Geologists’ Association Guide No. 69

THEROADSIDE GEOLOGY

OF WALES

Jim Talbot and John Cosgrove

Guide Series Editor: Susan B. Marriott

© THE GEOLOGISTS’ ASSOCIATION 2011

  i

CONTENTS PREFACE ACKNOWLEDGEMENTS LISTOF FIGURES

1.INTRODUCTION Wandering Wales                      Geological History of Wales                   The Ice Ages                           2.GWYNEDD,NORTHWESTWALESThe geological history of northwest WalesAnglesey(YnysMôn)   The A5/A55: Menai Bridge to Holyhead   The A4080: southwest coast road between Menai Bridge and Rhosneigr   Holy Island   The A5025: the north and east coast from Holy Island to Beaumaris   Southeast Anglesey TheLleynPeninsula   The A497, A499 and B4413: Porthmadog to Aberdaron    The B4417 and A499: Aberdaron to Caernarfon         SnowdoniaThe A55, A487 and A470: Conwy to Porthmadog   The A5: Betws-y-Coed to Menai Bridge   The A4086: Capel Curig to Caernarfon   The A4085 and A498: Caernarfon to PorthmadogTheHarlechDome   The A470: Dolgellau to Ffestiniog   The A496: Dolgellau to Ffestiniog (coast road)TheCadairIdrisRange   The A487: Machynlleth to Dolgellau   The A493: Dolgellau to Machynlleth (coast road)3.CLWYD,NORTHEASTWALES   The geological history of northeast Wales   The A5: Shrewsbury to Betws-y-Coed   The A55: Chester to Conwy   The A542 and A525: Llangollen to Rhyl and the Vale of Clwyd  TheClwydianRange   The A494: Mold to Ruthin   The A541: Mold to DenbighTheDenbighshireMoors

Page

ivvvi

134711111416182124283133373940455055575860656667696972747680808182

ii

The A548: Abergele to Llanrwst   The A543: Denbigh to Pentrefoelas4.CENTRALWALESThe geological history of Central Wales   The Palaeozoic Welsh Basin  TheCoastRoad   The A487: New Quay to Machynlleth  TheInlandRoads   The A44: Kington to Aberystwyth   Mountain road (B4574 in part): Devil’s Bridge to Rhayader   The A483: Builth Wells to Oswestry   The A470: Brecon to Dolgellau   The A470 and A4212: Ffestiniog to Bala   The B4391: Bala to Llanfyllin  TheWelshBorderland   The A458: Shrewsbury to Welshpool   The A490, A489 and A49: Welshpool to Church Stretton5.GLAMORGANANDGWENT,SOUTHEASTWALES   The geological history of southeast Wales TheForestofDeanandtheUskValley   The A4136: Mitcheldean to Monmouth   The A466: the Wye Valley, Monmouth to Chepstow   The A40 and A449: Monmouth to Newport  TheSouthWalesCoalfieldThe A470: Cardiff to Brecon   The A4059 and A470: Hirwaun to Brecon   The A465: Heads of the Valleys Road, Abergavenny to Swansea  TheWesternCoalfields   The A4067 and A40: Swansea to Brecon   The A4069: Llandovery to Brynamman   The A483: Swansea to Llandeilo   The A40: Ross-on-Wye to Carmarthen   The M48 and M4: Chepstow to Carmarthen  TheGlamorganCoastbetweenCardiffandSwansea   The A48: Cardiff to Bridgend  TheGowerThe north Gower coast6.SOUTHWESTWALESThe geological history of southwest WalesThePembrokeshireCoastSection   The A487: New Quay to St David’s   The south coast: St David’s to Carmarthen

838485858789899797101103105109111112113114117117121122123124125126130132135135139140141145148154155160161161165165174

  iii

  TheMarloesPeninsula    ThePembrokePeninsulaThe A478 and A4139: Pembroke to Begelly   The coast road: Begelly to Pendine   The back roads between Pendine and Carmarthen  SouthernPembrokeshireandCarmarthenshire   The A40: Carmarthen to Fishguard   The A477: St Clears to Pembroke and Haverfordwest   The A478: Tenby to CardiganREFERENCES AND FURTHER READINGCOMMON WELSH ROCKSINDEX OF PLACES

183189194197199203203204205206208211

iv

Roadside Geology of Wales

PREFACE

Wales is one of the classic areas of geology. It was here that over two centuries ago geologists recognized and named the three earliest subdivisions of the Palae-ozoic Era (from 570 to 410 millions of years ago). The first, or Cambrian Period, is named after the Cambrian Mountains; the second and third, the Ordovician and Silurian Periods, are named after ancient Celtic tribes of the region.

