Download THE ROADSIDE GEOLOGY OF WALES

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 PREFACE ACKNOWLEDGEMENTS LIST OF FIGURES Page
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1. INTRODUCTION
Wandering Wales 1
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Geological History of Wales The Ice Ages 2. GWYNEDD, NORTHWEST WALES
The geological history of northwest Wales
Anglesey (Ynys Môn)
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
The Lleyn Peninsula
The A497, A499 and B4413: Porthmadog to Aberdaron
The B4417 and A499: Aberdaron to Caernarfon Snowdonia
The 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 Porthmadog
The Harlech Dome
The A470: Dolgellau to Ffestiniog
The A496: Dolgellau to Ffestiniog (coast road)
The Cadair Idris Range
The A487: Machynlleth to Dolgellau
The A493: Dolgellau to Machynlleth (coast road)
3. CLWYD, NORTHEAST WALES
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
The Clwydian Range
The A494: Mold to Ruthin
The A541: Mold to Denbigh
The Denbighshire Moors
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The A548: Abergele to Llanrwst
The A543: Denbigh to Pentrefoelas
4. CENTRAL WALES
The geological history of Central Wales
The Palaeozoic Welsh Basin
The Coast Road
The A487: New Quay to Machynlleth
The Inland Roads
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
The Welsh Borderland
The A458: Shrewsbury to Welshpool
The A490, A489 and A49: Welshpool to Church Stretton
5. GLAMORGAN AND GWENT, SOUTHEAST WALES
The geological history of southeast Wales
The Forest of Dean and the Usk Valley
The A4136: Mitcheldean to Monmouth
The A466: the Wye Valley, Monmouth to Chepstow
The A40 and A449: Monmouth to Newport
The South Wales Coalfield
The A470: Cardiff to Brecon
The A4059 and A470: Hirwaun to Brecon
The A465: Heads of the Valleys Road, Abergavenny to Swansea
The Western Coalfields
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
The Glamorgan Coast between Cardiff and Swansea
The A48: Cardiff to Bridgend
The Gower
The north Gower coast
6. SOUTHWEST WALES
The geological history of southwest Wales
The Pembrokeshire Coast Section
The A487: New Quay to St David’s
The south coast: St David’s to Carmarthen
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The Marloes Peninsula The Pembroke Peninsula
The A478 and A4139: Pembroke to Begelly
The coast road: Begelly to Pendine
The back roads between Pendine and Carmarthen
Southern Pembrokeshire and Carmarthenshire
The A40: Carmarthen to Fishguard
The A477: St Clears to Pembroke and Haverfordwest
The A478: Tenby to Cardigan
REFERENCES AND FURTHER READING
COMMON WELSH ROCKS
INDEX OF PLACES
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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 Palaeozoic 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 tectonic 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 described 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 Gwalchmai, 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
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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 Sedgwick 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: Geological 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 bilingual, 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
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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.
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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.
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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.
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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. ix
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.
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1. Introduction
1. INTRODUCTION
Palaeozoic Mesozoic Cenozoic
Summary of the Geological History of Wales
PERIOD AGE in Ma LOCAL EVENTS LATITUDE of WALES
- 0 Glaciers cover Wales
Quaternary
52oN
Uplift produces present day
plateaux
Tertiary
Cretaceous
Jurassic
- 100
- 200
- 300
- 400
Silurian
Ordovician
- 500
Cambrian
Desert sand and mud - the Old Red
Sandstone
Caledonian orogeny as
Iapetus Ocean closes to north
At Equator
20oS
Welsh basin collects mud and turbidite
sands
Volcanoes in Snowdonia and Southwest
Wales
Welsh Basin forms; shallow shelf along
Welsh Borderland. Sands and siltst collect on edge of continent
- 600
Precambrian
30oN
Hercynian orogeny as
Rheic Ocean closes to south
Deltas with forests, later form coal.
Warm seas form thick grey limestone
Carboniferous
Devonian
Limestones collect in warm seas
along the Bristol Channel
Desert sand and mud to east and north the New Red Sandstone
Triassic
Permian
Wales a plain with low hills
Volcanic island arcs in Wales
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1. Introduction
Figure 1. Simplified geological map of Wales.
The geology of the British Isles is a 3,000 million-year story of moving and colliding 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 backbone of Wales; and the Ordovician and Silurian Periods, named after ancient
Celtic tribes of the region.
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1. Introduction
Figure 2. The changing position of Wales. Wales spent much of its past south of the equator, separated from Scotland and southern Europe until the oceans between them closed.
Wandering Wales
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 continents, at other times attached to North America. Wales has drifted steadily north
during the past 600 million years (Fig. 2).
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1. Introduction
Geological History of Wales
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 northwestern 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 continent 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 Snowdonia.
Continued stretching of the crust caused coalescence of the isolated basins 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 metres 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
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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 Caledonian 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
Figure 3. The Welsh Basin in Ordovician to Silurian times.
5
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. Geologists 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 Mountains 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 collected 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 between 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 flourished 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
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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 rumblings 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.
The Ice Ages
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 million 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 equatorial 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 during 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.
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1. Introduction
Figure 4. The Ice Ages of Wales.
During the latest episode of glaciation, the Loch Lomond advance between 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 impressive 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 completely, 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.
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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 advanced 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, followed 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.
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1. Introduction
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