.0 0.0 76.0 43.9 0.0 43.9 62.3 0.0 62.3 56.3 0.0 56.3 12.0 0.0 72.0 136.6 0.0 66.6

WATER CONTENT 0.10 0.33 15 BAR BAR BAR ------WTX------

30.0

.. • 50.9

VCS C • , V, TOTAL C , TOTAL -----------RATIOS-----------

4034 4035 4036 4037 4038 4039 4040 4041 4042 4043 4044

----------------------%----------------------O.G 1.' 7.2 25.9 24.7 59.6 16.5 23.9 40.4 0.2 2.1 14.9 34.6 24.0 75.8 ••• 14.3 24.2 0.0 0.4 4.3 34.8 32.7 72. t 13.3 14.6 27.9 O. ( 3.6 17 .8 39.6 24.5 85.6 4.' 10.0 14.'3 0.1 '.7 19.8 21.2 t7 .3 62.1 15.6 22.3 37.9 0.7 12.5 40.8 25.1 8.7 87.8 3.3 B.' 12.2 0.1 1.2 10.6 2 t. 1 24.3 57.4 16.2 26.6 42.8 O. , (.A 16.1 39.7 t7.7 75.0 B.4 16.6 25.0 0.4 2.2 12.8 24.7 20.9 60.8 13.8 25.4 39.2 6.6 lB.l 29.9 19.7 '.6 83.9 3.' 12.7 113.1 (.6 10.0 28.7 22.6 12.3 75.2 '.2 15.6 24.8

2-32

s/st FSI/CSI VFs/FS FCITC CEclCLAY 1.. LA 1.0 O.G 0.a3 ,. , 1.' 0.7 0.6 0.70 2.G (., a .• 0.6 0.69 5.' 2.2 0.6 0.6 0.135 (,6 (.4 0.8 0.6 0.74 7.2 2.7 0.3 0.7 0.64 1.3 1.6 (,2 0.6 0.67 3.0 2.0 0.4 0.6 0.58 1.6 1.8 0.' 0.6 0.60 '.2 3.7 0.' 0.' 0.58 3.0 1.7 a .• 0.6 0.61

CO FE % 0.2 0.2 0.2 0.2 0.2 0.2 0.' 0.2 0.2 0.2 0.2

PEDON 2 SOIL SERIES: BOSQUE VARIANT PEDaN: 59QrX-099-009 COUNTY = CORYELL

PEDaN CLASSIFICATION: PACHIC CALCIUSTOlLS: FINE-LOAMY, CARBONATIC. THERMIC

LOCATION: FT. HOOD. rEXAS: 10 M N OF CQWHOUSe CREEK; 300 M W OF WEST RANGE AOAD.

LANDFORM: STREAM TERRACE ELEVATION (104): SLOPE: 0-11. SLOPE ASPECT:

PARENT MATERIALS: ALLUVIUM FORMATION: HOLOCENE ALLUVIUM (UNIT 0)

TOPOGRAPHY: NEARLY LEVEL DRAINAGE: WELL DRAINED LANOUSE: NATIVE

COLLECTORS: NOROT ANO HALLMARK DATE: 07/03/90

HORIZON OEPTH (eM)

• I 0-15

'2 15-34

aKI 34-53

aK2 53-85

aKa 85-118

BK4 118-155

aKe 155-166

aK6 166-188

BK7 _ !a8-2IS

BKe 215-230

SOIL DESCRIPTION (COLORS FOR MOIST SOIL UNLESS STATED)

VERY DARK GRAYISH BROWN (IOVR 3/2) LOAM, GRAYISH BROWN (fOVR 5/2) DRY: MODERATE COARSE SUBANGULAR BLOCKY PARTING TO MODERATE MEDIUM SUBANGULAR BLOCKY STRUCTURE' VERY HARD: MANY FINE AND MEDIUM ROOTS; MODERATELY ALKALINE; STRONGLY EFFERVESCENT' CLEAR SMOOTH BOUNDARY, '

VERY DARK GRAYISH BROWN (IOYR 3/2) LOAM, VERY OARK GRAYISH BROWN (IOYR 3/2) DRY; WEAK COARSE PRISMATIC PARTING TO WEAK MEOIUM AND COARSE SUBANGULAR BLOCKY STRUCTURE: HARD; COMMON FINE ROOTS; COMMON FAUNAL CASTS; MODERATELY ALKALINE; STRONGLY EFFERVESCENT; CLEAR WAVY BOUNDARY,

VERY DARK GRAYISH BROWN (tOYR 3/2) LOAM, OARK GRAYISH BROWN (IOYR 4/2) ORY~ WEAK COARSE PRISMATIC PARTING TO MODERATE MEOIUM AND COARSE SUBANGULAR BLOCKY STRUCTURE: HARD; COMMON FINE ROOTS; FEW FAUNAL CASTS: MANY VERY THIN CARBONATE MYCELIA ON VERTICAL PEO FACES OF PRISMS; SECONDARY CARBONATE IS <11. OF HORIZON VOLUME: MODERATELY ALKALINE: STRONGLY EFFERVESCENT: GRA~UAL SMOOTH BOUNDARY.

VERY DARK GRAYISH BROWN (tOYR 3/2) LOAM, GRAYISH BROWN (10YR 5/2) DRY; WEAK COARSE PRISMATIC PARTING TO MODERATE MEDIUM SUB ANGULAR BLOCKY STRUCTURE; HARD: COMMON FINE ROOTS: FEW FAUNAL CASTS: MANY vERY THIN CARBONATE MYCELIA ON VERTICAL PEO FACES OF PRISMS; SECONDARY CARBONATE IS 110 OF HORIZON VOLUME; MODERATELY ALKALINE: STRONGLY EFFERVESCENT; GRADUAL WAVY BOUNOARY,

DARK GRAYISH BROWN (fOYR 4/2) LOAM, GRAYISH BROWN (IOYR 5/2) DRY; WEAK COARSE PRISMATIC PARTING TO MOOERATE MEDIUM SUBANGULAR BLOCKY STRUCTURE; HARD: FEW FINE AND MEDIUM ROOTS: FEW FAUNAL CASTS: COMMON (1510) FINE CARBONATE FILAMENTS AND THREADS ON SURFACES OF PEDS; COMMON (5%) FINE CARBONATE FILAMENTS AND THREADS IN INTERIORS OF PEDS: MODERATELY ALKALINE; VIOLENTLY EFFERVESCENT; GRADUAL WAVY BOUNDARY.

BROWN (10YR 4/3) LOAM, BROWN (fOYR 5/3) DRY; WEAK COARSE PRISMATIC PARTING TO MODERATE MEDIUM SUBANGULAR BLOCKY STRUCTURE: HARD; FEW FINE AND MEDIUM ROOTS; FEW FAUNAL CASTS; FEW SHELL FRAGMENTS; COMMON (15%) FINE CARBONATE FILAMENTS AND THREADS ON PEO SURFACES; COMMON (510) FINE CARBONATE FILAMENTS AND THREADS IN INTERIOR OF PEDS: MODERATELY ALKALINE: VIOLENTLY EFFERVESCENT; GRADUAL WAVY BOUNDARY.

BROWN (tOYR 4/3) lOAM, BROWN (tOYR 5/3) DRY: WEAK COARSE PRISMATIC PARTING TO MODERATE MEDIUM SUBANGULAR BLOCKY STRUCTURE; HARD; VERY FEW FINE ROOTS: FEW FAUNAL CASTS; FEW SHELL FRAGMENTS: COMMON (1010) FINE CARBONATE FILAMENTS AND THREADS ON PED SURFACES: COMMON (5%) FINE CARBONATE FILAMENTS AND THREADS IN INTERIOR OF PEDS: MODERATELY ALKALINE: VIOLENTLY EFFERVESCENT; CLEAR SMOOTH BOUNDARY,

BROWN (10YR 4/3) LOAM, BROWN (10YR 5/3) DRY; MODERATE MEDIUM PRISMATIC PARTING TO MODERATE MEDIUM SUBANGULAR BLOCKY STRUCTURE; HARD: VERY FEW FINE ROOTS: FEW FAUNAL CASTS: FEW SHELL FRAGMENTS: MANY (20%) FINE CARBONATE FILAMENTS AND THREADS ON PED SURFACES; COMMON (10%) FINE CARBONATE FILAMENTS ANO THREADS IN PED INTERIORS: MODERATELY ALKALINE: VIOLENTLY EFFERVESCENT; GRAOUAL SNOOTH BOUNDARY,

BROWN (10YR 4/3) LOAM, BROWN (IOYR 5/3) DRY: MODERATE MEDIUM PRISMATIC PARTING TO MODERATE MEDIUM SUBANGULAR BLOCKY STRUCTURE; HARD: VERY FEW FINE ROOTS; FEW FAUNAL CASTS: FEW SHELL FRAGMENTS: MANY (2010) FINE CARBONATE FILAMENTS AND THREADS ON PED SURFACES: COMMON (10%) FINE CARBONATE FILAMENTS AND THREADS IN PED INTERIORS: MOOERATELY.ALKALINE: VIOLENTLY EFFERVESCENT.

BROWN (10YR 4/3) LOAM. BROWN (10YR 5/3) ORY; WEAK COARSE PRISMATIC PARTING' TO WEAK MEDIUM SUBANGULAR BLOCKY STRUCTURE; HARD: VERY FEW FINE ROOTS: FEW FAUNAL CASTS; COMMON (10%) FINE CARBONATE FILAMENTS AND THREADS ON PEO FACES; COMMON (10%) FINE CARBONATE FILAMENTS AND THREADS IN PED INTERIORS: MODERATELY ALKALINE; VIOLENTLY EFFERVESCENT

REMARKS: BKS AND BK7 MAY HAVE BEEN A BURIED A. HORIZON BUT HAS CHARACTER OF B HORIZON TODAY,

2-33

Pedon 2 SOIL CHARACTERIZATION LABORATORY

SOIL AND CROP SCIENCES QEPT .. THE TEXAS AGRICULTURAL eXPERIMENT STATION

SOIL SERIES: BOSQUe VARIANT SOIL FAMILY: PACHIC CALC{USTOLlS: FINE-LOA~Y. CARBONAT[C, THERMIC LOCATION: CORVELL COUNTY

PEDON NUMBER: S90TX-099-009

LAB NO

4165 4166 4167 416B 4169 4170 4171 4172 4173 4174

LAB NO

DEPTH HOR I ZON (CIon 0- 15 A f

15- 34 A2 34- 53 81(1 53- 84 8K2 85-118 aK3

118-155 8K4 155-166 eK5 166-188 eK6 188-215 8K7 215-230 aK8

PARTICLE SIZE DISTRIBUTION (~Ml

-----------------SAND---------------- -----SILT----- -----CLAY-----VC eMF VF TOTAL FINE rOTAL FINE rOTAL

{2.a- (f.O- (0.5- (0.25- (0.10- (2.0- {O.02- (0.05- « (<

1.0) 0.5) 0.251 0.10) 0.05) 0.05) 0.002) 0.002) 0.0002) 0.002) ----------------------------------%----------------------------------0.4 0.6 7.0 23.0 18.0 49.0 18.1 31.2 10.5 19.8 0.4 1.8 7.4 21.3 15.5 46,4 21.5 34.3 9.0 19.3 0.10.64.523.619.748.5 21.1 .31.4 10.8 20.1 0.0 0.7 5.8 26.4 19.8 52.7 20.6 28.6 11.0 18.7 0.1 0.4 3.2 22 . .3 2L3 47 . .3 22.7 .31.8 12.6 20.9 0.2 0 . .3 4.0 20.1 21.2 45.8 19.6 31.7 13.4 22.S 0.00.22.017.522.642.3 19.4 .34.6 14.0 23.1 0.1 0.2 1.2 15.3 21.8 38.6 21.9 36.5 14.8 24.9 0.1 0.4 3.0 16.4 17.4 37.3 23.4 37.0 15.6 25.7 0.00.55.421.117.044.0 20.6 33.1 13.8 22.9

TeXTURE CLASS

L L L FSL L L L L L L

ORCiN PH -------NH40AC EXTR 8ASES------ KCL EXTR NAOAC 8ASE CAL- . DOLO- CAC03 C (H2O) CA "G NA K TOTAL AL CEC fCEC 5A T ESP SA' CITE MITE EO

COARse FRAG­MENTS

% o o o o o o o o o o

GVP SUM

r. 1: f ----------------------MEO/lOOG--------------------- ----y.--- -----------y.-----------4165 4166 4167 4168 4169 4170 4171 4172 4173 4174

1.17 I. 89 0.97 0.94 0.70 0.69 0.72 0.62 0.73 0.66

S. I 47.2 S.O 47.0 S., 47.0 S.J 45.5 S.J 47.8 S.' 47.9 B.' 47.9 B.' 50.1 S.J 50.8 S.J 45.5

LAB fLEC H20

,. I 0.1 0.4 49.8 , .0 0.1 0.7 49.9 ,. , 0.1 0.' 49.8 '.J 0.1 O.J 48.2 '.5 0.1 O.J 50.7 '.B O. I O.J 51.1 J.J O. I O.J 51.5 J.' O. I 0.4 54.5 4.' O. I 0.4 55.5 4. I O. I O.J 49.9

SATURATED PASTE EXTRACT

14.6 16.3 13.2 10.7 1f.5 t 1. 7 12.4 13.1 .!4. I 12.2

100 100 100 100 100 100 100 100 100 100

8ULK OEN 0.33 AIR

46. I 0.0 46. I 46.4 0.0 46.4 46.2 0.0 46.2 49.7 0.0 49.7 47.5 0.0 47.5 46.0 0.0 46.0 42.6' 0.0 42.6 42. f 0.0 42.1 44.4 0.0 44.4 49.1 0.0 49. f