Many examples of geological activity are illustrated along the roads and cliffs of Wales, including basin sedimentation, volcanic activity, the effects of plate tec-tonic movements in opening basins and folding and metamorphosing the rocks in the two major continental collisions recorded in the mountains.

This book is written for persons with little geology background. As the title of this guide indicates, it focuses on the geology of Wales that can be seen from a car. The  descriptions  are  organized  following  the main  roads,  though  in most cases the best outcrops are in the sea cliffs and on the beaches. We have not de-scribed rocks on private property. Non-geologists are referred to descriptions of common rocks on pp. 208–210.

In the interest of preserving the outcrops it is suggested that hammers are not used and that specimens of either rocks or fossils are not removed.

For those using 1:250 000 road maps with 10 km grids, the third digit of each grid reference can be ignored. For example, SH 389 763, the grid reference for Gwal-chmai, gives the position to 100 metres, but it is difficult to estimate locations to that level using a road map, so 38 76 should be adequate. 

Jim Talbot and John Cosgrove

  v

Roadside Geology of Wales

ACKNOWLEDGEMENTS

This book derives much of its impetus from the 200 years of studies of Wales, one of the finest areas illustrating the Palaeozoic era, and we are indebted to all the studies that provide the background for this book.

Two early workers had defined the Cambrian and Silurian periods, Adam Sedg-wick and Roderick Murchison, though the boundary became a source of conflict between  them  until  John  Lapworth  resolved  the  issue  by  proposing,  in  1879, the Ordovician period between the Cambrian and Silurian. These authors set the stage  for  later workers who have provided  further details  and  later placed  the geology in terms of plate tectonics.

We have relied on this great body of work to describe the rocks that the curious traveller will see and to select those areas that best illustrate the geology of the region. The selection and descriptions of the rocks, however, are ours and we are solely responsible for any errors in interpretation.

Early versions of this book were made available on the web and we are grateful for those who took the time to correct errors we made.

We are particularly grateful to Trevor Greensmith and Susan Marriott, editors of the Geologists’ Association who have provided an enormous amount of feedback and support.

The maps, unless otherwise acknowledged, are based upon ‘Rocks of Wales: Ge-ological Map of Wales 1:250 000, 1994’, reproduced by permission of the British Geological Survey IPR/134-25CY. © NERC. All rights reserved. This map is bi-lingual, English and Welsh. The maps based on the Geological Map of Wales are:

Northwest Wales: Figures 5, 21, 29, 33, 38, 45, 47, 56Northeast Wales: Figures 60, 62, 63, 65, 68, and 69;Central Wales: Figures 71, 72, 85, 86, 92, 94, 97, 98, 100, 101, 102, and 104;Southeast Wales: Figures 107, 108, 112, 113, 124, 131, 135, 136, 137,  and 144.Southwest Wales: Figures 149, 150, 161, 162, 166, 169, and 170.

