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69. The Roadside Geology of Wales (2011)


by Jim Talbot and John Cosgrove

This guide book is written for those who have little background knowledge of geology. It focuses on the geology of Wales that can be seen from a car and the descriptions are organised in relation to the main roads. The best outcrops are in the sea cliffs and on beaches and these are also described where they are easily accessible from a main road. Many examples of geological activity are illustrated along the roads and cliffs of Wales including basin sedimentation and volcanic activity. The effects of plate tectonics in opening basins and folding and metamorphosing the rocks in two major continental collisions are recorded in the Welsh Mountains.

Look inside:

Table of Contents

Overview of Itineraries


Table of Contents


PREFACE
ACKNOWLEDGEMENTS
LIST OF FIGURES

1. INTRODUCTION
Wandering Wales
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
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
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


Itineraries
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This guide book is written for those who have little background knowledge of geology. It focuses on the geology of Wales that can be seen from a car and the descriptions are organised in relation to the main roads. The best outcrops are in the sea cliffs and on beaches and these are also described where they are easily accessible from a main road. Many examples of geological activity are illustrated along the roads and cliffs of Wales including basin sedimentation and volcanic activity. The effects of plate tectonics in opening basins and folding and metamorphosing the rocks in two major continental collisions are recorded in the Welsh Mountains.

Gywnedd North West Wales
Anglesey
Itinerary 1 A5/A55 Menai Bridge to Holyhead 20 miles
Visiting the Marquis of Anglesey and his blue schist one of the world’s rare rocks, possibly the oldest in the world, a signature of high pressure low temperature metamorphism marking the site of a Cambrian subduction zone. Excellent Precambrian granites and interesting glacially sculpted landscapes abound.
Start point Menai Bridge SH 555 720

Itinerary 2 Southwest Coast Road on the A4080 Menai Bridge to Rhosneigr 23 miles
Carboniferous Limestone, deposited over the Precambrian basement peneplain, forms flat farmland. See pillow lavas and magnificent sand dunes. The road crosses a major melt water channel which exposes fine Precambrian rocks which have suffered greatly. Find evidence for the Caledonian and Variscan Orogenies and thus continental collision.
Start point Menai Bridge SH 555 720

Itinerary 3 Holy Island to Beaumaris on the A5025 33 miles
Spectacular tight, accordion-like folds in Precambrian schists support the lighthouse and underlie much of Holy Island. On a clear day the Lleyn Peninsula and Snowdonia’s mountains can be seen. Much of Holy Island is metamorphosed volcanic ash, mudstone and shale but Holyhead mountain is sparkling quartzite. At Cemaes Bay, the spectacular Gwna Melange is exposed, the result of submarine landslides whilst the rocks were still soft. This road passes the Wylfa nuclear power station and the Parys mountain copper mines originally worked by the Romans. Experience the raised beach overlying Carboniferous Limestone with interesting dissolution features in the cliffs and a wave cut platform.
Start Point car park at South Stack SH 202 823

Lleyn Peninsula
Itinerary 1 Porthmadoc to Aberdaron: South coast A497/A499 and B4413 29 miles
This itinerary is a tale of glacial cover and solid rock. A fine Ordovician sill, with columnar joints, supports Criccieth Castle, but the town of Criccieth sits dangerously on glacial till which needs support.
Start point Porthmadog SH 567 388

Itinerary 2 Aberdaron to Caernarfon: North coast on the B4417 and A499 32 miles
See Precambrian pillow lavas at the Lifeboat Station. Later volcanic history can be seen in the Ordovician volcanic rocks associated with the story of the closure of the Iapetus Ocean. Quarries in granites of Ordovician age can be seen from the road, on the mountain Yreifl. The itinerary finishes on the raised marine terrace into Caernarfon.
Start point Aberdaron SH 172 264

Snowdonia
Itinerary 1 Circular route around Snowdonia: Conwy to Porthmadoc A55/A487/A470 89 miles
Crossing Ordovician volcanic and intrusive rocks wonderfully exposed by the re-routing of the A55. Cambrian slates, including the largest slate quarries in Wales, are also visible and a wealth of glacial landscape features.
Start point Conway SH 780 774