NO COND CONT

"''''Hos/c''' " CA MG NA K COJ HC03 CL S04 BAR OR'" COLE

---o/cc-- CM/c,,",

WATER CONTENT 0.10 0.33 15 BAR BAR BAR

4165 4166 4167 4168 4f69 4170 4171 4172 4173 4174

LAB NO

4165 4166 4167 4168 4169 04170 04'71 4172 4173 4174

---------------------MEO/L-------------------- ------WT%---- .. -1.38 1.570.044 1.31 1.500.046 1.22 1.450.059 1.21 1.440.060 1.26 1.500.060 1.27 1.58 0.076 1.291.460.042 1.30 1.4B 0.044 1.29 1.50 0.052 1.28 1.470.047

PARTICLE SIZE DISTRIBUTION (CLAY-FREE BASIS) ------------SAND------------- -----SIlT-----

VCS C N F VF TOrAl C F TOTAL

----------------------%----------------------0.5 0.7 8.728.722.461.1 15.623.338.9 0.5 2.2 9.2 26.4 19.257.5 15.926.642.5 O. I 0.8 5.629.524.760.7 12.926.439.3 0.0 0.9 7.1 32.5 24.4 64.8 9.925.335.2 0.1 0.5 4.0 28.2 26.9 59.B It.5 2B.7 40.2 0.3 0.4 5.225.927.459.1 15.625.340.9 0.0 0.3 2.6 22.8 29.4 55.0 19.8 25.2 45.0 O. I 0.3 1.6 20.4 29.0 51.4 19.4 29.2 48.6 O. f 0.5 4.0 22.1 23.4 50.2 IB.3 31.S 49.8 0.0 0.6 7.0 27.4 22.1 57.1 16.226.742.9

2-34

-----------RATIOS-----------51st FSI/CSI VFS/FS Fc/TC

1.6 1.5 0.8, 0.5 1.4 1.7 0.7 0.5 1.5 2.0 0.8 0.5 1.8 2.6 0.8 0.6 1.5 2.5 1.0 0.6 1.4 1.6 1.1 0.6 1.2 1.3 1.3 0.6 1. f 1.5 1.4 0.6 1.0 1.7 1.1 0.6 1.3 1.6 O.B 0.6

26.6 30.4 35.1 35.7 33.2 31.8 32.5 32.5 33.2 33.9

CEC/ClAY 0.74 0.84 0.66 O.!H 0.55 0.52 0.54 0.53 0.55 0.53

co FE

• O.J O.J 0.3 o.J o.J O.J 0.' o.J 0.' 0.'

PEDON 3 SOIL SERIES: LEWISVILLE VARIANT PEDON: S90TX-Q99-002 COUNTY: CaRVELL

PEDON CLASSIFICATION: VERTIC CALCIUSTOLLS; FINE. MONTMORILlONITIC. THERMIC

LOCATION: FT. HOOD, TEXAS. CHSi8TRf.

LANOFOR~: STREA~ TERRACE ELEVATION (M): SLOPE: 0- 1% SLOPE ASPECT:

PARENT MATERIALS: ALLUvIUM FORMATION: HOLOCENE ALLUVIUM (UNIT C)

DRAINAGE: WELL DRAINED TOPOGRAPHY: NEARLY LEVEL LANOUSE: NATIVE

COLLECTORS: NOROT AND HALLMARK OA TE: 12/20/89

HORIZON

At

AB

BW

BKl

BK2

BK'

BK4

Be

REfIIARKS:

DEPTH (eM)

0-16

16-32

32-59

59-94

94-122

1:22-153

153-189

189-252

330-350

SOIL DESCRIPTION (COLORS FDA MOrST SOIL UNLESS STATED)

VERY OARK BROWN (IOVR 2/2) SILTV CLAY LOAM, VERY DARK GRAYISH BROWN (IOVR 3/2) CAY; MODERATE COARSE SUB ANGULAR BLOCKY PARTING TO MOOERATE FINE SUBANGULAR BLOCKY STRUCTURE VERY HARO: COMMON FINE ANO MEOIUM ROOTS; NANY WORN CASTS: FEW SANO SIZED CARBONATE GRAINS: NOOERATELY ALKALINE; STRONGLY EFFERVESCENT: CLEAR SMOOTH BOUNOARY.

VERY OARK BRO~N (IOVR 2/2) SILTY CLAY. VERY OARK GRAyISH BROWN (IOVR 3/2) ORY: ~EAK COARSE SUB ANGULAR BLOCKY PARTING TO MOOERATE FINE SUB ANGULAR BLOCKY STRUCTURE: HARD: COMMON FINE ANO NEO{UN ROOTS; CONMON WORM CASTS: FEW SHELL FRAGMENTS: FEW SANO SIZEO CARBONATE GRAINS: MOOERATELY ALKALINE: STRONGLY EFFERVESCENT; GRADUAL SMOOTH BOUNOARV.

VERY OARK GRAVISH BROWN (lOVR 3/2) SILTY CLAY, VERY OARK GRAVISH BROWN (lOYR 3/2) ORV; NOOERATE COARSE SUBANGULAR BLOCKV PARTING TO MODERATE MEOIUM SUBANGULAR BLOCKV STRUCTURE: HARD: FEW FINE ANO MEOIUN ROOTS: COMMON WORM CASTS; FEW SHELL FRAGMENTS: FEW SANO SIZEO CARBONATE GRAINS; MOOERATELV ALkALINE; STRONGLV EFFERVESCENT; GRAOUAL ~AVV BOUNOARV.

DARk BROWN (lOVR 3/3) SILTV CLAY, OARK BROWN (laVR 3/3) ORV; WEAK COARSE PRISNATIC PARTING TO MODERATE NEOIUN ANGULAR BLOCkY STRUCTURE; VERY HARD: FEW FINE ANO MEDIUM ROOTS: COMMON WORM CASTS: FEW SHELL FRAGNENTS; FEW SAND SIZED CARBONATE GRAINS: OCCASIONAL MEDIUM (1 eM) HARD IRREGULAR CARBONATE NODULES THAT ARE SONEWHAT PITTED: MOOERATELY ALkALINE; STRONGLV EFFERVESCENT; CLEAR WAVY BOUNDARY.

DARk VELLOWISH BROWN (lOVR 4/4) SILTV CLAY LOAN, DARK YELLOWISH BROWN (laVR 4/4) ORV: WEAk NEOIUN PRISNATIC PARTING TO MOOERATE MEDIUM ANGULAR BLOCKV STRUCTURE: VERy HARD: VERY FEW FINE ROOTS; FEW WORM CASTS; FEW SHELL FRAGMENTS; FEW SAND SIZED CARBONATE GRAINS: COMMON (3~) THREADS ANO FILAMENTS OF CARBONATE CONCENTRATED ON PED SURFACES BUT ALSO IN SOME PEO INTERIORS; SOME SMALL TO MEOIUM WEDGE-SHAPED PEDS WITH DISTINCT PRESSURE FACES; MOOERATELV ALKALINE: STRONGLV EFFERVESCENT; GRADUAL WAVY BOUNDARY.

DARk YELLOWISH BROWN (lOVR 4/4) SILTV CLAY LOAM, DARk YELLOWISH BROWN (IOVR 4/4) ORV: MODERATE MEDIUM ANGULAR BLOCKY STRUCTURE: VERY HARD; VERY FEW FINE ROOTS: FEW WORM CASTS; FEW SHELL FRAGMENTS: FEW SAND SIZED CARBONATE GRAINS: COMMON (5%) THREAOS AND FILAMENTS OF CARBONATE BOTH ON PED SURFACES AND IN PEDS: SOME SMALL TO MEDIUM WEDGE-SHAPED PEOS WITH DISTINCT PRESSURE FACES; MOOERATELY ALKALINE: STRONGLV EFFERVESCENT; GRAOUAL WAVY BOUNOARy.

DARK VELLOWISH BROWN (lOVR 4/4) SILTV CLAy LOAM; MOOERATE MEDIUM PRISMATIC PARTING TO MODERATE MEDIUM ANGULAR BLOCKV STRUCTURE: VERY HARD; VERY FEW FINE ROOTS; FEW SHELL FRAGMENTS: FEW SAND SIZED CARBONATE GRAINS: COMMON (7%) THREADS AND FILAMENTS OF CARBONATE BOTH ON PED SURFACES AND IN PEDS: MOOERATELV ALKALINE: STRONGLY EFFERVESCENT: GRADUAL WAVY BOUNOARV.

DARK YELLOWISH BROWN (IOYR 4/4) SILTV CLAY LOAM: MOOERATE MEDIUM PRISMATIC PARTING TO ~OOERATE MEDIUM ANGULAR BLOCkY STRUCTURE: VERY HARD: NO ROOTS: FEW SHELL FRAGMENTS: FEW SAND SIZED CARBONATE GRAINS; COMMON (9%) THREADS AND FILAMENTS OF CARBONATE BOTH ON PED SURFACES AND IN PEDS: MODERATELY ALKALINE: STRONGLY EFFERVESCENT.

YELLOWISH BROWN (10YR 5/4) SILTV CLAY; HARD: NO ROOTS: FEW SOFT SEGREGATIONS OF CARBONATE ABOUT 1-2 CM IN DIANETER; MODERATELV ALKALINE: STRONGLV EFFERVESCENT.

THE BC HORIZON WAS SAMPLED AND DESCRIBED BY HANO AUGER.

2-35

Pedon 3 SOIL CHARACTERIZATION LABORATORY

SOIL AND CROP SCIENCES DEPT .• THE TEXAS. AGRICULTURAL EXPERIMENT STATtON

SOIL SERIES: LEWIsvILLE VARIANT PEDaN NUMBER: 590TX-099-002 SOIL FAMILY: VERTIC CALCtUSTOLL5: FINE. MONTMQRIlLONITIC. THERWIC LOCATION: CQIlYELL COUNn

LAB NO

4018 4019 4020 4021 4022 4023 4024 4025 4026

LAB NO

4018 4019 4020 4021 4022 4023 4024 4025 4026

LAB NO

4018 4019 4020 4021 4022 4023 4024 4025 4026

LAB NO

4018 4019 4020 4021 4022 4023 4024 4025 4026

PARTICLE SIZE DISTRIBUTION !101M)

-----------------SANO---------------- -----SlLT----- -----CtAY-----VC C " F VF TOTAL FINE TOTAL fINE TOTAL COARSE

(2.0- (j .0- (0.5- (0.25- (0.10- (:Z.o- (0.01- (0.05- C < C< TEXTURE FRAG-OEPTH HORI ZON La I 0.5) 0,25) O. to) 0.05) O.OS) 0.002) 0.002) 0.0002) 0.002) CLASS IoIENTS {C"'~ ----------------------------------X---------------------------------- % 0- IG A I 0.2 O .• 0.5 2.G 7 .• , 1. I 36.8 51.8 17.6 37. f SteL 0

16- J> A2 0.3 O .• O.G 2.9 7. I 11.3 l6.9 47.7 22.6 41.0 SIC 0 ,,- 59 AB 0.4 0.3 0.7 2.7 7.2 If.3 36.4 45.7 23.4 4l.0 SIC 0 59- 9' BW 0.4 0.7 0.9 '.5 8.7 14.2 34.0 43.7 23.1 42. I SIC 0 94-122 BKI 0.2 0.5 1.0 '.' If. 1 17.2 31.9 43.5 21.8 39.3 SICL 0

122-153 BK2 0.2 0.4 O.G '.5 12.0 17 .1 32. I 44.5 21.5 37.8 SICL 0 153-189 BK3 0.1 0.4 0.4 '.5 13.3 18.7 lO.5 .. .3 22. t 37.0 SICL 0 189-252 , .. 0.2 0.' 0.' '.8 12.1 16.8 lO.8 .. .B 22.5 38.4 SICL 0 330-350 C 0.1 0.2 0.' 2.0 B.G 11.2 29.5 " .G 25.l 41.2 SIC 0

ORGN PH C {H20}

~ 1: t

-------NH40AC EXTR 8ASES------ KCl EXTR NAOAC BASE CAL- OOLO- CAC03 GYP CA 0.1:0 NA K TOTAL AL CEC ECEC SAT ESP SA. CITE ~ITE EO SUM

----------------------MEO/IOOG--------------------- ----1,--- -----------/.-----------1.817.9 1.37 8.0 1.04 8.1 0.798.3 0.58 8.3 0.44 8.3 0-.34 8.4 0.31 8.3 0.l4 8.4

51.8 2.4 0.0 t.l 55.5 26.8 47.7 2.2 0.1 0.9 50.9 27.1 49.5 2.3 0.1 0.7 52.6 28.5 47.3 2.4 0.1 0.6 50.4 26.2 48.1 3.0 0.1 0.5 52.3 2t.9 47.6 3.3 0.1 0.5 51.5 25.3 45.3 3.9 0.1 0.6 50.0 21.1 43.6 4.8 0.1 0.6 49.2 21.5 44.0 8.8 0.1 0.6 53.6 25.4

ELEC H20 CONO CONT M~HOs/cM %

0.5 61 0.5 61 0.4 61 0.3 51 0.2 56 0.2 59

SATURATEO PASTE EXTRACT

CA IotO NA I( C03 He03 CL S04 ---------------------MEO/L--------------------

4.4 0.3 0.2 0.3 0.0 4.4 0.2 0.3 4.4 0.3 0.2 O. t 0.0 4.5 0.0 0.3 3.5 0.3 0.2 O. I 0.0 3.9 0.0 0.1 2.1 0.2 0.2 0.0 0.0 2.6 0.0 0.3 1.1 0.2 0.2 0.0 0.0 1.9 0.2 0.1 1.5 0.2 0.2 0.0 0.0 1.8 0.2 0.2

PARTICLE SIZE DISTRIBUTION (CLAY-FREE BASIS) ------------SANO------------- -----SILT-----

100 0 0 100 0 0 100 0 0 100 0 0 100 0 0 100 0 0 100 0 100 0 100 0

BULl( OEN 0.33 AlR BAR DRY u-o/cc--1.23 1.54 1.25 1.51 t .31 1.61 1.43 1. 74 1.39 1.13 1.41 1.16 1.421.76 1.35 1.18