Jim Talbot and John Cosgrove

vi

Roadside Geology of Wales

LISTOFFIGURES

1. Simplified geological map of Wales.2. The changing position of Wales.3. The Welsh Basin in Ordovician to Silurian times.4. The Ice Ages of Wales.5. The geology of northwest Wales.6. Geological map of Anglesey.7. Section along the A5 across Anglesey.8. Geological map of southwest Anglesey.9. Sketch of Roche Moutonée.10. Llanddwyn Island. Pillows in Precambrian lava flow. 11. Geological map of Holy Island.12. Section across Holy Island showing its two sets of folds.13. Holy Island. Sketch of folds on South Stack. 14. Trearddur Bay. Early folds in volcanic ash. 15. Geological map of northern Anglesey.16. Geological map of Cemaes Bay.17. Parys mine map.18. Geological map of southeast Anglesey.19. Red Wharf Bay. Large sandstone pipes in Carboniferous Limestone. 20. Red Wharf Bay. A section of a large pipe in the limestone in cliff. 21. Geological map of the Lleyn peninsula.22. Criccieth Castle on an Ordovician dolerite sill.23. Columnar jointing in Ordovician lavas. Beach below Criccieth castle.24. The town of Criccieth above a cliff of glacial till.25. The geology of the headland at Uwchmynydd.26. Uwchmynydd. Large blocks of quartzite in mixture of schists and volcanics. 27. Morfa Nefyn lighthouse. Pillows in Precambrian basalt (Gwna group). 28. Trefor. Granite quarry in Ordovician granite on Yr Eifl. 29. Geological map of Snowdonia.30. Sketch of the hills west of Conwy. 31. Blaenau Ffestiniog. Slate cribs. 32. Llanrwst Bridge.33. Geological map of Nant Ffrancon pass.34. Detailed cross section from Capel Curig through Nant Ffrancon pass.35. Nant Ffrancon pass. The Pitt’s Head Tuff near Roman Bridge. 36. Nant Ffrancon valley from the Youth Hostel at west end of Llyn Ogwen.37. Fence of slate from the Penrhyn quarries.38. Geological map of Llanberis Pass.39. Cross section along the Llanberis pass.40. View west along Llanberis pass.41. Llanberis Pass. Volcanic bombs in Ordovician volcanic ash. 

  vii

Roadside Geology of Wales

42. Dinorwic slate quarries.43. Igneous dyke in slate: Vivian slate quarry Llanberis.44. Llanberis. Precambrian Padarn volcanic ash outcrops.45. Geological map of Pass of Rhyd-Ddu.46. Mynydd Mawr.47. Geology of the Harlech Dome.48. The Rhinog Mountains.49. Barmouth. Convolutions in a turbidite sandstone of the Maentwrog Beds. 50. Barmouth. Cambrian slate with vertical cleavage below Porkington Terrace.51. Barmouth. Coarse sands of the Rhinog Grits on path above St John’s Church. 52. Barmouth. Thick bedded sandstone of the Rhinog Grits. 53. Harlech Castle on an old sea cliff eroded into the Rhinog Grits.54. Harlech Castle. Turbidite sandstone in slate of the Cambrian Rhinog Grits. 55. Morfa Harlech. A fault separates Cambrian rocks from a low plain.56. Geological map of Cadair Idris Range.57. Tal-y-Llyn valley.58. Cadair Idris from the Barmouth estuary.59. Machynlleth. Silurian slates in road cutting.60. Geological map of northeast Wales.61. Geological cross section of northeast Wales.62. Geological map of area from Llangollen to Betws-y-Coed.63. Geological map of area from Chester to Conwy.64. Llandudno. Cliffs of Carboniferous Limestone. 65. Geological map of area from Llangollen to Rhyl and the Vale of Clwyd.66. View northeast from a quarry in Silurian slate near Horseshoe Pass.67. View from Craig Fawr. 68. The geology of the central Clwydian Range.69. Geology of the Denbighshire Moors.70. Denbigh. Banded Silurian siltstone. 71. Geological map of Central Wales.72. Geology of the area of the A487 between New Quay and Machynlleth.73. New Quay. Turbidite sandstone at the west end of the harbour. 74. New Quay. Turbidite sandstone.75. Aberaeron. Turbidite  sandstones. South end of beach.76. Aberarth. Sea cliffs in glacial till. 77. Aberarth. Ripped up and contorted turbidite layer. 78. Aberystwyth. The cliff at the north end of the esplanade. 79. Aberystwyth. Alternating sandstone and mudstone layers. 80. Aberystwyth. Tracks of small animals on deep water Silurian sandstone. 81. Aberystwyth. Scour marks on the underside of a sandstone bed. 82. Colour-banded Silurian slates at Borth.83. Borth. Close up of the banded Silurian slate. 84. Machynlleth. Colour-banded Silurian slate. 