Itinerary 2 The Snowdon Passes: Betws-y-Coed to Menai Bridge, The Nant Ffrancon A5 22 miles
Follow Watling Street through the Ordovician outcrops and see the effect of glaciation on the more resistant volcanics.
Start point Betws-y-Coed SH 795 565

Itinerary 3 The Pass of Llanberis: Capel Curig to Caernarfon A4086 23 miles
Admire the glacial lakes preserved in Ordovician volcanics and a classic U-shaped valley with waterfalls from hanging valleys. Worth a visit, the Dinorwic hydro-electric power station which makes use of old Cambrian slate quarries.
Start point Capel Curig SH 721 581 Itinerary 4 The Pass of Rhyd-Ddu: Caernarfon to Porthmadoc A4085/ A498 19 miles
If you wish to climb Snowdon this is the route to choose. This a good opportunity to see the confluence of two glacial valleys at Beddgelert. Road cuttings show good exposures of the Snowdon Volcanic Group, rocks which filled the Snowdon caldera, and you can also visit the old Sygun copper mine which shows how many metallic ore deposits were formed.
Start point Caernarfon SH 482 627

Harlech Dome
Intinerary 1 Dolgellau to Ffestiniog A470 9 miles
A feast of Cambrian rocks whose hardness is reflected in the landscape.
Start point Dolgellau SH 728 178

Itinerary 2 The Coast Road Dolgellau to Ffestiniog A496 31 miles
See some of the finest exposures of the oldest sedimentary rocks of the Welsh basin, travel through 70 million years of Cambrian rock. These beds were deposited as turbidity currents poured off-shelf into the Welsh basin. Harlech Castle stands on the Rhinog grits formed from coarser facies of the turbidite sequence. This castle built by Edward I guards the water gate, one of his supply routes.
Start point Dolgellau SH 728 178

The Cadair Idris Range
Itinerary 1 Eastern Cadair Idris: Machynlleth to Dolgellau A487 16 miles
Travel a forested gorge of Silurian slates and cross one the largest faults in Wales, the Bala fault in Ordovician slate and pick up changes in rock type in the walls and buildings.
Start point Machynlleth SH 745 008

Itinerary 2 The Cadair Idris Coast Road: Dolgellau to Machynlleth A493 34 miles
See the Dolgoch Falls, a series of cascades cutting into Ordovician slates. The Silurian slates showing steep cleavage in many road cuttings which have been used as roof slates, as have the Cambrian Ffestiniog flags.
Start point Dolgellau SH 728 178

Clwyd North East Wales
Itinerary 1 The A5: Shrewsbury to Betws-y-Coed 61 miles
An excursion into the Palaeozoic. Westwards off the rich Permo-Trias farmland, into the Vale of Llangollen which exposes the Carboniferous Millstone Grit deltaic sandstones then the older, grey Carboniferous Limestone scarp and dark Silurian Slates, on which stands Castell Dinas bran. These slates mark the compression of the region during the Caledonian Orogeny and they can be seen in large quarries and also are used in the local buildings. Conwy Falls tumble from a hanging glacial valley over the dark Silurian slates. The road follows the glacial valley with Ordovician volcanic on the West and the slates on the East.
Start point Shrewsbury SJ 487 096

Itinerary 2 The A55: Chester to Conwy 38 miles
A good opportunity to admire the red Permo-Trias, New Red Sandstone (NRS), which was quarried to build Chester. Passing from the Dee Valley over the Clwydian range, Carboniferous Limestone blocks are scattered on the fields. Thick limestone layers mark the fault bounded Clwyd Valley graben, part of the opening Atlantic story and the break-up of Pangaea. The contrast in agriculture clearly reflects the underlying geology. The coast road follows a wave-cut terrace caused by sea-level changes associated with the last glaciation. Conwy Castle exploits the local Carboniferous Limestone as its building material.
Start point: Chester SJ 425 642