COLE C~/CM 0.078 0.079 0.084 0.06B 0.016 0.017 0.014 0.097

36.5 0.0 36.5 37. I 0.0 37. I 38.6 0.0 38.6 39.6 0.0 39.S 39.4 0.0 39.4 39.6 0.0 39.6 31.3 0.0 31.3 35.5 0.0 35.5 33.6 0.0 3l.6

WATER CONTENT 0.10 0.33 15 BAR BAR BAR ------WT%-- ----

32.8 32.5 34. I 21. I 29.1 30.5 29.3 35.2

ves e " F VF TOTAL e F TOTAL -----------RATtOS-----------

----------------------%---------------------- S/St FsIICSI Vfs/FS FCtTC CEC/CLAY 0.3 O.G O.B 4. I 11.8 17 .6 23.9 58.5 82.4 0.2 2.4 2.' 0.5 0.72 0.5 0.7 1.0 4.' 12.0 19.2 18.3 62.5 80.8 0.2 3.4 2.4 O.G 0.66 0.7 0.5 1.2 '.7 12.6 19.8 16.3 63.9 80.2 0.2 3.' 2.7 0.5 0.66 0.7 1.2 1.6 G.O 15.0 24.5 16.8 58.7 75.5 0.3 3.5 2.5 0.5 0.62 0.3 O.B I.G 7.2 18.3 28.3 19. I 52.6 71.1 0.' 2.B 2.S 0.6 0.56 0.3 O.G 1.0 7.2 19.3 28.5 19.9 51.6 71.5 0.4 2.6 2.7 0.6 0.67 0.2 0.6 0.6 7. I 21. t 29.7 21.9 48.4 10.3 0.4 2.2 3.0 O.G 0.57 0.3 0.5 0.7 G.2 19.6 21.3 22.7 50.0 12.7 0.4 2.2 3.2 O.G 0.56 0.2 0.3 0.5 3.' 14 .6 19.0 .30.8 50.2 81.0 0.2 I.G 4.3 0.6 0.62

2-36

CO FE % 0.' 0.5 0.5 0.5 0.5 0.5 0.' 0.5 0.4

PEDON 4 SDIL SERIES: LEWISVILLE VARIANT PECCN: S90TX-099-001 coUNn: CORHLL

PEDON CLASSIFICATION: UOIe CHRO~STERTS; FINE. MONTWORILLONtT[C, THERMIC

LOCATION: FT. HOOD, TEXAS. STRATIGRAPHIC UNIT A.

LANOFORM: STREAM TERRACE ELEVATION (N): SLOPE: 1% SLOPE ASPECT:

PARENT MATERIALS: ALLUVIUN FORMATION: PLEISTOCENE ALLuvIUN (UNIT A)

ORAINAGE: WELL DRAINED TOPOGRAPHY: GENTLY SLOPING LANOU5E: NATIVE

COLLECTORS: NeRDT. HALLMARK, WILDING, DREES, .lGRD 606 CLASS CATE: 01/20/90

HORIZ.ON

.A.

BKSS f

BKSS2

.. ,

.'2

.'3

•••

•••

.e

OEPYH (eM) SOIL DESCRIPTION (COLORS FOR ~IST SOIL UNLESS STATED)

o·a VERY DARK BROWN (IOVA 2/2) CLAY LOAM, VERY DARK BROWN {IOVR a/a} DRY: ~OOERATE FINE AND MEDIUM SUB ANGULAR BLOCkY AND WEAK FINE GRANULAR STRUCTURE: VERY HARD: FEW FINE PORES: CO~MON FJNE ANO MEOIUM ROOTS: FEW SANO-SIZEO CARBONATE GRAINS; NONCALCAREOUS: CLEAR S~OTH BOUNDARY.

8~1" VERY DARK BROVN (101'R 2/2) CLAY LOAM, VERY DARK BRowN (IOYR '2/2) ORY: MODERATE FINE AND MEDIUM SUB ANGULAR BLOCKY STRUCTURE: VERY HARD: COMMON FINE AND MEOIUN ROOTS; FEW SAND-SIZED CARBONATE GRAINS: NONCALCAREOUS: GRAOUAL SNOOTH BOUNOARY.

14-38 VERY OARK BROWN (IOYR 2/2) CLAI', VERY DARK BROWN (IOYR l/2) DRY: ~OOERATE MEDWM PRISMATIC PARTING TO ."I00ERATE COARSE ANGULAR BLOCKY STRUCTURE: VERY HARO; CONMON FINE ANO MEDIUM ROOTS; COMMON BROWN (7.5I'R 4/4) VORM CASTS; FEW TO COMMON (:IX) MEorUM (f CM) HARO HIGHLY PITTEO CARBONATE NOOULES; SLIGHTLY CALCAREOUS; GRADUAL WAVI' BOUNOARY.

3B-66 DARK BROWN (7.5YR 3/4) CLAY, DARK BROWN (7.5YR 3/4) ORY; MODERATE COARSE PRISMATIC PARTING TO MODERATE COARSE ANGULAR BLOCKY STRUCTURE; VERY HARD: COMMON FINE ANO MEDIUM PORES: FEV FINE ROOTS; COMNON VERY OARK BROWN (10YA 2/21 CHANNEL FILLINGS ON VERTICAL PEO FACES; CO"~"'ON BROWN (7.5YR 4/4) WORM CASTS IN MATRIX: FEW sANQ-SIZEO CARBONATE GRAINS; FEW TO COMMON (1%) MEOIUI4 (I CM) HJGHLI' PITTED CARBONATE NOOULES: FEW PRESSURE FACES ANO SLiCKENSIOEs IN LOWER PART OF HORIZON; ROOTS FLATTENEO AGAINST PEO FACES: STRONGll' EFFERVESCENT; GRAOUAL WAVY BOUNDARY.

66-98 STRONG BROWN (7.5YR 4/6) CLAY, BROVN (7.5YR 4/4) DRY: VERY HARO: COMMON FINE ANO MEDIUM PORES: FEW FINE ROOTS; FEW VERY DARK BROwN (IOYR 1/1) CHANNEL FILLINGS ON VERTICAL PEO FACES; FEW BRO'ol"N (1.SYR 4/4) WORM CASTS IN !04ATRIX; FEW SANO-sIZEO CARBONATE GRAINS: FEW (1)(,) MEOIUM (1-2 CI4) HIGHLY PITTEO CARBONATE NOOULES; COM~ON INTERSECTING SLIGKENStOES AND PARALLELIPIPEOS; FEW MEOIUM 1.5YR 5/6 POCKETS OF CARBONATE-RICH MATERIAL SIMILAR TO HORIZON BELOw: FEW FINE THIN CARBONATE FILAMENTS ON SOI4E PEa FACES; ROOTS FLATTENEO AGAINST PEO FACES: STRONGLY EFFERVESCENT; CLEAR WAVY BOUNDARY.

98-120 STRONQ BROWN (7.5YR 4/6) CLAY LOA~. STRONG BROWN {7.5YR 5/6} DRY: WEAK COARSE SUBANGULAR BLOCKY PARTINO TO WEAK FINE AND MEDIUM SUB ANGULAR BLOCKY STRUCTURE: HARD; FEW FINE PORES; VERY FEW FINE ROOTS: COMMON BROWN (7.5~R 4/4) WOR~ CASTS IN MATRIX; ABOUT ~O~ OF HORIZON COMPOSEO OF 7.5YR 6/6 MATERIAL OF SIMILAR TEXTURE BUT APPEARS TO BE HIGHER IN CARBONATE CONTENT: COMMON (15~) MEDIUM (2-3 CM) HARO CARBONATE NODULES: CO~NON (15-20~) CARBONATE FILAMENTS ANO THREAOS THROUGHOUT; HARD NOOULES ARE NOT AS PITTED AS ABOVE: STRONGLY EFFERVESCENT: CLEAR WAVY BOUNOARY.

120-'50 REODISH YELLOw (7.5YR 6/6) CLAY LOAM: NANY COARSE FAINT REDDISH YELLOW (1.5YR 7/6) ANO I4ANY COARSE FAINT REDDISH YELLOW (7.5YR 6/B) MOTTLES; WEAK COARSE SUBANGULAR BLOCKY PARTING TO 'oI"EAK FINE ANO MEDIUM SUBANGULAR BLOCKY STRUCTURE: HARO: VERY FEW FINE ROOTS: COKNON BROWN (7.5YR 4/4) WORM CASTS IN MATRIX: COMMON (5-10%) MEDIUM (1-2 CN) HARD CARBONATE NOOULES; COMMON (10%) CARBONATE FILAMENTS ANO THREADS: STRONGLY EFFERVESCENT; GRAOUAL WAVY BOUNOARY.

1~-190 STRONG BROWN (7,5YR 5/6) SANOY CLAY LOAM: MANY COARSE FAINT REDDISH YELLOW (7.5YR 6/6) ANO CO~N COARSE DISTINCT YELLOwISH REO (SYR 4/6) MOTTLES: WEAK COARSE SUBANCULAR BLOCKY PARTING TO WEAK MEDIUM SU8ANGULAR BLOCKY STRUCTURE; VERY HARD: VERY FEW FINE ROOTS: COMMON BROWN (7.SYR 4/4) WORN CASTS IN MATAIX; FEw TO COfolMON (1%) MEOIUM (1-2 CN) KlRO CARBONAT[ NOOULES: COIolM(]N (3%) CARBONATE FILAMENTS ANO THREADS: COMHON '-2 MI4 ~OOT CHANNELS: 30-40% OF HORIZON ~EWORKED BY BIOTIC ACTIVITY: STRONGLY EFFERVESCENT; GRAOUAL wAVY BOUNOARY.

1&0-213 YELLOWISH B~OWN (IOYA 5/6) SANOY CLAY LOAM:' MANY COARSE FAINT YELLOWISH BAOWN (IOYR 5/4) ANO CONNON COARSE OISTINCT BROWN {7.9YA 4/4} MOTTLES; WEAK COARSE SUBANQULAR BLOCKY PARTINO TO WEAK MEOIUM SUB ANGULAR BLOCKY STRUCTURE; VEIIY HARD: VERY FEW FINE ROOTS': CONNON BROWN {7.SYR 4/4} WORM CASTS IN MATRIX: 'E~ THIN PATCHY CLAY FtL~s: VERY FEw ~EoIU~ HARD CARBONATE NOOULES; FEW (IX) CARBONATE FILAMENTS ANO THREAOS; COfolMON 1-2 NM ROOT CHANNELS: 30-40~ . OF HQIIIZON REWORKEO BY BIOTIC ACTIVITY; STRONGLY EFFEIIVESCENT: GRAOUAL WAVY BOUNOARY.

213-250 YELLOWtSH BROWN (IOYA 5/6) SANOY CLAY LOA~; MANY COARSE FAINT YELLOWISH BAOWN (IOYR 5/4) AND BROWN (7.5YR 4/4) MOTTLES; WEAK COARSE SUSANGULAR BLOCKY PAATING TO WEAK MEDIUM SUSANGULAR 8LOCKY STRUCTURE: VERY HARO; VERY FEW FtNE AND MEOIUM ROOTS; MANY BROWN (7.5YR 4/4) WORM CASTS IN MATRIX; FEW THIN PATCHY BROWN (7.5YR 4/4) CLAY FILMS: VERY FEW MEDIUM HARD CARBONATE NODULES; FEw (I~) CARBONATE FILAMENTS AND THREAOS: COMMON 1-2 ~ ROOT CHANNELS: 30 TO 40~ OF HORIZON REWORKEO BY BIOTIC ACTIVITY: STRONOLY EFFERVESCENT; GRAOUAL WAVY BOUNOARY.

250-270 REDDISH YELLOW (7.5YR 6/6) LOAM; CONYON M~IUM FAINT BROWNISH YELLOW (IOYR 6/8) ~TTLES: STRUCTURELESS MASSIVE: HARO; VERY FEW FINE AND MEDIUM ROOTS: COMMON BROWN (7.5YR 4/4) WORM CASTS IN ~ATRIX: FEw THIN pATCHY BROWN (7.5YR 4/4) CLAY FILMS; CO~N BIOCHANNEL ANO BIOPORES I MM OIAMETER: LARGE BIOCASTS wiTH WHITE FUNGAL GRowTH; FEW LINESTONE CLASTS 0.5 TO I CM OIAMETER: STRONGLY EFFERVESCENT.