viii

Roadside Geology of Wales

85. Geology of the area along the A44 between Kington and Llangurig.86. Geology of the area along the A44 between Rhayader and Aberystwyth.87. Ponterwyd. The Castell mine. Fractured rock invaded by mineral veins. 88. The gorge at Devil’s Bridge.89. Parson’s Bridge. The River Rheidol gorge with large potholes. 90. Devil’s Bridge. High moor plateau about 600 metres above sea level. 91. Plynlimon, the highest mountain (752 metres) in central Wales.92. Geology along the A483.93. Newtown on the A483. Pencil slate in Silurian silt and mudstone. 94. Geology of the A470 between Brecon and Newtown.95. The River Wye south of Builth Wells.96. Builth Wells. The bridge over the River Wye. 97. Geology between Newtown and Dolgellau.98. Geology between Ffestiniog and Bala.99. The Arenig Hills northwest of Llyn Celyn.100. Geology of the B4391 from Bala to the intersection with the A495.101. Geology of the Welsh borderland.102. Geology of the Shrewsbury/Welshpool area.103. Middletown. Thick Ordovician volcanic ash beds. 104. Geology of the Welshpool/Church Stretton area.105. The Stiperstones.106. The stone stripes below the Stiperstone Ridge.107. The geology of southeast Wales.108. Geological map of the lower Usk valley and the Forest of Dean.109. Staunton, Forest of Dean. Old Red Sandstone conglomerate.  110. Symonds Yat. The River Wye.111. Wynd Cliff, N of Chepstow. Carboniferous Limestone with vertical joints.112. Geological map of the Valleys with the eastern part of Coalfield syncline.113. Geological map of the area between Cardiff and Brecon.114. Geological cross section from Cardiff to Brecon.115. Head of Rhondda valley, south of Hirwaun.116. One of the peaks in Fforest Fawr, west of the A470.117. Quarry on A40 north of the Brecon pass in Devonian red sandstone.118. Ystradfellte. Millstone Grit, lying above ribs of Carboniferous Limestone. 119. Porth yr Ogof: caves near Ystradfellte.120. Clydach Gorge, A470 E of Brynmawr. Coal Measures shale and sandstone.121. Black Rock Quarry. East of Brynmawr. Thick Coal Measures sandstone.122. Pontneddfechan. Dinas Rock, Craig y Ddinas.123. Section across Dinas Rock.124. Geological map of the western part of the Coalfield.125. Abercraf. Pennwyllt quarries.126. Pennwyllt quarries. Carboniferous Limestone with sea lily fossils, crinoids. 127. Pennwyllt quarries. Millstone Grit.

  ix

Roadside Geology of Wales

128. Henrhyd waterfall near Coelbren.129. A4069 south of Llangadog. Road cutting in Devonian Old Red Sandstone.130. Viewpoint on Black Mountain. Uppermost layer of the Old Red Sandstone. 131. Geology between Ross-on-Wye and Brecon.132. Ross-on-Wye. Road cutting in Devonian Old Red Sandstone.133. Abergavenny. Sugar Loaf, capped by Old Red Sandstone.134. Abergavenny. Blorenge.135. Geology between Brecon and Carmarthen.136. Geology of the M4.137. Geology of the Glamorgan coast between Cardiff and Swansea.138. Lavernock Point. Jurassic limestone cliffs. 139. Lavernock Point. Oyster fossils in Jurassic limestone. 140. Barry Island. Triassic scree on Carboniferous Limestone erosion surface.  141. Llantwit Major. Wave-cut platform and cliff cut into Jurassic limestone.  142. Southerndown. Jurassic limestone lying on Carboniferous Limestone. 143. Southerndown. Folds in Jurassic limestone.144. Geological map of the Gower.145. Three Cliffs Bay.146. Three Cliffs Bay. Weathered Carboniferous Limestone.147. Rhossili Beach.148. Worms Head, southwest corner of the Gower.149. The geology of southwest Wales.150. Geological map of the north coast of the St David’s Peninsula.151. Llangrannog. Ordovician sand and mud contorted by submarine slides. 152. Aberporth. Ordovician mudstone and thin layers of sandstone. 153. An esker at viewpoint on the B4582 west of Cardigan. 154. Dinas Head. Glacial meltwater channel. 155. Sketch map of the meltwater channels near Fishguard.156. Scleddau. Meltwater channel.157. Strumble Head, west of Fishguard. Massive Ordovician lava flows. 158. Ordovician lavas. View east from near the Strumble Head lighthouse.159. Geology of Abereiddy Bay.160. Abereiddy Bay. Ordovician volcanic ash beds alternating with softer slate. 161. Geology of St David’s to Carmarthen.162. Geological map of St David’s Peninsula.163. Whitesands Bay. Cambrian sandstone with ripple marks. 164. St David’s Head. Massive intrusive gabbros. 165. Caerfai Bay. Cambrian sandstones in cliffs. 166. Geological map and section across St Brides Bay.167. Broad Haven. Faults and folds. 168. Broad Haven. Fold in Coal Measures.  169. Geological map of Pembroke and the Marloes Peninsula.170. Geology of the Marloes Peninsula.       