Itinerary 3 The A452/A525: Llangollen to Rhyl and the Vale of Clwyd 35 miles
Cross over the folded dark Silurian slates and gently tilted Carboniferous Limestone, pass Valle Crucis Abbey where the slates show paper-thin sandstones with rippled surfaces, which emphasise an angular relationship. Slate quarries show interesting colour banding, in well cleaved, ideal roofing and billiard table material. A nearby area shows lead, zinc and silver mineralization of Variscan age, which has been worked from Roman times and produced one-eighth of Britain’s lead during the Industrial Revolution. Sands derived from desert environments in NRS times provide material for the beaches. Glacial features can be detected and reddish colouration has stained the limestones. Above Prestatyn, a good view of the general landscape features is worth stopping for.
Start point Llangollen SJ 215 419

The Clywdian Range
Itinerary 1 The A494: Mold to Ruthin 10 miles
Road cutting shows bladed and folded Silurian siltstones and slates overlain by Carboniferous limestone in angular unconformity. Millstone grit can be seen in the buildings.
Start point Mold SJ 238 639

Itinerary 2 The A451: Mold to Denbigh 16 miles
A feast of Carboniferous Limestone.
Start point Mold SJ 238 639

The Denbighshire Moors
Itinerary 1 The A548: Abergele to Llanrwrst 17 miles
3000 m of Silurian sediments, produced by submarine slides and turbidity currents underlie this sector of the Cambrian Mountains; gentle easterly dips interrupted by some large folds are again topped with Carboniferous limestone in an angular unconformity. Glacial melt water channels incise these rocks, now showing tiny misfit streams.
Start point Abergele SH 945 785

Itinerary 2 The A543: Denbigh to Pentrefoelas 17 miles
Small quarries expose finely banded turbidite sediments which show weak cleavage developed during the Devonian Acadian Orogeny
Start point Denbigh SJ 058 665

Itinerary 3 The B5113 Colwyn Bay to Llanrwrst 19 miles
Here you cross one of the high Welsh plateaux originally a Permian erosional surface. Now deeply incised, it gives great panoramas of Snowdonia.
Start point Colwyn Bay SH 860 789

Central Wales The Palaeozoic Welsh basin
The Coast Road -
The A487: New Quay to Machynlleth 39 miles
Track some of the finest exposures of Silurian turbidites, moving north into the Welsh Basin and losing bedload and energy, as mudstone deposition dominates the deeper part of the basin. The Caledonian Orogeny resulted in tilting and folding of these sediments, near vertical in places, and showing an angular unconformity with the overlying Irish Sea glacial till, a real mixed bag of a sediment. Trace fossils show evidence of life in Silurian times. Near journey’s end the rocks show “concertina folds” and pervasive cleavage resulting from compression in the Caledonian Orogeny.
Start point New Quay SN 385 598

The Inland Roads -
Itinerary 1 The A44: Kington to Aberystwyth 61 miles
Silurian rocks intruded along faults by dolerite at the end of the Caledonian collision are valued for road aggregate. At Ponterwyd, visit mines exploited in the 19th century for zinc, silver and lead ores, the tips are worth picking over. Devil’s Bridge, where glacial melt water cuts a deep gorge with fine pot holes in the Ordovician slates, is a place where graptolite fossils can be found.
Start point Kington SO 302 568

Itinerary 2 Mountain Road (B4574 in part) Devil’s Bridge to Rhayader 23 miles
An adventurous road over Silurian turbidites with good cleavage in fine sediments, passing Jubilee Arch (memories of George III), and scour waste heaps at Cwmwyswyth mine, which has been worked for zinc, lead and silver since the Bronze Age. Over the moors, you can see excellent views of the high Welsh plateau and work out the geological story.
Start point Devil’s Bridge SN 738 769

Itinerary 3 The A483: Builth Wells to Oswestry 58 miles
A good opportunity to find “pencil” slate in the Silurian slates, good for building not for writing. Overlook the Severn Valley plain and contrasting red Triassic rocks which were deposited in a down-faulted valley associated with the Atlantic opening.
Start point Builth Wells SO 040 510