2-37

Pedon 4 SOll CHARACTERIZATION lABORATORV

SOIL ANO CROP SCIENCES DEPT., THE TEXAS AGRICULTURAL EXPERIMENT STATION

SOIL SERIES: LEWISVILLE VARIANT PEDON NUMBER: S90rX-099-001 SOIL FAMILY: UDIC CHROMUSTERTS: FINE. MONTMQRtLlONtT[C. THERMIC LOCATION: CORVELL COUNTY

PARTICLE SIZE OISTRIBUTION o~ ... ) -----------------SANO---------------- -----SILT-----

VC C N , V, TOTAL FINE TOTAL LAB {:2 .0- f 1 .Q- {O.S- fO.2S- (0.10- (2.0- {O.Ol- (0.05-NO CEPTH HORIZON 1.0) 0.5) 0.25) 0.10) 0.05) 0.05) 0.002.) 0.002)

-----CLAV-----FINE TOTAL

, < ,< TeXTURE 0.00021 0.002) CLASS

!C"'! ----------------------------------~----------------------------------J995 D- e AI O. , 0 3 0.4 7.0 12.6 20.1 22.2 39.6 15.9 39.1 CL 3996 a- 14 ., 0.2 0.3 0.3 6.' 13.3 20.6 22.0 34.5 23.5 44. • C 3997 14- 3B BAK 0.1 0.' 0.' 6 .. 13.8 '21.6 18.6 31.1 28:7 46 .7 C 3998 3B- 66 BKSSI 0.4 0.4 O.B 6 .. IJ.6 22. f 20.1 33.0 27.2 44 .. C 3999 66- 'B BKSS2 0.6 0.7 1.3 a .2 13.8 24.6 20.4 32.2 26.5 43. 2 C 4000 98-120 BKl 1. 1 1.4 3.' 12. 3 14 .8 JJ.l 22.4 33.1 20.0 33. 2 CL 4001 110-150 BK2 0.6 2 1 7.0 17 .7 18.6 46.0 20.3 30.3 14.7 23.7 L '002 150-190 BK3 2.2 3.0 7.B 18.1 19.3 51.0 16.9 29.7 ILl 19.3 L 4003 190-213 BK4 1.. 2.' '.6 20.:3 n.9 52.6 IS.8 29.5 10. I 17.9 FSL 4004 213-250 BK' 1.B 2.' 7.1 16. , 18.5 46.0 20. :2 35.0 10. J 19.0 L 400' 250-270 BC 0.' 1.7 6.' 15.2 19.3 44.0 20.7 36.5 10. f 19.5 L

LAB ORGN PH -------NH4QAC EXTR BASES------ KCL EXTR NACAC BASE CAL- OOlO- CAeOl NO C (H2O) CA "G NA K TOTAL AL CEC ECEC SAT ESP SA' CITE MITE EO

COARSE FRAG-MENTS

Yo 0 0 0 0 0 B

• 6 2 , ,

GYP SUN

% 1:1 ----------------------NEOllOOG--------------------- ----y.--- -----------1.-----------3995 3996 :3997 3998 3999 4000 4001 4002 4003 '004 'oos

LAB Nt

3995 3996 3997 3998 3999 4000 '00' '002 '003 4004 400'

LAB NO

3995 3996 3997 3998 3999 4000 '001 '002 '003 '004 'oos

2.25 7.6 56.7 2.0 0.0 2.2 60.9 33.9 100 0 0 4.2 0.0 4.2 1. 13 7.6 58.5 1.7 0.0 1.B 62.0 34.8 100 0 0 3.0 0.0 3.0 '·.30 7.6 62.7 1.6 0.0 1.2 65.4 32.8 100 0 0 4.3 0.0 4.3 0.97 7.7 62.9 1.2 0.2 0.6 GS.a 29.8 100 1 11.4 0.0 11.4 0.12 7.B 65.8 1.3 0.1 o.s 67.8 21. , 100 0 20.0 0.0 20.0 0.35 7.8 54.8 0.' 0.2 0.' 56.4 18.8 100 , 38.6 0.0 38.6 0.26 7.' 54.9 (.0 O. , 0.3 56.3 11.5 100 1 48. I 0.0 48.1 0.42 7.' 52.6 0.9 0.0 0.3 53.8 ••• 100 0 51.1 0.0 51.1 0.23 B.O 52.0 1.0 0.1 O.J 53.4 B.a 100 1 52.8 0.0 52.8 0.00 B.O 51.7 0.' O.d 0.3 52.9 '.2 100 0 51.3 0.0 51.3 0.06 B.O 52.2 1.1 0.0 0.3 53.§ 9.0 '00 0 49.8 0.0 49.8

ELEC. H20 CONO CO NT

MMHOS/CN r.

SATURATED PASTE EXTRACT

CA MG NA 1< C03 HC03 CL S04

BULK OEN 0.33 AIR BAR DRY ~--G/CC--

COLE CM/CN

WATER CONTENT 0.10 0.33 15 BAR BAR BAR

----~-------------~--MEO/L-------------------- ------WT%------0.5 73 0.4 79 0.4 76

4.8 0 . .2 0.1 0,3 0.0 4,1 0.0 0.0 4.2 0.2 O. I 0.2 0.0 2.9 0.0 0.0 3.9 0.2 0.1 0, I 0-,0 3.0 0.0 0.0

1.12 1.56 1.25 1.68 1.30 1.71 1.33 1.75 1.40 1.78 1.43 1.56 1.47 f .61 1.47 1.62

0.117 0.104 0.096 0.096 0.083 0.029 0.031 0.033

1,41 1.59 0.041 1.55 1.70 0.031

PARTICLE SIZE DISTRIBUTION (CLAY-FREE BAStS) --------~---SANO--------~---- -----SlLT-----

YCS C J4 F YF TOTAL C F TOTAL ---------------------~%----------------------0.7 0.5 0.7 11.6 20.9 34.3 28.9 36.8 65.7 0.4 0.5 0.5 11.824.1 37.4 22.739.962.6 0.2 0.8 0.8 12.9 2S.9 40.5 24.6 34.9 59.5 0.7 0.7 1.512.524.740.1 23.436.559.9 1.1 1.2 2.314.424.343.3 20.835.956.7 1.6 2.1 5.2 18.422.249.6 17.0 33.5 50.5 0.8 2.8 9.223.224.460.3 13.126.639.7 2.7 3.7 9.723.223,963.2 IS.9 20.9 36,8 2.3 3.5 11.724.721.864.1 IS.4 20.5 35,9 2.2 3.1 8.8 19.922.856.8 18.324.943.2 1.1 2.1 8.618.924.0 54.7 19.6 2!L7 45.3

2-38

-----------RATIOS-----------S/SI FSI/cSI YfS/FS Fc/TC 0.5 1.3 1.8 0.4 0.6 1.8 2.0 0.5 0.7 1.4 2.0 0.6 0.7 1.6 2.0 0.6 0.8 1.7 1.7 0.6 1.0 2.0 1.2 0.6 1.5 2.0 1.1 0.6 1. 7 1.3 1.0 0.6 1.8 1.3 0.9 0.6 1.3 1.4 I. I 0.5 1.2 1.3 1.3 0.5

47.4 40.0 34.9 33.7 31.3 25.3 29.2 24.8

27.5 22,8

CEC/CLAY 0.85 0.78 0.70 0.66 0.63 0,57 0.48 0.51 0.49 0.48 0.46

CO

" % O.B O.B 0.' 0.' 0.' 0.7 0.4 0.3 O.J O.J O.J

CaIcoIaICd Carbonate-Free Partlcle Sim Distribution of Pedon 4

Clay-free basis Horizon Depth Sand Silt Clay Sand Silt

em %

Al 0-8 19.6 44.6 35.8 305 69.5

A2 8-14 18.4 39.1 42.5 32.0 68.0

BAk 14-38 19.8 36.3 43.9 35.3 64.7

Bkssl 38-66 20.2 36.2 43.6 35.8 64.2

Bkss2 66-98 21.3 32.2 46.5 39.8 60.2

Bkl 98-120 24.9 30.4 44.7 45.0 55.0

Bk2 120-150 30.5 27.7 41.8 52.4 47.6 N .:.,

Bk3 150-190 41.1 27.3 31.6 60.1 39.9 '"

Bk4 190-213 43.1 29.3 27.6 59.5 40.5

BkS 213-250 30.7 33.4 35.9 47.9 52.1

BC 250-270 32.5 31.7 35.8 50.6 49.4

Horizon Depth

em

Al 0-8

A2 8-14

BAle 14-38

Bless1 38-66

Bless2 66-98

'" Btl 98-120 1-0

Bk2 120-150

Bk3 150-190

Bk4 190-213

BkS 213-250

Be 250-270

Elemental Analysis of CaIbonate-free Sands and SillS of Pedon 4

Sand (2-.05 mm)

Ti K Fe Zr

%

0.083 0.71 0.37 0.029

0.077 0.62 029 0.029

0.083 0.68 0.39 0.035

0.059 0.62 0.30 0.021

0.058 0.60 0.43 0.017

0.104 : 0.69 0.77 0.023

0.109 0.75 1.09 0.012

0.101 0.73 1.04 0.013

0.094 0.67 0.97 0.009

0.149 0.86 1.11 0.028

0.120 0.76 0.92 0.021

Silt (.05-.002 mm)

Ti K Fe Zr

0.48 1.86 1.44 0.074

0.49 1.77 1.30 0.078

0.49 1.71 1.23 0.062

0.49 1.62 1.25 0.063

0.54 1.48 1.26 0.053

0.49 1.28 1.11 0.075

0.54 1.31 1.23 0.097

0.48 1.22 1.22 0.083

0.48 1.28 1.26 0.089

0.45 1.01 1.06 0.109

0.47 1.27 1.08 0.074

ReoollStmctIon ADaJ,ysfs of Pedon 4

Gain-Lnss of Constituents Gain-Lnss of Carbonat~ Coarse Fine

Horizon Sand Silt Clay Clay Total Sand Silt Clay Total

g/cm'

Al -9.2 -5.0 .().4 -2.0 -16.5 -6.8 -5.2 -1.1 -13.1

A2 -7.0 -3.9 .().1 0.3 -10.7 -5.3 -4.0 '().9 -10.2

BAk -26.5 -15.5 -1.3 3.7 -39.6 -20.5 -15.7 -2.9 -39.1

Bkss1 -26.8 -14.4 '().5 4.7 -36.9 -20.9 -14.9 -1.7 -37.5

Bkss2 -26.0 -12.9 0.2 6.0 -32.7 -20.3 -11.7 -1.5 -33.5

Bkl -5.6 -2.1 0.9 2.7 -4.1 -4.3 -1.7 '().1 -6.1

't Bk2 '().4 '().8 '().2 1.7 0.3 0.2 0.7 -1.7 '().8

- Bk3 7.2 '().6 '().2 1.6 7.9 4.0 0.7 0.0 4.7

Bk4 4.7 '().4 '().6 0.3 4.0 2.8 0.3 '().2 2.9

Bk5 0.7 0.4 -1.0 0.1 0.1 1.9 1.1 -2.4 0.6

BC '().1 0.0 .().1 '().2 '().5 0.1 0.6 -1.4 .().7

Sum (:t) -89.12 -55.08 -3.41 18.76 -128.85 -69.05 -49.70 -13.97 -132.72

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Rabenhorst, M. C. 1983. Genesis of soils and carbonate enriched horizons in a c1imo­sequence developed over Cretaceous limestone in Central and West Texas. Ph.D. Diss., Texas A&M Univ., College Station.

Rabenhorst, M. C., L. P. Wilding, and L. T. West.. 1984. Identification of pedogenic and lithogenic carbonates using stable carbon isotope and micro fabric analyses. Soil Sci. Soc. Am. 1. 48:125-132.

Rose, P. R. 1979. Edwards group, surface and subsurface, Central Texas. Bur. of Econ. Geo!. Rep. Invest. No. 74. Univ. of Texas, Austin.

2-45

Salomons, W. and W. G. Mock. 1976. Isotope geochemistry of carbonate dissolution and reprecipitation in soils. Soil Sci. 122: 15-24.

Sobecki, T. M. and L. P. Wilding. 1983. Formation of calcic and argillic horizons in selected soils of the Texas Coast Prairie. Soil Sci. Soc. Am. J. 47:701-715.

Soil Survey Staff. 1981. Soil survey manual. U.S.D.A.-Soil Conservation Service Agric. Handbook 18. U. S. Gov. Print. Office, Washington, D. C.

Soil Survey Staff. 1990. Keys to soil taxonomy. 4th Edition. Soil Manage. Supp. Servo Techn. Mongr. No.6. Cornell Univ., Ithaca, NY.

Stricklin, F. L., Jr., C. I. Smith, and F. E. Lozo. 1971. Stratigraphy of Lower Cretaceous Trinity deposits of Central Texas. Bur. Econ. Geo!. Rep. Invest. No. 71. University of Texas, Austin.

Tonkin, P. J. and L. R. Brasher. 1990. Soil-stratigraphic study of soil and landform evolution across the Southern Alps, New Zealand. In Knuepfer, P. L. K. and L. D. McFadden (eds.) Soils and landscape evolution. Geomorphology. Vo!' 3, Nos. 3/4. pp. 547-555.

U. S. Department of Agricultural (1981). Land resource regions and major land resource areas of the United States. Soil Conservation Service Agriculture Handbook 296. U. S. Government Printing Office, Washington, D. C.

Walker, 1. R. 1978. Geomorphic evolution of the Southern High Plains. Baylor Geol. Studies Bull. No. 35. Baylor Univ., Waco.

West, L. T. 1986. Genesis of soils and carbonate enriched horizons associated with soft limestones in Central Texas. Ph.D. Diss., Texas A&M Univ., College Station.

West, L. T., L. R. Drees, L. P. Wilding, M. C. Rabenhorst. 1988. Differentiation of pedogenic and lithogenic carbonate forms in Texas. Geoderma. 43:271-287.

Amino Acid Racemization Analysis of the Chronology and Integrity of Archeological Sites in Central Texas

Abstract

GlennA. Goodfriend G. Lain Ellis

James T. Abbott

Amino acid racemization analysis (D-alloiso-Ieucine /L-isoleucine, or . A/I, values) of shells of the land snails Rabdoills and OligJ)ra were used to carry out detailed chronostratigraphic studies of archeological sites at Fort Hood and San Angelo, in Central Texas. Because large numbers of racemization analyses can be performed at a relatively low cost, the method lends itself to the assessment of age uniformity within and among archeological strata. Racemization analyses of a series of individual radiocarbon-dated shells show that A/I values have good age-predictive ability, especially for middle to late Holocene samples.

A series of examples is discussed in which a range of patterns of distribution of A/I values is found. Some proveniences show uniform A/I values, indicating uniformity of age and high site integrity. However, other proveniences show nonuniform A/I values, which indicates that reworking, slow burial, or disturbance has compromised site integrity. Heating from fire produces anomalously high A/I values, and fire sometimes can be identified as a probable source of nonuniform A/I values.

[Adapted from poster presented at the Annual Meetings of the Geological Society of America, 7 November 1996,

New Orleans 1

3-1

...,

.:.,

Introduction

Site Integrity

Problem: Before we invest in archeological data recovery, we need to know how confident we can be about the integrity of the data we recover.