x

Roadside Geology of Wales

171. St Brides Haven. Horizontal Upper Carboniferous rocks on Silurian rocks.  172. Silurian sandstones under 60-metre terrace that continues to Skomer Is.173. Martin’s Haven. Silurian sandstone dips south toward Gateholm Island. 174. Marloes Sands. Old Red Sandstone conglomerate near Gateholm Island. 175. Marloes Sands. Quartz veins in Old Red Sandstone. 176. St Ann’s Head. Devonian mudstone in cliffs.  177. Geological map of region between Manorbier and St Govan’s Head.178. Stackpole Quay. Carboniferous Limestone thrust over Old Red Sandstone. 179. Geological map of west part of Pembroke peninsula.180. Geological map of Freshwater West.181. West Angle Bay. Carboniferous Limestone lies on muddy limestone. 182. Manorbier. Vertical sandstone and mudstone of the Old Red Sandstone. 183. Geological map and cross section between Pendine and Penally.184. Saundersfoot. Anticlinal fold at Ladies’ Cave. 185. Amroth West. Coal Measures rocks. 186. Amroth East. Coal Measures sequence in the cliffs. 187. Geological map of area between Carmarthen, Llansteffan  and Pendine.188. Pendine. Carboniferous Limestone in the cliffs.189. Pendine.  Rubble breccia in collapsed cave in Carboniferous Limestone. 190. Llansteffan. Red and green Old Red Sandstone rocks overlying calcrete. 191. Geological map of the area from Carmarthen to Fishguard.

  1

1. Introduction

1.INTRODUCTIONSummaryoftheGeologicalHistoryofWales

                       Palaeozoic 

  

       M

esozoic 

         Cenozoic

 PERIOD    AGE in Ma     LOCAL EVENTS        LATITUDE of                           WALES Quaternary

 Tertiary

 Cretaceous

 Jurassic

 Triassic

 Permian

 Carboniferous

 Devonian

 Silurian

 Ordovician

 Cambrian

 Precambrian

   - 0   

   - 100

   - 200

   - 300

   - 400

   - 500

   - 600

Glaciers cover Wales

Uplift produces present dayplateaux

Wales a plain with low hills

Limestones collect in warm seasalong the Bristol Channel

Desert sand and mud to east and north - the New Red Sandstone

Hercynianorogenyas RheicOceanclosestosouth

Deltas with forests, later form coal.Warm seas form thick grey limestone

Desert sand and mud - the Old Red Sandstone

Caledonianorogenyas IapetusOceanclosestonorth

Welsh basin collects mud and turbidite sandsVolcanoes in Snowdonia and Southwest Wales

Welsh Basin forms; shallow shelf along Welsh Borderland. Sands and siltst col-lect on edge of continent

Volcanic island arcs in Wales

52oN

30oN

At Equator

20oS

2

1. Introduction

The geology of the British Isles is a 3,000 million-year story of moving and col-liding continents, stretched and broken crust, and rivers and glaciers shaping the landscape.   The rocks of North Wales (Fig. 1) are famous among geologists. It was here  that  almost  two centuries ago geologists  recognized and named  the  three subdivisions  of  the Early Palaeozoic Era,  from 570  to  410 million  years  ago. They are: the Cambrian Period, after the Cambrian mountains that form the back-bone  of Wales;  and  the Ordovician  and  Silurian  Periods,  named  after  ancient Celtic tribes of the region.