Itinerary 4 The A470: Brecon to Dolgellau 104 miles
Here you cross the deepest part of the Welsh Basin with the longest record of Ordovician and Silurian sand and mud deposition, indicating off-shelf turbidite facies. In the South you meet the other red sandstone the Devonian, Old Red Sandstone (ORS), much is neither red nor sandstone; these are flood plain sediments formed when the Welsh Basin had finally filled. A broad anticline outlines Builth Wells and several large folds can be traced as you drive north, with Ordovician slates stained red by rusted pyrite in the original mudstone. Follow the River Wye as it jumps down steps formed of resistant sandstone forming little rapids. Near Cemmaes, sparse, poorly consolidated mud and silt outcrops means few stone boundaries and subdued landscapes. Rugged moors form on Ordovician volcanic rocks of the Cadair Idris Group.
Start point Brecon SO 045 285

Itinerary 5 The A470 and A4212: Ffestiniog to Bala 26 miles
This route crosses Cambrian and Ordovician rocks, with few outcrops apart from masses of dark dolerite. Typical hummocky moraine left by Ice Age glaciers can also be seen. Horizontal volcanics are eroded to form the Arenig hills; crystals can be seen, as can columnar joints in places, these are ignimbrites. Towards Bala, landscape reverts to rolling grazing land.
Start point Ffestinog SH 701 419

Itinerary 6 The B4391: Bala to Llanfyllin 28 miles
Here you cross the Berwyn Mountains formed of Ordovician and Silurian turbidite rocks mostly covered by glacial debris. Road cuttings show some sandstone and shale clasts ripped up by successive turbidites. Ribs of Ordovician volcanic rocks stand proud on some slopes. Much of the southern part of this route shows subdued landscape.
Start point Bala SH 929 362

The Welsh Borderland
Itinerary 1 The 458: Shrewsbury to Welshpool 18 miles
The Severn Valley is floored with Triassic mudstone. The road climbs onto Carboniferous sandstone soils and then back onto folded Ordovician and Silurian slates. You can find volcanic bombs at Middletown in the steeply dipping Ordovician volcanics, mostly of fine ash. Look out for the landscape contrasts here.
Start point Shrewsbury SJ 490 127

Itinerary 2 The A490, A489 and A49: Welshpool to Church Stretton (Shropshire) 27 miles
A classic geological area with rocks of Precambrian to Silurian ages and the tectonic and structural relationships are worth disentangling. A knob of dolerite forms a prominent hill at Churchstoke. Mining for lead and zinc and quarrying are an important part of the history of the area, with waste tips in abundance. Jagged teeth of the white quartzite Stiperstones stand proud on the skyline. They mark the position of a nearshore sand bar as Ordovician sea-level rose. You can recall the Pleistocene tundra landscape when looking at the stone stripes which stretch out on the slopes below. The Longmynd, a Precambrian mountain shows few outcrops, visit the car park west of Church Stretton to reveal these Longmyndian rocks. Make use of the National Trust car parks to see Silurian Wenlock limestone, the same age as the Welsh Basin, but showing rocks from a different environment, an ancient barrier reef.
Start point Welshpool SJ 225 074

Glamorgan and Gwent, Southeast Wales
The Forest of Dean and the Usk Valley -
Itinerary 1 The A4136: Mitcheldean to Monmouth 10 miles
Encounter a different scale of coal mining exploiting the Carboniferous Coal Measures, which with the Carboniferous Limestone, form the bedrocks under the Forest of Dean. The rim of the basin is formed from an ORS conglomerate, speckled with round white quartz pebbles and as the road drops down to the Wye Valley, the soil reddens as it crosses the less resistant ORS beds.
Start point Mitcheldean SO 665 183

Itinerary 2 The A466: The Wye Valley, Monmouth to Chepstow 16 miles
Follow the River Wye as it cuts down into the plateau of Carboniferous rocks, forming the boundary between England and Wales. The meandering river originally developed on a plain near sea level, incising to form the deep valley regardless of rock type as the plain rose. It crosses ORS rocks until, south of Tintern Abbey, cliffs have formed as the vertical joint pattern in the Carboniferous Limestone controls the way the rocks erode.
Start point Monmouth SO 510 130