Inferences from artifacts to human behavior are only as reliable as judgments that artifacts in a given provenience are or are not behaviorally related. Such judgments themselves are reliable only insofar as:

(a) the assemblage was buried rapidly enough to prevent inclusion of behaviorally unrelated artifacts in the same provenience; and

(b) disturbance, redeposition, or other processes did not introduce beha viorally unrelated artifacts.

Integrity assessment: evaluating the extent to which artifacts from a given provenience can be known to have been buried contemporaneously.

Temporal Diagnostics: A conventional index for assessing integrity.

Diagnostics of different ages frequently are recovered from the same provenience. Typically, this is interpreted as assemblage mixture.

Perdiz A.D. 1200-1500

Castroville 800-400 B.C.

Pedernales 2000-1200 B.C.

Diagnostics in Hypothetical Column A

i" ~ ~ 6666 i· 7!;-~lf6tilS7S-Problem: Is the assemblage mixed, or were Perdiz users avid projectile point collectors? Co-occurrence of non-contemporaneous artifacts does not mean that artifacts were not deposited and buried contemporaneously.

'-' (.,

Introduction (cont'd)

Deposition Rate: Another conventional index for assessing integrity.

Deposition rate typically is used as an index of integrity. This assumes that deposition rate is a direct measure of integrity. However, if we run ...

Hypothetical Column S, One Date per level ,

-3 ~ I ~ - - -1 !~~~i~rn:i~- - - - - -c 4 ~ r .e _______ .!:!!.o~r~..!!~ __ re 5 ~ "~ .. ' Inl .. "y ~ _________ Falilst!!~lilitIO~

w 6 ~ ~ ~h.'~I", Y § ~w!!"$'P.2S~OIJ! ________ _ o lower mtGgnty ~7 ~

I

... more dates, another pattern might emerge. Rilpid deposition and high integrity, although usually related, are not the same thing.

Hypothetical Column S, Three Dates per level

31 ~ Mixed assemblaga - - -~ - - - - - - - - - - -~

41 ___ ;; ~ ~~~ as:e:b~ge __ _ 5 r 1"+1 Unmlxod assemblage ____ _ ':!'1 ________ _

61 Mixed assemblage 1"+11"+1 ~ ------------

7 Mixed a.ssembla go

, I - ,-- I

~ ~

~

o 1000 2000 3000 0 1000 2000 3000

Radiocarbon Years B.P. Radiocarbon Years B.P.

It takes at least two measurements of absolute or relative age per provenience to provide an index of the contemporaneity of burial of the artifacts in the assemblage. The more measurements, the greater one's confidence in the index. Obviously, integrity assessment can get expensive quickly, especially if one has several columns to evaluate.

Amino Acid Racemization: A new index for assessing integrity.

Amino acid racemization dating is an economical tool that provides a useful complement to other indices for assessing integrity.

This study applies racemization dating of land snail shells to integrity assessment of archeological sites in Central Texas.

Here, we:

------, /C:f1trw T6);&:> \

l. * \ • San Fort 1 t Angelo Hood /

"", ,J ... _---,

• review the principles of amino acid racemization dating;

• show that amino acid racemization can be calibrated to provide absolute and, hence, relative dates; and

• provide a series of cases studies with examples of high-integrity deposits, compromised integrity, influences from fire, and multicausal interpretation of integrity.

Amino Acid Racemization Dating

Principles of Amino Acid Racemization Dating

• Mollusk shells contain small amounts of protein (ca. 0.03% by weight in land snails) which are preserved through time.

• The proteins are constructed of amino acids, which are initially all in the L- configuration.

• Over time, the L- form converts (racemizes) to the D- form, up to an equilibrium ratio of D:L. In this study, the racemization of L-isoleucine to D-isoleucine is analyzed.

• The D/L value of the amino acids (here, the D-alloisoleucine/L-isoleucine, or A;1, value) can be used as a measure of relative age.

• The rate of racemization depends on temperature and varies among different taxa. Thus, estimation of absolute ages from A/l values requires a rate calibration. This is usually accomplished through radiocarbon analysis of selected samples.

Random and Systematic Error

• Analytical (measurement) error is random. Analytical error is a percentage of A;1 (typically ca. 5%). In this study, analytical error is ca. 4%.

• Temperature variation introduces systematic error.

• Calibrate to control for climatic temperature history. Supplement with other chronometric analyses under conditions of divergent slope, aspect, and depth.

• Fire can induce racemization. This effect sometimes can be identified during integrity assessment.

• Amino acids can be leached. Interpret with caution when samples come from contexts with evidence of long-term saturation.

3-4

Principles of Integrity Assessment

• Variation among A/l values of shells from a given provenience is an index of variation of the relative ages of the shells.

• Variation of the relative ages of the shells is an index of the potential for variation of the relative ages of artifacts in the same provenience.

• If A;1 values are tightly clustered within analytical equality, the pattern reflects contemporaneity of burial of shells. Artifacts in the same provenience also are likely to be contemporaneously buried.

• If A/I values are not tightly clustered, the A/I values and other evidence are examined to evaluate the likely cause(s) of A/I variability. In many cases, evaluation is ambiguolls, but specific reasons for doubting integrity can be identified.

• In this study, most A/I values are obtained from shells of Rabdotus mooreanus. For one site, shells of Oligyra orbiculata also are used.

Why Use Racemization Dating?

• Although use of bone for racemization dating has been discredited, mollusk shells have bee/! shown to yield reliable racemization dates when properly calibrated.

• Racemization analyses are performed with off­the-shelf technologies, and are relatively inexpensive.

• Because racemization analyses are relatively inexpensive, more dates can be budgeted at a given funding level.

• The ability to use more dates improves one's ability to assess integrity by providing a more robust data set.

w ("

Calibration

Calibration of A/Ion Radiocarbon Age

0.25 -f1-'--'-.L....l-'--'-.L....ll.....a....L-..L...Jl.....a..-L-............... -+

0.20

0.15 -I

~J 0.05

AJ1 = 0.008 + 2.28. < 10"5 (RYBP) R2 = 0.906 • High R2

Points >5000 BP scattered

Points in middle are above regression line: /. •• linear model not best fit • •

• • G.

• --Points <5000 BP show

tr:iJ8 strong linear trend

0.00 -fl-,--.--,-,-..,.-,rr-r-r-r-.-r-r-r-TO-r-+ o 2000 4000 6000 BOOO

Radiocarbon Years B.P.

Regression of A/Ion radiocarbon age shows strong relationship. However, linear model may not be best as result of climate change and/ or other systematic error"

Calibration of All for Last 5000 BP Cali bra tion of All on Calendric Age

0.15 -f'~~<-1.-'-~<-1.-'-~-'--'-"'-'-'--'-"'-'-+ 0.15 t' ~:'::->.-:-,-""",--,--,--,-:--,--,--,-""",-,-""",--,-+

0.10

:ii

0.05

AI~ = -0.004 + 2.96 x 10"5 (RYBP) A = 0.978

Very high R2

• Intercept = -0.004:

belowmeasured modem

Points well distributed around regression line

0.10

:ii

0.05

AI~ = 0.0122 + 2.67 x 10"5 (cal B.P.) R = 0.966 •

Very high R2

•

Calibration predicts calendric age for last 5000 years

Intercept = expected

0.00 Iii I I v~lu,e <p,Oj12l O I modern value

0.0 iii i' o 1000 2000 3000 4000 5000 a 1000 2000 3000 4000 5000

Radiocarbon Years B.P. Calendric Years B.P .

For last 5,000 years, linear relationship is very strong. However, intercept and radiocarbon measurements on modem shell indicate radiocarbon age anomaly for Rabdotus.

Calibrating radiocarbon ages with correction for 445 year average age anomaly predicts measured modem value in regression of A/I on calendric age.

Results:

• Calibration provides calendric dates for last 5000 years from All values.

• Strong age predictive value supports use of A/I values for relative dating.

• All values as relative dates provide an index of contemporaneity of burial and, hence, integrity.

'" 0,

High Integrity

Theoretical Signature of High Integrity

.c i5. Ql

Q

.~ ~

-- . -- . ... - -E----- . -

All values tightly clustered, many within analytical error

of youngest shell

Excavation error or intrusive shells: material from newer • deposits included

~ I .£ Rodent ~ midden- ----

• • Redeposited shells stand out

as isolated higer values among • clustered lower values

Increasing All :>

Clusters of analytically equal AlI values are very strong evidence of rapid burial and low levels of vertical mixing. Samples from rodent middens should be avoided since they reflect rodent behavior, not conditions of burial. Presence of a few high values among well clustered values is likely to reflect low levels of vertical displacement or redeposition of shells eroded from older contexts.

A/I Values from Test Pit 2, 41CV1200, Fort Hood

500 -90

'E-100 -8. .c i5. Ql

Q -110

-120

>-+-0

~

Calendric Years B.P. 1500 2500

She All values clustered within analytical error with

two older shells

Clustered racemization dates consistent with radiocarbon dates

/000 ........ • Three Two shells t<:=:>I shells

ca. 250 yea",

......... He, Charcoal ~ 14C,ShoU

0,. R.acomlUroIiDfl

Interpretation:

3500 -,

•

Clustered A/I values imply that the assemblage was rapidly buried with no detectable introduction of materials that post-date burial. Duration of burial is ca. 250 years or less for 10 em of deposits.

Presence of scattered older shells without scattered newer shells implies that redeposition-not disturbance-is source of older materials. If the sediment source contains small artifacts, they could have been transported to tlte site. Small artifacts deposited at the site may have been rearranged during burial.

Overall, integrity of the artifact assemblage is probably high for any problem requiring temporal resolution of 250 years or less per 10 em level.

u.> .:..,

High Integrity (cont'd)

A/I Values from Test Pit 6, 41BL532, Fort Hood

2000 -60

E -70 .3-.c: a. Q)

o -80

-90

Interpretation:

Calendric Years B.P. 3000

• Intrusive or excavation

error

Seven /VI values clustered within analytical error

o 0000 0 I

Two shells

1< >1 ca. 430 years

4000 -.

All values clustered at analytical equality imply that burial occurred over a period of ca. 430 years or less for a 10 em level. Youngest shell either is intrusive or was introduced by excavation error.

Absence of older shells implies that detectably older materials were not introduced by redeposition or disturbance.

Overall, integrity of the artifact assemblage is probably moderately high. For problems requiring relatively low temporal resolution « ca. 430 years per 10 em level), integrity can be regarded as high.

A/I Values from Test Pit 1, 41BL513, Fort Hood

1000 -50

E -60 .3-.c: a. Q)

o -70

-80

Interpretation:

Calendric Years B.P.

2000

Five- analytically equal shells

o O<lX> •••

1=1 Three older ca. 325 shells years

~: :>1 ca. 735 years

3000 -,

All values clustered at analytical equality imply burial over a period no longer than ca. 325 years. However, older shells may reflect relatively slow burial, systematic error, and/or redeposition.

If slow burial accounts for older dates, artifacts may be in situ, but temporal resolution (10 em over ca. 735 years) is fairly low. If older shells are redeposited, temporal resolution improves, but not reliability: artifacts occur upslope at this site. If systenzatic error accounts for older dates, limits of temporal resolution are unknown.

Integrity is ambiguous, but there are good reasons to doubt that artifact associations are reliable.

Whereas a pattern of highly clustered A/I values is very strong evidence of rapid burial and an unmixed assemblage, less clustered patterns are ambiguous. However, even ambiguous patterns can point toward specific reasons for doubting integrity.

...., , 00

Compromised Integrity

Theoretical Signature of Compromised Integrity

L::

C. Q)

o '" c: ·w '" ~ " .EO

t

••••• ----..... -Distribution

randomized by disturbance

-_ .. ••••••

••••• • ._ ... •••• •

•• •• • •

Redeposited, or buried together in situ?

• . _ ... ••••• •

Increasing All ~

Disturbance, redeposition, and slow burial have similar signatures since all involve mixing of shells of different ages. Inability to distinguish between these may not be important: all three processes lead to low confidence in assemblage integrity. Other kinds of evidence are necessary to reduce ambiguity.

A/I Values from Test Pit 2, 41BL754, Fort Hood

0.00 0.05 All 0.10 0.15 0.20

-20 r, --'--... --r--y-~r--,---"--,

E -30 ~ .c. 15. '" 0-40

-50

Youngest shells are effectively modem

........... "-

Total of four analytically equal shells

Very wide range of

All values

Interpretation:

•

. Poorly clustered A/I values imply assemblage mixture. This site is a rock shelter with internally derived sediments as the predominant fill. Redeposition of shells is unlikely.

Lowest A/I values are effectively modem (ca. 0.5% net racemization). Presence of very young shells 30-40 cm deep implies disturbance, but there was no evidence of vandalism or natural agents.

Sediments were powdery, and unit walls collapsed repeatedly during excavation. Excavation error may have mixed old and new materials. Integrity of the shelter is unknown, but the excavated assemblage is severely compromised regardless of the cause.

w , '"

Compromised Integrity (cont'd)

A/I Values from Test Pit 2, 41BL168, Fort Hood

All 0.00 0.05 0.10 0.15 0.20 Orl--~--'---~--r---~-'--~--,

E -10 ~ so 15. " o -20

-30

Wide range of values

f: H 00 •••

I Two analytically

equal shells

•

Interpretation:

Poorly c/ustl?red A/I values imply assemblage mixture.

This site is a rock shelter with externally derived sediments as the predominant fill, with no evidence of disturbance by vandals. Redeposition of sheils is likely to account for A/I variation.

A lithic production locale is on a mesa top 3-4 meters above the shelter opening, and artifacts are scattered on the slope from the mesa to the sheltl?r. The likelihood of redeposited shells is accompanied by a high likelihood of redeposited artifacts. Confidence in assemblage integrity can be regarded as low.

A/I Values from Test Pit 5, 4181.532, Fort Hood

All 0.00 0.05 0.10 0.15 0.20 0.25 0.30

40.