Figure1.Simplified geological map of Wales.

  3

1. Introduction

WanderingWales

Continents wander, carried as passive passengers on moving plates. Wales spent much of its past south of the equator, at one time attached to the southern conti-nents, at other times attached to North America. Wales has drifted steadily north during the past 600 million years (Fig. 2).

Figure2.The changing position of Wales. Wales spent much of its past south of the equator, sepa-rated from Scotland and southern Europe until the oceans between them closed.

4

1. Introduction

GeologicalHistoryofWales

  The geological history of the British Isles begins in Precambrian time, nearly 3,000 million years ago. Rocks almost this old are at the surface in north-western Scotland and elsewhere they occur as the basement beneath the younger rocks. The oldest rocks in Wales are nearly 1,000 million years old. They poke through younger rocks in a few places in Anglesey, St David’s Peninsula and the Welsh Borderland (Fig. 1). These Precambrian rocks are basalt of an old ocean floor and sediments that were deposited near the edge of a small continent that lay about 60 degrees south of the equator. These Precambrian rocks can best be seen in the spectacular cliffs of Anglesey.  About 600 million years ago a broad ocean, Iapetus, separated Scotland from Wales. Scotland was part of North America on the northwestern shore of Iapetus. Wales lay on the southeastern side of the ocean, on part of a small conti-nent called Avalonia that included much of England, Wales, southern Ireland and Belgium. Avalonia was close to the supercontinent Gondwana, which contained all the southern continents of today.   Near the end of Precambrian time, Avalonia began to drift slowly away from Gondwana  and  the  Iapetus  Ocean  began  to  close.  The  ocean  floor  slab plunged  beneath  the  northwestern  edge  of  the Avalonian  continent  through  a trench near the present-day Solway Firth. Volcanoes erupted above the sinking slab, dumping a huge  thickness of ash onto  the barren  landscape  -  the Padarn Tuffs of North Wales. The sinking slab pulled Avalonia towards it, as a sinking ship drags survivors towards it, and the crust of Avalonia stretched to form the Welsh Basin that collected sediments from the late Precambrian to near the end of the Silurian period.   Avalonia’s drift speeded up in Ordovician times (Fig. 2), narrowing the Iapetus Ocean, but opening another, the Rheic Ocean, to the south. The Iapetus Ocean floor continued to sink into the oceanic trench and volcanoes still erupted violently above the sinking slab, accumulating thick piles of volcanic rocks that now form the Lake District of England and the spectacular mountains of Snow-donia.  Continued stretching of the crust caused coalescence of the isolated ba-sins  of Cambrian  time  to  create  a  single Welsh Basin  in  central  and  northern Wales (Fig. 3). Sand and silt accumulated near the edges of the basin, but most of it collected mud black with organic matter. In some parts of the basin, 4,000 me-tres of mud and silt accumulated in the 70 million-year span of Ordovician time. That is about a half-millimetre per year. The violent volcanic eruptions probably occurred sporadically, but the volcanic rocks dominate the modern landscape of Snowdonia because these rocks resist erosion more than the surrounding slates.  About 450 million years ago, in early Silurian time, the North American plate began to impinge on the Avalonian plate. The ocean floor stopped sinking, and  the volcanoes died. However,  sediments continued  to pour  into  the Welsh 

  5

1. Introduction

Basin. In contrast to the fine-grained mud and silt of Ordovician age, the Silurian rocks are mostly sandstone. Fine examples of these sandstones are exposed in the sea cliffs between Newquay and Aberystwyth. Faults along the eastern margin of the basin separated deep from shallow water and limestone formed in the shallow water east of  the fault  -  the  limestones of Wenlock Edge near Church Stretton (Fig. 3). By the end of the Silurian Period the Welsh Basin was full of sediment.   The  Iapetus  Ocean  had  closed  by  the  end  of  the  Silurian,  about  420 million years  ago. That was when Scotland  and  southern Britain,  once on  the opposite sides of the Iapetus Ocean, finally collided. The join between the two continents lies in southern Scotland. The slow, grinding collision raised the Cal-edonian Mountains  in Scotland,  folded  the  sedimentary  and volcanic  rocks of the Welsh Basin  and  thickened  the  crust  to produce  the Caledonian highlands covering most of Scotland and Wales. As a result, the shoreline retreated south to northern Devon, with the sea to the south.  Once  the Caledonian collision was over,  sediment washed down  from the highlands in Late Silurian and Devonian times to collect in valleys and the 

Figure3.The Welsh Basin in Ordovician to Silurian times.