Itinerary 3 The A40 and A449: Monmouth to Newport 25 miles
Here you cross the anticline which divides the synclinal South Wales Coalfield and Forest of Dean. Keeping an eye on the soil colours helps decide whether you are on the Silurian or Devonian rocks, as outcrops are rare.
Start point Monmouth SO 510 130

The South Wales Coalfield -
Itinerary 1 The A470: Cardiff to Brecon 44 miles
Crossing the South Wales Coalfield, the broad sweep of the Coal Measures rocks forms a basin, with roadside outcrops of Pennant Sandstone. Valleys also show the clear effects of the last glaciations, as well as relics of the great mining industry which exploited the Coal Measure rocks for both coal and iron. The North part of the route exposes Millstone Grit, cuts down into Carboniferous Limestone, then the underlying ORS and the Usk Valley. The ice sculpted Brecon Beacons National Park with its many road cuttings gave geophysical evidence that these ORS flash flood sandstones were deposited 20 ̊ south of the Equator.
Start point Cardiff ST 193 747

Itinerary 2 The A4059 and A470: Hirwaun to Brecon 21 miles
Rock outcrops appear gradually in road cuttings and as isolated blocks of Carboniferous Limestone which gives a fine opportunity to see the effects of acid rain at Porth-yr-Ogof caves. The overlying Millstone Grit scarp, with white quartz pebbles, shows clearly, with ribs of limestone below it. ORS forms the bedrock for the moors before the Brecon Beacons are seen, capped by more massive ORS sandstones.
Start point Hirwaun SN 950 055

Itinerary 3 The A465: Heads of the Valleys Road, Abergavenny to Swansea 44 miles
An opportunity to reflect on the relics of the mining and steel industries, forged from the Carboniferous Coal Measures. Associated lower, thick, gently dipping, massive sandstones and shales, deposited from huge, fluvial deltas interleaved with marshy areas, are easily seen in road cuttings. West of Hirwaun, are more complex areas of faults and folds, the ‘disturbances’. The Neath Disturbance is best seen near the Waterfalls Centre which tells the geological and cultural story of the area.
Start point Abergavenny SO 305 128

The Western Coalfields -
Itinerary 1 The A4067 / A4109 and A40 : Swansea to Brecon 41miles
Across the Swansea Valley, where towns cluster round old mines which exploited the Coal Measures. Travelling north, the rocks dip gently south, so you travel back in geological time until the white Millstone Grit caps the mountains to the North. An interesting diversion takes in the Henrhyd Waterfall, South Wales’ highest, which drops over the ‘Farewell Rock’, the miners’ name for the youngest Namurian sandstone below the lowest coal seam. Dan-yr-Ogaf caves mark the last of the Carboniferous Limestone, before the Devonian ORS gives the landscape a reddish hue and rippled surfaces indicate sandstones deposited on an ancient flood plain.
Start point Swansea SS 721 961

Itinerary 2 The A4069: Llandovery to Brynamman 15 miles
Good views of the landscape, outcrops of bedrock and the oldest rocks in the region, Ordovician slates of the Welsh Basin. The younger Devonian rocks are similar to the Silurian silts below, but the landscape changes as Black Mountain is reached; typical ORS moorland with gullies in glacial till and large sandstone boulders scattered across the surface. Use the car park to examine the top of the ORS with its magnificent white, vein-quartz pebbles set in a sandy matrix. Towards the top of the mountain, Carboniferous Limestone has been quarried.
Start point Llandovery SN 766 343

Itinerary 3: The A483: Swansea to Llandeilo 26 miles
Crossing unproductive Coal Measures which produce a subdued landscape, a cutting through a south dipping ridge of Carboniferous Limestone indicates that here it is the north limb of the syncline. Pastureland towards Llandeilo is underlain by ORS and Silurian siltstones.
Start point Swansea SS 721 961

Itinerary 4 The A40: Ross-on-Wye to Carmarthen 107 miles
Skirting the South Wales Coalfield, the road cuttings show ORS coarse sands and conglomerates, deposited from fast flowing rivers across coastal plains. Grey cliffs at Symonds Yat expose Carboniferous Limestone tilting gently west. In Monmouth, colour difference shows where red sandstones from the ORS, below the Carboniferous Limestone, are used in the buildings. Soils give clues to the underlying ORS where outcrops are absent.
Start point Ross-on-Wye SO 598 242