E -50. ~ so 15. " o -60

-70

Best case f=i

Heated or redeposited

shell? «IN' • •• •

V Two analytically

equal shells ~: >1

Worst case

Interpretation:

.. Poorly c/ustl?red A/I values imply assemblage mixture. Test pit is on a colluvial bench, and the spread of values is consistent with redeposition of shells-and artifacts-from the ancient upslope surface. If systematic error does not affect values, then best-case interpretation is compromised integrihj.

However, the highest value could be anomalously high as a result of fire. If it is, then all A/I values could be affeded, and the range of A/I values would under-estimate integrity.

Upshot: Integrity is .mknown_

w , 0

Effects of Fire

Theoretical Signature of Influence by Fire

s: li " 0

'" c: .,. nI ~

" .!:

t

----\ \

May have \ •

Range of values will be ·stretched- as a result of

differential distance from fire

• • • an apparent stratigraphic \- ... • •

• reversal if all shells were affected

, -------

Some values may be Mextreme- (l.e., may have

Implausibly old apparent age)

Increasing N1 ----7

Combination of apparent stratigraphic reversal, "stretched" values, and "extreme" values is very strong evidence for influence by fire. If other evidence (e.g., oxidized sediments, burned rock) is available, it reinforces this conclusion.

Extreme Values Reflecting Influence of Fire

As yet, there is no chemical or other test that can unambiguously identify effects of fire.

A robust calibration is the best currently available means for identifying A/I values that are far too high to be plausible candidates for redeposition. Scatter at the old end of the Fort Hood calibration prevents confidently using it to determine A/I values that pre-date human occupation.

At 4ITG307 in San Angelo, three shells with identical radiocarbon ages had widely divergent A/I values. Shells were recovered near a rock hearth and scattered burned rocks. Higher A/I values surely reflect influence of fire.

8900

0.. CD

~ nI

" >-§ 8700 -e nI

" .2 "C

'" II:

14Cdates [ • identical

All values widely divergent

• ......... / All surely increased

by fire

8500 LI ------~----~~----~~----~ 0.05 0.10 0.15

N1

0.25 0.20

'f ~

Effects of Fire (cant'd)

Examples: Shells Recovered from Levels Near Hearths

A/I Values from Test Pit 2, 4ITG307, San Angelo

= '"

• Shells can be as far as , this from fire in Feature:

J .. :. "4 .. , ... Scattered.

, -. bwned , Excavation rock. • Nine shells

Um~s \ _ recovered from

Backhoe Trench

Nt 0.00 0.05 0.10 0.15 0.20 0.25

-1001 110

11

and 'stretched" • • • • § -120 pattem probably ~ .. ~ --;; show effects of •

12,...

Ci. differential dIStance • ~ -130 from flr8

-140

-150

Interpretation:

Shells from same levels as hearth and scattered rock

• .. Feature 2 elevation

~ m

13 -

14

15

A/I values probably are affected by fire. Integrity cannot be confidently assessed with these values.

A/I Values from Test Pit 8, 4ITG307, San Angelo

Excavation limits

-" '"

Feature 5 Rock. Hearth

Nine Shells recovered from this

1 xl mtestpit

Shells can be as far as from fire in Feature 5

Backhoe Trench

Nt 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

-40 I 14

'~t Extreme values and

H ·stretched" pattern Feature 5 probably show eHecis of elevation r-

:[-60 differential distance 6~ from fire

10-20 em vertical m. m distance from Cl

·70 Feature 5 to shells 7

• • • •• • •• 0 L !.U •

Interpretation:

A/I values probably are affected by fire. Integrity cannot be confidently assessed with these values.

Prospecting for Fire-Oriented Features

A/I Values from Test Pit 6, 4ITG307, San Angelo

Nt 0.0 02 0.4 0.6 0.8 1.0 1.2

-110 I 111

Four Extreme values and ·stretched" pattem

:[,'20 r sIlt'S probably show effects of fire r[ '" o. ." • ~ -130 \

/ These values are 13

Two especially high shells

_140 L ...J -14

o Analytically Equal Values

No evidence of fire was visible in Test Pit 6. However, pattern of values and the especially high values of two shells imply that fire may have been nearby. The area near Test Pit 6 may be a good place to look for a hearth or similar feature.

Multicausal Interpretation

Profile of Burned Rock Midden at 41B1598 Bumed rtldI:

Racemization and Radiocarbon Data from Test Pit 1, 41B1598

c:::::=::J Excavation em 7 Level 0

a-<lj> 0.. (I)

3 0> b OC 0

\- -4 Modem

shells

oo>-d

o

A/I Values

Patterns lor Rabdotus 10 Rabdolusl and Oligym ara similar Q Oligyra I

---------

13 10 5 maiMS

o 9--000 0- f All for e and f ara widely divergent

Very hIgh NI '0

• Test Pit 1 excavated adjacent to the central hearth.

6

• Levels 3 and S in burned rock midden matrix.

7 ~o o-j

• h , ~o 0

Patterns for Rabdotus and O/igyra are similar

• Level 7 in underlying midden.

Problem: How extensive was assemblage mixture during formation of the burned rock midden?

0.0

'" ,; > ~ ~

"'

~

.~

b~ ,~

~

, 0.1

, 0.2 0.3

Radiocarbon Dates

Pattern of radiocarbon dates for a-d is similar to

pattern of All values d~

0.4

,; > ~ ~

Radiocarbon dates for e and f e lOt

are Identical, unlike NI values I 0

~ g~

"ii Pattern of radiocarbon ~ I >--0-< Ralxiotus I dates for g.j Is similar to

. .--. ChM:oaI. pattern of All values

o 2000 4000 6000 Ca!endric Years B.P.

Interpretation:

• Level 7 has a mixed assemblage judging from A/I values and radiocarbon dates. Mixture apparently results from slow burial and/or reworking. Absence of newer shells shows that deposits were sealed ca. 6000 cal BP.

• Level 7 deposits are reliable for problems requiring "Early Holocene" assemblages, but unreliable for problems requiring finer temporal resolution.

• Rabdotus in levelS have signature of slowly buried assemblage. But, the extreme (!) value of Oligyra implies that the AlI values under-estimate integrity due to heat from the nearby hearth. A/I -radiocarbon pairs "e" and ':f' support this. Absence of lower A/I values implies that dated charcoal filtered down. Integrity is probably moderately high.

• Level 3 has signature of a mixed assemblage. Radiocarbon dates and absence of extreme A/I values imply that the pattern was not induced by fire. Although most radiocarbon dates are ca. 1000-2000 cal BP, A/I values show that younger and older materials are mixed in. Integrity of level 3 is low.

3-12

l~ ho

J~

6000

0.5

10000

Conclusions

• Amino acid racemiZiltion analysis has allowed us to identify sites where we can have high confidence in the integrity of archeological deposits.

• Because it is relatively inexpensive, amino acid racemiZiltion analysis of mollusk shells can be an important addition to the tools we already use. In conjunction with other lines of evidence, it is possible to provide robust interpretations of the likely conditions of burial and preservation of archeological assemblages.

• Although some cases of ambiguitycould not be resolved, other assessments allowed us to detennine that there were good reasons to doubt integrity. The ability to know whether we can be confident about the integrity of archeological data is an important aspect of the general problem of determining whether our data are sllitable for the analyses we want to perfonn.

References

Ellis, G. L., G. A. Goodfriend, J. T. Abbott, P. E. Hare, and D. W. Von Endt (in press). Assessment of integrity and geochronology of archeological sites using amino acid racemization in land snail shells: Examples from central Texas. Geoarchaeology.

Goodfriend, G. A. (1987a). Radiocarbon age anomalies in shell carbonate of land snails from semi-arid areas. Radiocarbon 29:159-167.

_____ (1987b). Chronostratigraphic studies of sediments in the Negev Desert, using amino acid epimerization analysis of land snail shells. Quaternary Research 28:374-392.

__ -,-__ (1987c). Evaluation of amino-acid racemization/ epimerization dating using radiocarbon-dated fossil land snails. Geology 15:698-700.

_____ (1989). Complementary use of amino acid epimerization and radiocarbon analysis for dating of mixed-age fossil assemblages. Radiocarbon 31:1041-1047.

_-:--::-__ (1991). Patterns of racemization and epimerization of amino acids in land snail shells over the course of the Holocene. Geochimica et Cosmochimica Acta 55:293-302.

____ (1992). The use of land snail shells in paleoenvironmental reconstruction. Quaternary Science Reviews 11:665-685.

Goodfriend, G. A., and V. R. Meyer (1991). A comparative study of amino acid racemization/epimerization kinetics in fossil and modem mollusk shells. Geoc"imica et Cosmochimica Acta 55:3355-3367.

Goodfriend, G. A., and J. J. Stipp (1983). Limestone and the problem of radiocarbon dating of land-snail shell carbonate. Geology 11:575-577.

Johnson, B. J., and G. H. Miller (in press). Archaeological applications of amino acid racemization dating: A review. Archaeometry.

3-13

ARCHAEOLOGY AT FORT HOOD by David L. Carlson

Introduction

Cultural resources investigations began at Fon Hood in 1978 (Skinner, et. al. 1981). From 1978 to 1991 surface surveys of 95 percent of the surveyable area of the post are complete. Unsurveyable areas include the anillery impact areas, the permanently dudded area, and the cantonment. These areas are either heavily disturbed or too dangerous to survey. The total number of sites recorded by post archaeologists and contractors is 2,150. Since 1992, systematic shovel testing and geomorphic investigations have been conducted at 571 prehistoric sites and National Register eligibility testing has been conducted at approximately 100 sites.

Prehistory

The prehistory of Fon Hood may be broadly divided into six periods spanning the last 12,500 years:

Paleoindian (10,550-6,550 B.C.). The Paleo indian period includes the earliest inhabitants of the Texas. During the beginning of this period, central Texas was cooler and moister than todays climate. Extinct animals such as manunoth, mastodon, horse, and camel occupied the area. The earliest people in the area made a distinctive projectile point called the Clovis point. The sites occupied by these people often contain extinct fauna. Later Paleo indian people where hunters and gatherers who adapted to changing environmental conditions as the last Ice Age ended about 10,000 B.C. This period is poorly studied because sites of this age are very rare. Prewitt (1983: 210) lists only three sites in central Texas with radiocarbon dates falling into this period.

Early Archaic (6,550-3,050 B.C.). The Early Archaic represents as shift in projectile point styles from the lanceolate styles of the Paleoindian to a variety of barbed and notched styles. Population is assumed to have been low, but many sites have undoubtedly been destroyed over the years by geomorphic processes. This period spans the Altithermal, a climatic period during which the environment was apparently warmer and drier than today. The specific details of environmental change during the Altithermal are unknown. This period is only slightly better understood than the Paleo indian. Prewitt (1983: 209-210) lists only six sites in central Texas with radiocarbon dates falling into this period.

Middle Archaic (3,050-650 B.C.). While the initial appearance of burned rock features begins in the Early Archaic, they are abundant and extensive during the Middle Archaic. These large accumulations of burned rock called burned rock mounds or middens appear throughout central Texas. The burned rock was apparently used repeated over a considerable period of time. Whether the burned rock mounds represent the cooking of root foods, the processing of acorns, or some other purpose is still unknown. Some archaeologists believe that the Middle Archaic represents a peak in the population of the hunter-gatherers who occupied central Texas (Weir 1976). Plant gathering may have become more imponant during this period. At Fon Hood there are more Middle Archaic than Early Archaic sites, but there are considerable more Terminal Archaic sites. Bison which had been absent during the Altithermal move back into central and even southern Texas. Eight sites in central Texas are reponed by Prewitt to have radiocarbon ages within this period (1983: 206-208).

Late Archaic (650 B.C.-A.D. 200). Burned rock accumulations are fewer in number. Bison reinvade Texas from the southern Plains and were hunted although their imponance may not have been that great in the Fon Hood area. The Late Archaic is slightly better studied than the previous periods, but only nine sites in central Texas have radiocarbon ages within this period (Prewitt 1983: 207-208).

Terminal or Transitional Archaic (A.D. 200-700). This period has the highest site density at Fon Hood. Rockshelters often contain evidence of Terminal Archaic occupations, which may indicate a change in settlement pattern. Bison are absent from central Texas during this period and foodgathering probably shifted

4-1

toward dispersed animals such as deer and heavy dependence on plant foods. Fourteen sites in central Texas have radiocarbon dates falling in this range (Prewitt 1983: 206-207).

Late Prehistoric (A.D. 700-1780). The Late Prehistoric (called the Neoarchaic) is associated with the introduction of the bow and arrow into the area. There may be linle or no change in hunting and gathering practices. At Fort Hood and other parts of central Texas this period contains substantially fewer sites which may reflect a population shift out of the area. Alternatively it could reflect a shift in settlement location. Small rockshelters often have Late Prehistoric occupations, but these sites rarely have artifacts on the surface which would allow their assignment to a period. Much of the Late Prehistoric occupation may be hidden in small rockshelters and in the now flooded floodplain of the Leon River. The Late Prehistoric is divided into the Austin and Toyah phases. The Toyah phase may represent a migration of Bison hunters from the southern Plains of Oklahoma and the Texas panhandle into the area. The introduction of ceramic into the area indicates interaction and trade with the Caddo to the east. Twenty sites with radiocarbon ages falling into this period are known from central Texas (Prewitt 1983: 203-206).

History

Table 1 identifies the major events in the history of Bell and Coryell counties). The following summary is based on the Fort Hood Historic Preservation Plan:

Pioneer Era (1830-1850). European senlement of the Fort Hood area did not begin until Rohert Leftwich was awarded an impresario contract from the Mexican government in 1825. When Leftwich died in 1830, the administration of the colony passed to Sterling Robertson. In 1834 six families were granted title to land in what is now Bell County. Robertson lost control of the colony in 1835 because he was unable to establish 100 families in the area. Control of the colony then passed io Austin and Williams. In 1849 the U.S. Army built Fort Gates on the Leon River to protect settlers. Bell County was established in 1850.