6

1. Introduction

lowland of what is now South Wales. Most of this sediment, which is commonly red, collected as sand and gravel in valleys, and as mud in low alluvial plains. Ge-ologists call this collection of rocks the Old Red Sandstone. The Brecon Beacons, north of Cardiff, expose fine examples of this vast cover of sand and mud.   By the Carboniferous Period, Wales was near the equator and the remains of the Caledonian hills had eroded to a low plain. Warm, shallow seas invaded the edges of the landmass and thick layers of limestone accumulated as sea shells settled on the sea floor. There are fine examples of this limestone in the sea cliffs on Anglesey and nearby Llandudno, at Llangollen in Central Wales, and in the spectacular sea cliffs on the Gower and Pembroke peninsulas of South Wales.  The Rheic Ocean, which had separated the North American and northern European  supercontinent  from  the  southern Gondwana  supercontinent,  closed during Carboniferous  times as  the oceanic crust  sank beneath Gondwana. The continental collision began  in  the Late Carboniferous, about 320 million years ago and raised the continental crust of Wales above sea level. Large deltas formed as sediment washed down from  the hills. Forests flourished on  the deltas, and thick layers of peat accumulated. As more sediment buried the peat, the moisture was gradually squeezed out to convert it into the coal of South Wales.   The  collision  of  Gondwana  with  the  northern  continents  continued and they all merged to form the supercontinent Pangaea that included all of the present-day continents. This collision folded the rocks of South Wales and raised foothills in southwestern England and Wales that flanked the Hercynian Moun-tains of Europe.  In Permian and Triassic times, the supercontinent of Pangaea began to break up. The North American plate separated from Europe to begin its westerly drift away from Europe that continues to this day. Minor spreading also stretched the edges of the continents, resulting in down-dropped valleys in Cheshire and Cardigan Bay. At this time, Wales was about 30 degrees north of the equator - in desert latitudes. Flash floods wore down the Hercynian hills and the debris col-lected in the faulted valleys as sand and mud to form the New Red Sandstone. Shifting sand dunes collected in many areas and are best preserved in Cheshire.  The New Red Sandstone rocks are of Permian and Triassic age, which are be-tween 290 and 200 million years old. Chester, the Vale of Clwyd, and the coast near Cardiff show fine outcrops of these rocks.   Continued stretching resulted in further subsidence and the flooding of parts of England and Wales with a shallow sea in Jurassic times. Animals flour-ished in the warm, shallow sea and their shells collected as limestone on the sea floor. Examples of this Jurassic limestone and mud occur in the cliffs near Cardiff and in the Cotswold Hills in England. Wales began to take on its present shape. The Irish Sea, parts of Cardigan Bay, and the Bristol Channel foundered, leaving Wales surrounded by water on three of its four sides.   Wales continued as an eroded surface, parts of  it flooded from time to time, until rising in some places to nearly 600 metres above sea level. Remnants 

  7

1. Introduction

of this surface form the plateaux of Central and North Wales. Geologists do not agree on the cause of this uplift. Some believe it to be the result of distant rum-blings of the Alpine collision to the south. Others believe light granite intruded near the base of the crust made the crust lighter and caused it to rise. Still others believe the surface was never raised tectonically, but is simply the result of long and continuous erosion.