Itinerary 5 Motorway geology, M48 and M4: Chepstow to Carmarthen 90 miles
Motorways skirt the South Wales Coalfield, crossing ORS, Carboniferous Limestone, Triassic New Red Sandstone (NRS) and Jurassic rocks, often difficult to distinguish when flashing past at 70mph, as all contain limestones and sandstones and there are not many road cuttings. Generally, ORS shows thick, reddish-brown sandstones, whereas NRS has thinner, bright, orange-red beds. Carboniferous Limestone is blueish, ‘battleship’ grey, often in 1m or thicker beds, whereas Jurassic limestones tend to be thinner, 10 – 20cm layers in buff, yellow-brown.
Start point Chepstow ST 535 915

The Glamorgan Coast south of the M4: Cardiff to Swansea
A48/B4232/B4265/B4247 Cardiff to Bridgend
Here are sediments from fault-bounded, Triassic valleys with red mudstones, part of the NRS. The Triassic and Jurassic rocks are part of the story of the opening of the Atlantic Ocean, when warm seas flooded the basin, now traced by the Bristol Channel. Cliffs along the coast expose these rocks to good advantage. At Lavernock Point, see where Marconi made the first overwater radio transmission. Here alternate layers of thin limestones, separated by dark grey mudstones, are evidence of environmental, and climatic changes, dipping gently to the south and ageing to the north. Barry Island beach shows the unconformable relationship between the Carboniferous Limestone, folded in the Variscan Orogeny and the Triassic green and red mudstones, polygonal cracks are evidence of geological episodes of drying out, as happens today. Along the inland plain, quarries indicate the working of Carboniferous Limestone for road aggregate and an iron mine can be seen north of Cowbridge.
Start point Exit 33 M4, Lavernock Point ST 189 679

The Gower
The Gower Peninsula: A4067 / A4118 / B4295
A marine erosional plain, now at 100m above sea-level, cutting rocks of Devonian and Carboniferous age, folded during the Variscan Orogeny, and now clearly seen at Mumbles Head in the east and Rhossili in the west of the Gower. Another platform, about 7m above sea-level, probably resulted from changing sea-levels during Pleistocene times is clear at Three Cliffs Bay. Today, a third platform is developing on the modern foreshore. Look for cycles of shallowing and deepening water during Carboniferous times as the pulses of collision occurred during the Variscan Orogeny. Glacial sands from the Pleistocene can be seen in the inland dunes, which moved inland in the 14th century storm episodes to engulf the village and castle. Another dune field can be seen at Rhossili, to the north of the lovely sandy beach, which fronts hills of ORS and Carboniferous Limestone.
Start point Exit 47 M4, Mumbles Head SS 636 871

Southwest Wales
North Pembrokeshire Coast -
Itinerary 1 The A487: New Quay to St David’s 50 miles
Road builders avoided the deep inlets, sticking to the inland plateau. Excursions to coastal inlets will reveal wonderfully slumped Ordovician beds beneath Silurian alternating siltstones and mudstones, the results of fast moving turbidity currents into the Welsh Basin. This journey also takes in the effects of the Caledonian Orogeny and slaty cleavage in the fine-grained mudrocks and chevron folds can be found. The last glaciation has also left its mark in the form of meltwater channels, frost shattered misfit drainage patterns, ‘head’, fluvio-glacial sand bodies and eskers deposited by rivers under the glaciers. Dark igneous rocks, some of which can be seen at Stonehenge, form the Preseli Hills. At Strumble Head, Ordovician volcanic rocks, consisting of ‘pillow lavas’ and ashes - greenish in colour because of later alteration, are interleaved with the slates. Dark slates at Abereiddy are easy to appreciate from the Pembrokeshire Coast Path, as less resistant slates form deep inlets with volcanic rock ribs between. Fossil graptolites can be found if you look carefully in the beach pebbles.
Start point New Quay SN 383 598