RanchIng Era (1850-1880). Population grew steadily in the area except for the Interruption of the Civil War. The early settlers in the area were largely self-sufficient. Based on the population density of Bell and Coryell Counties we can estimate that 936 people lived within the boundaries of what is now Fort Hood in 1860. That number nearly doubled to 1715 in 1870 and doubled again to 3819 in 1880 (Mueller-Wille and Carlson 1990b: 40). Domestic structures were often constructed of logs. At least seventeen small communties were established withIn the confines of Fort Hood during this time. Very rew historic sites at Fort Hood can be conclusively assigned to this period. Only three sites have components dating to 1860, 24 to 1870, and 27 to 1880 (Mueller-Wille and Carlson 1990a: 32).

Railroad Era (1880-1915). In 1880, the Gulf, Colorado, and Santa Fe railroad built a line through Bell county. In 1882, the Missouri, Kansas, and Texas railway crossed tracks with the Santa Fe at Temple which then became the major town in the area. Many communities foundered when the railroad passed them by. The railroad connected Bell and Coryell counties to markets allowing them to obtain lumber, barbed wire, and other goods more economically. Production for cash rather than home consumption grew as some ralsed sheep for wool and others raised cattle. Rural population growth slowed and peaked as the estimated population of Fort Hood grew from 3819 in 1880 to 6985 in 1900 and then declined to 6868 in 1910 (Mueller-Wille and Carlson 1990b: 40).

Automoblle Era (1915-1942). After a brief boom in conan prices, the area entered a post World War I agricultural depression. The automobile facilitated travel and accelerated the shift from rural to urban areas. During this time the estimated Fort Hood population dropped from 6868 in 1910 to 5409 in 1940. The towns and communities of Fort Hood declined to the point that they were simply rural neighborhoods which depended on towns like Gatesville, Copperas Cove, and Killeen for most services.

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Date

1687

1698

1801

1825

1830

1835

1836

1841

1849

1850

1852

1853

1854

1859

1866

1870s

1880

1882

1890s

1893

1904

1907

1911

1913

1914

1920

1923

1930

1935

1936

1942

1951

Table I. Summary of Bell County and Coryell County History (from Anonymous [1893], Newcomb [1961]. Scott [1965]. and Tyler [1936]).

Summary

Henri Joulel recorded Tonkawa and Mayeye Indians in Central Texas.

Missions were established in northeast Mexico for the Ervipiame.

Phillip Nolan went on hunting expedition in Brazos Falls region.

Robert Leftwich granted empresario contract by Mexico.

Leftwich's contract passed to Sterling Robertson; HamIet of Tenoxtitlan became first settlement in Robertson's Colony.

Nashville-on-the-Brazos founded; James Coryell given a headright grant in the Nashville Colony in present-day Coryell County.

Bell County residents fled eastward in "Runaway Scrape"; Milam County created out of the Milam Land District; Coryell County was later created out of Milam County.

Governor Sam Houston pacified Indian problems for settlers in Bell County.

Fort Gates established as last garrison along the frontier line from Fort Duncan, near Eagle's Pass, to Coffee's Station on Red River.

Bell County officially organized; "Nolandsville n (renamed "Belton n in 1852) designated as county seat.

Fort Gates was abandoned.

Fort Gates was temporarily used as a quartermaster depot.

Coryell County created; GatesvilJe later designated county seat.

Belton (pop. 300) the only town of significance.in Bell County; Governor Houston gives direct aid to settlers to repulse Indians~ First cattle drive out of Coryell County to Shreveport, Louisiana.

Cattle business developed in Texas and trails to northern mark~ts passed through Bell County.

Wends settle The Grove.

Gulf, Colorado and Sanra Fe railroad passed through Bell County.

Missouri, Kansas, and Texas railway passed through Temple; Missouri Pacific CKaty") branch passed through Belton; Texas and St. Louis Railway Company completed tracks to Gatesville; Gulf, Colorado, and Santa Fe Railway Company reach southwestern Coryell County from Galveston.

Wends settle Copperas Cove; Cotton and wheat prices declined as the availability of manufactured goods increased.

Panic began and lasted until 1899.

Boll weevil reached Bell County and destroyed crops.

Stephenville North and South Texas Railway Company laid tracks from Stephenville to Hamilton.

Stephenville North and South Texas Railway Company extended lines to both Comanche and Gatesville.

Bond issue passed in Bell County for construction of better roads.

Farm prices dropped with onset of World War I followed by a war-inflated boom.

Period of deflation in Ben County.

Federal aid for highway construction granted to Coryell County.

Community Natural Gas Company provided service for 500 customers.

Community Public Service provided electricity for 783 customers.

Rural Electrical Association available in Bartlett region of BeII County.

Camp Hood activated as a tank destroyer training center.

Camp Hood renamed Fort Hood.

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Military History

In 1942 the Army purchased most of the land which is now Fort Hood to establish a tank destroyer training center called Camp Hood. In 1943 the initial construction was completed with 5,630 buildings and 35 fIring ranges on 160,127 acres. Between 1942 and 1945 the peak wartime troop strength of Camp Hood reached 130,000. At the conclusion of World War II troop strength dropped to 14,000. In 1950 the Army designated Camp Hood as a permanent installation and changed the name to Fort Hood. In 1952 additional land was purchased around Lake Belton and added to the Government reservation bringing it up to 216,915 acres.

Research Topics

While a great many research topics have been investigated using the data from Fort Hood, they can generally be summarized under three broad areas: broad patterns in site distribution through time, settlement patterns, lithic procurement and technology, and burned rock midden/mounds. During the FOP fIeld trip we will have the opportunity to visit sites representing all of these areas of research.

Table 2 summarize the the distribution of prehistoric site components as of 1990. Two small surveys since then have not altered the overall picture. The sites on the fort can be divided into those within 10 km (a days foraging radius) from the Leon River, those within 10 km from the Lampasas River, and those more than 10 km from either river. Only small portion of Fort Hood lies within 10 km of the Lampasas River (West Fort Hood located below Highway 290). The low site density in this area relative to the other two areas is probably at least partly a reflection of this fact. Site density is highest near the Leon River. For most periods site density in the area more than 10 km away from the Leon River is half to two thirds the density for sites within 10 km of the Leon. The other striking feature of the site distribution is the dramatic decrease in site component density after the Terminal Archaic. This pattern has been noted by Prewitt (1983) for other areas of Central Texas. He suggested the the striking change in site density could be explained in terms of a population decrease in the area. Subsequent research suggests that other factors, particularly a change in mobility strategies, may account for much or all of the change. The Terminal Archaic is partly created by the short timespan estimated for the use of several dart point forms such as Ensor. If these point types were used longer, the Terminal Archaic peak would not be as large. Secondly, the pattern seems to hold only for upland surveys. Most of the reservoir surveys were conducted many years ago and did not include a systematic examination of the entire surface and are not published in the detail that modem surveys are. Late Prehistoric sites do seem to be common in the floodplains, however, and most sites tested in the floodplain contained Late Prehistoric components. Third, two site types appear to have been occupied during the Late Prehistoric, but this use is rarely detected during surface surveys. Burned rock mounds regularly produce radiocarbon age estimates in the Late Prehistoric range, but often do not contain Late Prehistoric arrow points which are the only way to identify sites of this period from the surface. Rockshelters also regularly produce evidence of Late Prehistoric occupation, but arrow points are rarely found on the surface. This means that upland surveys will systematically underestimate the number of sites occupied during the Late Prehistoric.

Table 3 shows the distribution of historic components at Fort Hood. Figure 2 shows that site densities increased rapidly between 1880 and 1910 where it peaks and that site density began to drop after 1910. Interestingly, the relative densities for sites near the Leon, near the Lampasas, and away from either river is similar to the prehistoric pattern. Comparison with historic population fIgures indicates that the site density underestimates the nineteenth century population. That is, there should be more sites occupied between 1860 and 1890 than we have found. While rural population in Bell and Coryell Counties did drop off after 1920, the site density drop is much more rapid.

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Period

Early

Late

Early

Early/Middle

Middle

Middle/Late

Late

Terminal

Unknown Archaic

Austin

Toyah

Table 2. Prehistoric Chronological Components (Source: Mueller-Wille and Carlson, 1990a)

Age

PALEOINDIAN

12,500 - 10,000 BP

10,000 - 8,500 BP

ARCHAIC

8,500 - 5,000 BP

5,000 - 2,600 BP

2,600 - 1,750 BP

1,750 - 1,250 BP

LATE PREHISTORIC

1,250 - 650 BP

650 - 200 BP

Unknown Late Prehistoric

Components

32

37

147

4

237

7

141

227

94

48

23

59

120 r-------------------------------------------------~ E ~

CJ (f) 100 o o ~

'-Q} 80 a.. E .2 ffi 60

~ '-Q} 40 a.. (f) -C Q}

c 20 o a. E

>10krn

< 10 krn Leon

••••••• < 10 km Lampasas

Terminal Ardlaic /1 , " , ,

I

, , late Archaic,' , , , ,

, , , , I

I I

, I I --. Middle Archa~ , , ,

..... . .••. Tnvah -- ...... -,

Paleoindian Pal~~~n .. __ .. .. .. .. . ., .•... , ... o o t~E;"~'Y~~~~~~'e~~~~~"Y~M~~~~-::-~-:-=-::::=::.~ .. :.~ .. ~ .. ~.~ .. ~.~ .. ~ .. ~.~ .. ~_A~V~S"~.n_.~.~J ------ ..... o ........... .

12,000 10,000 8,000 6,000

Years B. P. 4,000 2,000 o

Figure 1. Chronological and Geographic Distribution of Prehistoric Components (after Mueller-Wille and Carlson 1990a).

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Table 3. Historic Chronological Components (Source: Mueller-Wille and Carlson. 1990b)

Decade Estimated Population Components

1860 930 3

1870 1703 25

1880 3793 28

1890 5395 161

1900 6939 139

1910 6822 385

1920 6048 325

1930 6056 275

1940 5373 143

80 r--------------------------------------------------,

• 60 E

::.:: &

(f)

o 040 ~

~

Q)

Il.. ffJ Q) .t: (f) 20

-->10km

• - - - < 10 km leon

••••••• < 10 km lampasas

, , , ,

I, I ,

I ' , ·1 , I , , ......

I '\ I \

\ \

I • • ••••• I " ........................ .

'.

\ \ \

\ \

\

,----'.... . ..... . oL-__ ~~--~·~ .. ·~·~ __ ~ ____ L-__ ~ __ ~ 1840 1860 1880 1900 1920 1940 1960

Years A.D.

Figure 2. Chronological and Geographic Distribution of Historic Sites (after Mueller-Wille and Carlson 1990a).

4·6

Settlement pattern studies have focused on mobility strategies of hunter-gatherers. Archaeologists distinquish between residential mobility where the whole band moves from site to site and exploits the resources in the immediate vicinity of the site from tactical mobility where the band stays in one place and smaller task groups collect foodstuffs and bring them back to the base camp. The first involves moving the mouths to the food, while the second involves moving the food to the mouths. Tactical mobility allows greater settlement stability. This in turn permits a group to have more possessions and to have possessions which are difficult to move. The greater sedentism permitted by tactical mobility is often considered a prerequisite for the domestication of plants, the construction of substantial housing, and the use of pottery. For these reasons, archaeologists are particularly interested in documenting changes from residential to tactical mobility. The issue is complicated by the fact that there are no straightforward diagnostic criteria of these mobility strategies and by the fact that real hunter­gatherers can combine both strategies. One immediate consequence of mobility strategies is that the same number of people can generate very different numbers of sites. A pure tactical mobility strategy would generate a large base camp with relatively high visibility in the archaeological record and a number of specialized camps which might be very difficult to fmd. A pure residential strategy will generate numerous sites throughout the year that are all comparable in their visibility in the archaeological record. One way to interpret the dramatic decrease in site density during the Late Prehistoric is to propose that mobility shifted from residential camps scattered in the upland and floodplain environments (Archaic pattern) to base camps in the floodplains and much smaller (and harder to detect) specialized camps in the uplands. During 1990 and 1991 The Texas A&M field school investigated five rockshelters to search for data that might help to confirm or disprove this proposition. All of the rockshelters contained evidence of Late Prehistoric occupation (although we did not know this initially). Four of the rockshelters are quite small and could have housed only a single family or perhaps two. We thought that this might suggest that the rockshelters were occupied by specialized task groups, but analysis of the lithic tools does not support this expectation. In fact the range of tool types is quite diverse suggesting that the rockshelters were used by one or two families. Furthermore, one of the small rockshelters contained a deciduous tooth from a child of about 7 or 8. This evidence suggests that Late Prehistoric bands split into small groups (micro-bands) in narrow protected canyons during some time of the year and iIIen aggregated into larger groups in the floodplain during a different season. Further investigations at Fort Hood and elsewhere in central Texas might test this model by careful comparison of floodplain and upland sites.

Fort Hood is an ideal place to study the way in which stone was selected, manufactured into chipped and ground stone tools, worn out or broken, and discarded. The fort contains numerous lithic procurement areas where chert is readily available on the surface. The abundance of this critical raw material probably helps to explain why site density and size is so high on the post. Counting these large lithic exposures, which contain abundant evidence of prehistoric tool manufacturing, 7.2 % of the post has some evidence of prehistoric activity. [n addition, the chert is quite variable and there are some varieties that are found in fairly restricted parts of the post. Research by Dickens (1995) and by Frederick and Ringstaff (in Trierweiler, 1994) has defmed 17 different chert types for the fort. The spatial distribution of these types provides some indication of how far raw materials moved across the fort and to what degree chert selection was affected by quality and availability. Examination of chert diversity through time and by site type may help to clarify changes in mobility patterns over time, but these comparisons have not yet been made and the samples from dated contexts are only just beginning to be large enough to conduct such studies.

