Geologists’ Association Guide No. 69 THE ROADSIDE GEOLOGY OF WALES Jim Talbot and John Cosgrove Guide Series Editor: Susan B. Marriott © THE GEOLOGISTS’ ASSOCIATION 2011 CONTENTS Page PREFACE iv ACKNOWLEDGEMENTS v LIST OF FIGURES vi 1. INTRODUCTION 1 Wandering Wales 3 Geological History of Wales 4 The Ice Ages 7 2. GWYNEDD, NORTHWEST WALES 11 The geological history of northwest Wales 11 Anglesey (Ynys Môn) 14 The A5/A55: Menai Bridge to Holyhead 16 The A4080: southwest coast road between Menai Bridge and Rhosneigr 18 Holy Island 21 The A5025: the north and east coast from Holy Island to Beaumaris 24 Southeast Anglesey 28 The Lleyn Peninsula 31 The A497, A499 and B4413: Porthmadog to Aberdaron 33 The B4417 and A499: Aberdaron to Caernarfon 37 Snowdonia 39 The A55, A487 and A470: Conwy to Porthmadog 40 The A5: Betws-y-Coed to Menai Bridge 45 The A4086: Capel Curig to Caernarfon 50 The A4085 and A498: Caernarfon to Porthmadog 55 The Harlech Dome 57 The A470: Dolgellau to Ffestiniog 58 The A496: Dolgellau to Ffestiniog (coast road) 60 The Cadair Idris Range 65 The A487: Machynlleth to Dolgellau 66 The A493: Dolgellau to Machynlleth (coast road) 67 3. CLWYD, NORTHEAST WALES 69 The geological history of northeast Wales 69 The A5: Shrewsbury to Betws-y-Coed 72 The A55: Chester to Conwy 74 The A542 and A525: Llangollen to Rhyl and the Vale of Clwyd 76 The Clwydian Range 80 The A494: Mold to Ruthin 80 The A541: Mold to Denbigh 81 The Denbighshire Moors 82 i The A548: Abergele to Llanrwst 83 The A543: Denbigh to Pentrefoelas 84 4. CENTRAL WALES 85 The geological history of Central Wales 85 The Palaeozoic Welsh Basin 87 The Coast Road 89 The A487: New Quay to Machynlleth 89 The Inland Roads 97 The A44: Kington to Aberystwyth 97 Mountain road (B4574 in part): Devil’s Bridge to Rhayader 101 The A483: Builth Wells to Oswestry 103 The A470: Brecon to Dolgellau 105 The A470 and A4212: Ffestiniog to Bala 109 The B4391: Bala to Llanfyllin 111 The Welsh Borderland 112 The A458: Shrewsbury to Welshpool 113 The A490, A489 and A49: Welshpool to Church Stretton 114 5. GLAMORGAN AND GWENT, SOUTHEAST WALES 117 The geological history of southeast Wales 117 The Forest of Dean and the Usk Valley 121 The A4136: Mitcheldean to Monmouth 122 The A466: the Wye Valley, Monmouth to Chepstow 123 The A40 and A449: Monmouth to Newport 124 The South Wales Coalfield 125 The A470: Cardiff to Brecon 126 The A4059 and A470: Hirwaun to Brecon 130 The A465: Heads of the Valleys Road, Abergavenny to Swansea 132 The Western Coalfields 135 The A4067 and A40: Swansea to Brecon 135 The A4069: Llandovery to Brynamman 139 The A483: Swansea to Llandeilo 140 The A40: Ross-on-Wye to Carmarthen 141 The M48 and M4: Chepstow to Carmarthen 145 The Glamorgan Coast between Cardiff and Swansea 148 The A48: Cardiff to Bridgend 154 The Gower 155 The north Gower coast 160 6. SOUTHWEST WALES 161 The geological history of southwest Wales 161 The Pembrokeshire Coast Section 165 The A487: New Quay to St David’s 165 The south coast: St David’s to Carmarthen 174 ii The Marloes Peninsula 183 The Pembroke Peninsula 189 The A478 and A4139: Pembroke to Begelly 194 The coast road: Begelly to Pendine 197 The back roads between Pendine and Carmarthen 199 Southern Pembrokeshire and Carmarthenshire 203 The A40: Carmarthen to Fishguard 203 The A477: St Clears to Pembroke and Haverfordwest 204 The A478: Tenby to Cardigan 205 REFERENCES AND FURTHER READING 206 COMMON WELSH ROCKS 208 INDEX OF PLACES 211 iii 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 iv 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, 56 Northeast 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 v Roadside Geology of Wales LIST OF FIGURES 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. vi 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.