TheIceAges

The final chapter in shaping the Welsh landscape records the repeated advance of glaciers of the great ice ages of Pleistocene times, which began about two mil-lion years ago. Although we do not know what triggered the ice ages, geologists believe that the movements of the tectonic plates that closed the Straits of Panama between North and South America about three million years ago cut off the equa-torial currents and forced warm water along the east coast of North America into the Arctic. The warmed air carried moisture to middle and high latitudes where it precipitated as snow in winter. By about a million years ago, the climate cooled sufficiently  that  snow did not melt  completely  in  summer and accumulated  to build into glacier ice.  Glaciers covered Scandinavia and most of the British Isles at the peak of glaciation more than 600,000 years ago, and moved across Wales several times from centres in North Wales, Scotland and northern England. The northern ice lobes impinged on Wales from the Irish Sea. Glaciers covered Wales during the Anglian glacial period about 450,000 years ago (Fig. 4). Snowdonia had its own ice cap and ice spreading from centres in the Irish Sea flowed around the Welsh ice to cover Anglesey, the Lleyn Peninsula, most of Southwest Wales and to fill the Bristol Channel. Glaciers covered Wales again about 150,000 years ago dur-ing the Wolstonian glaciation (Fig. 4), but later glaciers obscured the evidence.  Sea level rose between the major glaciations as the water trapped in the ice melted and returned to the sea. The higher sea level cut into the cliffs in many places leaving raised beaches, particularly in Anglesey.  The  last major  glaciation,  the Early  to mid-Devensian,  that  reached  a maximum from about 122,000 to 24,000 years ago, destroyed many of the effects of earlier glaciations and covered much of Wales with a layer of glacial debris. The Irish Sea ice was smaller then and covered only Anglesey, northern Lleyn and the northern part of St David’s peninsula. The Welsh ice covered the rest of Wales, including some of the South Wales coast. Several major outflows of ice from the main collection areas scoured out the valleys of Snowdonia and some of the major valleys of Central Wales. The large glaciated valleys of Snowdonia, and the Towy, Usk and Wye valleys all carried large glaciers during the maximum advance of the ice. The deepened valleys leave their tributaries stranded on the sides of  the steep glacial valleys. Waterfalls cascade down from these hanging valleys to the main valley floor.

8

1. Introduction

  During  the  latest episode of glaciation,  the Loch Lomond advance be-tween 12,900 and 11,500 years ago (Fig. 4), ice collected only on the high peaks of Snowdonia to form small glaciers that gouged out the cwms, or cirques, high up on the sides of the earlier ice-deepened valleys. These are particularly impres-sive in Nant Ffrancon along the A5 in Snowdonia. In other parts of Wales, cwms formed at higher elevations as far south as the Welsh Coalfield.    When the ice melted, huge torrents of water flowed beneath the ice and channelled out valleys before they emerged from beneath the front of the glacier to make broad outwash plains of gravel and sand. After the glaciers melted com-pletely, these valleys were left with only tiny streams or no stream at all. The area around Fishguard has fine examples of dry melt-water valleys.   Wales  also  owes  its  indented  coastline  to  the  glaciers.  They  enlarged and deepened the main valleys. Then sea level rose as the great glaciers melted. 

Figure4.The Ice Ages of Wales.

  9

1. Introduction

The rising sea level flooded the valley mouths, converting the lower reaches of the streams into estuaries, such as those of Tremadog, Barmouth and Aberdovey Bays north of Aberystwyth.  Repeated freezing and thawing shattered the rocks in the region near the glaciers. Frost heaving pushed rocks into piles and low ridges that flowed slowly downhill. This disturbed soil  and shattered  rock  is called “head.” Glaciers ad-vanced over the head in many places and it can be seen beneath the till in many cliff sections.  For the past 10,000 years, soil creep, landslides and rivers have changed and softened this glacial landscape. With the melting of the glaciers at the end of the ice ages, sea level rose and flooded many of the coastal valleys. Birch, fol-lowed by pine trees, covered the landscape until about 4,000 years ago, when the climate became wetter and the forests died off. Moss grew profusely and decayed to peat that now covers the hills. Since then human activity has removed most of the remaining trees and changed the landscape to sheep-grazing grasslands.   Rocky outcrops are rare, so the roadside geology routes in the following chapters may take you somewhat out of your way. Many tours will send you to the splendid cliff sections along the coastal paths, and we hope you enjoy the sea vistas and the birds as well as the rocks.

10

1. Introduction