Burned rock mounds/middens are the enigmatic features of central Texas. Despite excavations of a large number of these features, we still do not really know what they represent. The classic burned rock mound is a circular or oval concentration of burned rock from 0.5 to about 2 m high. The mound often contains animal bone, chert flakes and tools, charcoal, and carbonized plant remains. Some have a central depression. Burned rock concentrations containing artifacts and ecofacts, but laCking distinct boundaries and are often buried in alluvial or colluvial contexts are also found. [t is not clear if they represent three, or more, different uses of burned rock for processing foodstuffs or a single use in different depositional environments. We do not know what they were used for although numerous suggestions can be found in the literature from roasting pits for meat, to leaching and cooking acorns, to cooking root foods. The features were apparently used repeatedly and may well have been used for several purposes. Understanding what they were used for would help to clarify mobility strategies since large-scale bulk processing of a foodstuff for storage should be part of a tactical mobility strategy, but not a residential one. Research at Fort Hood to date has focused on chronological issues regarding these features. Prewitt (1981) suggests that burned rock middens and mounds were constructed between about 5,000 and 2,250

4-7

years ago (the Middle Archaic). This inference is based on the fact that these features often contain dan points that have been dated to this interval from other sites, especially stratified midden sites. Recently archaeologists have begno to obtain multiple radiocarbon age determinations on these features with the result that many of them contain evidence of Late Prehistoric use. This discrepancy between the age estimate based on the anifacts and the age estimate based on the charcoal has not been explained. That the features represent multiple episodes of use and reuse is indicated by the fact that stratigraphic reversals in radiocarbon age estimates are common. One possibility is that the features are Late Prehistoric and the Archaic-age points were incorporated by Late Prehistoric people either accidenrally or deliberately (Steve Black, personal communication). Another possibility is that muliple re-use of the fearute oxidizes and breaks up the older charcoal so that the possibility of recovering a piece of charcoal from the earlier use of the feature is very small.

While this brief summary does not exhaust the research potential of sites at Fort Hood, it should provide some idea of how culrutal resources investigations at the fort are contributing to our understanding of prehistoric lifeways.

Sites

The following are brief descriptions of the results of investigations undertaken at six sites on the fort. All or only some of these sites will be visited depending on weather conditions and the availability of time.

Site 41CV960 consists of two burned rock concentrations or middens that were tested by TRC Mariah and Associates in 1993 (Abbott and Trierweiler 1995: 483-498). The site straddles the confluence of Cowhouse Creek with an unnamed tributary and includes portions of a Holocene terrace of Cowhouse Creek and the flooplain of the unnamed tributary (Figure 3). The burned rock middens are located on either side of the unnamed tributary. Test excavations consisted of six Ixl m test pits and six backhoe trenches. Artifacts were largely confmed to the upper meter of the deposit. Despite the overall similarity in the appearance of the burned rock fearutes on either side of the creek, radiocarbon age determinations indicate the the eastern midden is about 3200 years old (3200 ± 60 BP, Beta b-7(038) and the western midden is about 1700 years old (1730 ± 60 BP, Beta b-70039 and 1690 ± 60, Beta b-7(037). Projectile points recovered from this midden include Late and Terminal Archaic types (Ellis, Edgewood, Godley, Costroville).

4-8

, ,

! I N

I o " .",,,, "

CQ<'IIOU.-ed 01 1 l,Ielet Inle''tQAs. (fe..al;o", !:lOUd 0t'I ossumed datum.

Test Pit = Backhoe Trench

Site Boundary Subarea Boundary 2- track Rood

IMC,,), .... • ,~"

Mopped Ronaid Corroll Texos. 9-29-9J

Figure 3. Site plan for 41CV96O (Trierweiler 1994).

, ' , ' , ' , ' , ' , "-..... , , ... al-~

, ' , , A', " , ,

, ' , , , ' , , , ,

" '

Site 4IBL670 is a rockshelter that was tested by the 1990 TAMU Archaeological Field School (Carlson 1993a). It is located along Bull Branch and lies only 1.5 m above the floodplain. The shelter is 8 m wide by 10 m deep (Figure 4). Because it is triangular in plan section and because the ceiling is very low, the usable space is very small. The shelter was tested with a 1x2 m test pit excavated to 90 cm. Flake density was modest varying from 2 to 68 flakes per 10 cm level. Diagnostic artifacts recovered from the unit included one Seal/om (20-30 cm), one Bonham (30-40 cm), one Perdiz (40-50 cm), and two unidentifiable arrow points (30-40 cm). Radiocarbon age estimates were obtained on charcoal at 20-30 cm (1080 ± 80 BP) and 60-70 cm (2020 ± 110 BP). Fauna recovered from the shelter include deer, rabbit, opossum, and bear.

4-9

•

,/ /

...

".

DETAIL OF ROCKSHEL TER

41 BL 670 -==-'=- ..

Figure 4. Site plan for rockshelter at 41BL670 (Carlson 1993a).

Site 4lBL671 is a rockshelter that was tested by the 1990 TAMU Archaeological Field School (Carlson 1993a). The site has been heavily vandalized with obvious potholes covering about 25 % of the site (Figure 5). The surface of the shelter is 3.5 m above an unnamed tributary of Bull Branch. The shelter is about 40 m long and reaches a maximum depth of 4 m in the middle. Five different test pits were excavated into the site to determine what periods of occupaton were represented and how badly disturbed the site was. Very little of the site appears to be intact except for discontinuous patches near the bedrock. Human skeletal remains were found in several units and a complete burial was excavated in units 2 and 5. The burial appears to be an adult female between 22 and 29 years of age and about 5'2" tall. Portions of at least 5 other individuals were also recovered. Some 15,132 flakes were recovered in the excavations. Dart points include Darl, Ensor, Marcos, and Marshall. Arrow points include Perdiz and Scallom. The points indicate Late Arcliaic through Late Prehistoric use of the shelter, but the there is no stratigraphic order to the materials. For this reason, no radiocarbon age estimates were obtained for the shelter. Use of the shelter by non-human animals is indicated by numerous snake and rodent bones as well as probably prey species such as deer and rabbit.

4-lO

\ '\ 'V

\ --~---~---- ---

Figure 5. Site plan for rockshelter at 41BL671 (Carlson 1993a).

Site 4IBL233 includes two burned rock mounds that were tested by TRC Mariah Associates in 1993 (Trierweiler 1994: 207-213). The site contains at least five recognizable burned rock mounds. Two of these were tested. Feature 1 is a 6 by 8 m mound consisting of burned limestone rock with associated flakes and charcoal. A single lxl m test pit was excavated 50 cm into the mound adjacent to a backhoe trench. One Uvalde point (Early Archaic) was recovered from level 3 and seven radiocarbon age estimates were obtained (Figure 6). Four charcoal ages are statistically homogeneous and provide a pooled average age of 763 ± 43 BP. Feature 5 is a 7 by 7.5 m mound consisting of burned limestone rock with associated flakes and charcoal. A single Ixl m test pit was excavated 70 cm into the mound adjacent to a backhoe trench. Seven radiocarbon age estimates were obtained (Figure 7). Five charcoal age estimates form three distinct clusters: 2863 ± 70 BP. 1742 ± 62 BP. and 630 ± 70 BP. No diagnostic artifact types were recovered. but the dates suggest Late Archaic through Late Prehistoric use. Both mounds contained modern. unburned seeds at depths of 40-5Ocm indicating movement of small materials through the deposit is a continuous process.

4-11

_"

Backhoe Trench Profile

• 5 J

Feature 1 Plan Map

BHT Backhoe Trench

TP Test Pit (1x1m)

~ Treeline

2

t H

I o 2 meters

o

midden mlrtru:; eJay Ioem; YtK'y stony; ~ subengo.IIar ~ 7Bdin9 into massive; hatd ay: st1clty....-eot atu'IdanI: buned, lX'Ibune<Ilimestone clasts; aI'Ju"Idant fine f1JQtS ill ~ 20 ern; common WOOCy nxu (Me) ~ Yf¥.'( daOI. tJrown (10YR 211 o;nd-ng doWn into appro:lImately 7.5YR 211)

lmestone stabs w/day Joem in intentices (dismegn6ng beQ-odo;) stabs _ 15 ern ttlic:Io;. up to 0_5 m long. ~ dat1I; btown clay loam iI1 intentices

(m~sive, hatd dry; stic:Jo;.y wet. YtIry daf'k btown (7,5 YR 211 apprnximately)

cmbs West Wall Tn

O.-__ ~~~ ____ ~ 00 0 '"

0' <c. q '6 C> DD

20-

bo<>od<

17Ot508 860 t70C 870 t 70C

670 t 70 121" t "'" H 650 t 70C 15-4 t 1,2% S

S·SHd B -Bone

C'C~I H , Hunate

o limestone rock

level from which date was obtained

Zone containing mkfden mabix

Figure 6. Plan and Profile of Feature I at 41 BL233 (AbbatE and Trierweiler 1995),

4-12

"-1&.410

ilGC8SS rn.ad

--

meters

Zooo ,

Zone "

"- -~ Feature 5 Plan Map I "- ~.....-.

"-"- N \ Backhoe Trench

I BHT

J ST / TI' Test p< (1xlm)

/ ST Shovel Test I

/ ~ Treeline a 2 meteo /

Backhoe Trench Profile cmbs

middeomalrix: storry clay loam; ~sl.baogutl!ll bIodI.y Sb'\..JctU'9 9fiI'd.no:;J lI'ItomaS$Ne; clast-supported buT'.ed l,mestone rnodoenrIe 9fll.SS roots inwater 20 em; law 'oIrOOCy I1XlU ~; fitI.e to mednxn limu!OI'le clasts· _ailly mbnc.aled - except on c.omlll', wr«e $Iao$ up to -35 em diame1&r at8

sli'tlnQty ~bric3ted. slopinQ 10 S:SW:SE at IS.· 25>, Yf!!ry daf1c. ~sh tItOw!'Il0YR 312}

stony clay loam; ma.uive: hIt'd ay: sUdl.y wet aI:Iu"Idant medUn to l2Ige limestone coObIes: slMls (Il'10$1 ~): vf!!ry da1I. 9'2)'ish brown {tOVR 312).ry: v«y daf1t tItOw!'I (10YR 212) _t: common fll"\e.oot1y roou: some lan;>e honzontal slabs at hmestone' sl.lCstrate: h¥d Ifmestone

cmbs East Walt TP1

60

t850 t: S(I C

21340 t: 70 C 630t:70C 117 t: 0,9'10 S

2880 t 70 C - 1759 t 49 H

Zone containing midden matItx"

Limestone rock

Level from which date was obtained

// h Unexcavated

H -Hl$Tli!le C -Chan;oal

S-Seed

Figure 7. Plan and Profile of Feature 5 at 41BL233 (Abbott and Trierweiler 1995).

4-lJ

Site 41BL495 is a rockshelter that was tested by the 1991 TAMU Archaeological Field School (Carlson 1993b). The shelter faces southeast and is about 24 m long (Figure 8). At its deepest it is 6 m, but this is • deceptive since the deepest ponion is a narrow hole in the limestone that one would have to crawl into. Seven different test units were placed into the shelter and excavated to depths of 40 to 60 cm before reaching bedrock. Debitage counts ranged from 1 to 361 flakes with the highest density occuring in the top 20 cm. The shelter was occupied during the Late Prehistoric period as indicated by 8 Sea/lorn and 7 Perdiz points recovered from the excavations. Fauna recovered from the shelter included a variety of freshwater mussels, deer, rabbits, raccoon and other taxa.

- ..

Figure 8. Site plan for rockshelter at 41BL495 (Carlson 1993b).

4·14

\ \ \ '. '-.~.

Site 41BL497 is a rockshelter that was tested by the 1991 TAMU Archaeological Field School (Carlson 1993b). The shelter is located high on the valley wall of Spicewood Creek near the top of the escarpment (Figure 9). It is very small and could have provided shelter for only three or four people. Four test units were excavated to obtain information on the age and integrity of the cultural deposits. The occupation seems to center around Excavations Units 1 and 2 and is buried by 10-30 cm of relatively sterile fill. Seven Seal/om points were recovered from these two units. Debitage counts ranged from I to 235 flakes per level with peak densities occuring at 20-50 cm in Unit I and 30-60 cm in Unit 2. Faunal remains included freshwater mussels, deer, opossum and rabbit. Radiocarbon age estimates from Units I and 2 are statistically homogeneous and give a pooled age estimate of 1224 ± 74 BP. A single human fibula shaft fragment was also recovered from the shelter which may indicate burials that were not encountered by the test excavations.

I 1 ,_ "0" 10. 10 hOO_ 0', c •• (

r

41BL497 ,.

Figure 9. Site plan for rockshelter at 41BlA97 (Carlson 1993b).

4-15

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4-16

1988 Archaeological Survey at Fon Hood, Teras, Fiscal Year 1985, The Nonhwestern Training Area. Archaeological Resource Management Series, Research Report Number 15.

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1990a Archeological Survey at Fon Hood, Texas, Fiscal Year 1986, The Shoal Creek Watershed. United States Army, Fort Hood Archaeological Resource Management Series, Research Report Number 20.

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Appendix Pictoral Glossary of Fon Hood Projectile Point Types Mentioned in the Text

Uvalde (Early Archaic, Abbott 1994)

Castroville (Late Archaic, Carlson, Dockall, Olive 1994)

Marcos (Late Archaic, Carlson, Dockall, Olive 1994)

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Marslwll (Late Archaic, Carlson 1993a)

Ellis (Late-Terminal Archaic, Turner and Hester 1993)

Godley (Late-Terminal Archaic, Abbott and Trierweiler 1995)

Darl (TerminallTransitional Archaic, Abhott and Trierweiler 1995)

Edgewood (Terminal/Transitional Archaic, Abbott and Trierweiler 1995)

Ensor (Terminal/Transitional Archaic, Abbott and Trierweiler 1995)

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Bonham (Late Prehistoric, Carlson 1993a)

Perdiz (Late Prehistoric, Carlson 1993a)

Seal/om (Late Prehistoric, Carlson 1993a)