Itinerary 2 St David’s to Broad Haven and Little Haven 15 miles
St David’s Peninsula shows contrasting geology. To the north, Ordovician slates and igneous intrusions including gabbro occur beneath the landscape, whereas to the south, you find Cambrian sediments and Precambrian volcanics. Look out for the Lingula Flags in the Cambrian. Carn Llidi is worth the climb to see the speckled gabbro. At St David’s Head, see where the gabbro baked the Ordovician mudstones to form hornfels. Caerfai Bay is a good place to see the magnificent Cambrian and Precambrian rocks, dipping steeply to the south. Purple and green sandstones found here made good building stones for the Cathedral. Glacial sediments are easy to find at Newgale and see Cambrian rocks faulted against the Coal Measures which can be seen as a wave-cut platform. Precambrian rhyolite forms the foundations for Roch Castle. Cross the concealed Coal Measure rocks, folded and faulted during the Variscan Orogeny, but well exposed in the coastal cliffs. Nolton, Broad and Little Havens show these effects really well. Druidstone Haven is unique. A faulted ‘horst’ which brings Ordovician rocks to the surface. These have been affected by both the Caledonian and Variscan Orogenies. Everything to the south shows just Variscan effects.
Start point St. David’s SM 754 253

The Marloes Peninsula -
Marloes B4327 and C class roads
This area reveals the rocks only in the cliffs below the Pembrokeshire Coast Path. St Bride’s Haven shows multiple faults and folds in ORS mudstones and Carboniferous Limestone thrust at a very shallow angle on top. Silurian volcanics and sediments dip at a shallow angle at St Martin’s Haven, where the National Trust car park is the hub of several paths for exploration of the youngest Silurian volcanics of the Welsh Basin, the ending of the Iapetus Ocean plate. Both basalt and andesite show gas bubbles, definite indicators of lava flows. Another National Trust car park allows access to the beach at Marloes. Silurian fossil worm burrows, occur in some of the sandstones. The Silurian gives way to ORS opposite Gateholm Island, coarse conglomerates and sandstones, reddish in colour and resulting from flash floods.
Start point Talbenny SM 839 119

The Pembrokeshire Peninsula -
Itinerary 1 The A478 / A4319 / A4320: South of peninsula 17 miles
The landscape is very different and strongly influenced by the folded rocks of the ORS and Carboniferous Limestone, with a distinctive east-west Variscan fabric. Exposures are confined to the coast, where evidence for large folds can be discovered at Freshwater East and Stackpole Quay, where the harbour is the core of an eroded anticline in the Carboniferous Limestone. Freshwater West exposes Palaeozoic rocks unconformably overlain by the ORS basal conglomerate. Find calcrete nodules, which indicate development in desert soil horizons. They also show the offset of a large north-south fault which breaks the rocks along the beach. At West Angle Bay, both ORS and Carboniferous Limestone are exposed in tight, asymmetric folds formed during the Variscan Orogeny.
Start point Pembroke SM 985 014 or Freshwater East SS 018 984

Itinerary 2 A478 / A4139: Pembroke to Begelly 17 miles
The road from Pembroke to Manorbier, birthplace of Gerald of Wales, crosses a flat grassy plateau over Carboniferous rocks with no outcrops. At Manobier, you are back on ORS mudstones and thin sandstones, vertical in attitude but strongly folded. You can see rippled upper surfaces and calcrete nodules. Tenby, on the southern edge of the Pembrokeshire Coalfield, is on the Ritec Fault, which forms the inlet of Milford Haven at its west end, and thrusts the folded Carboniferous Limestone up over the Coal Measures. Good tight folds from Variscan disturbance can be seen. At Saundersfoot, Coal Measure sandstones and mudstones form the cliffs, with lots of sedimentary and structural interest including Ladies’ Cave, one of the most photographed anticlines in Britain.
Start point Pembroke SM 985 014

Itinerary 3 The Coast Road: Begelly to Pendine 10 miles
A gentle journey on one of Pembrokeshire’s C class roads over a high grassy terrace with no outcrops but the cliff sections take a journey telling a different story. Carboniferous Coal Measure rocks are tightly folded and broken up by faults which disrupt the repetitions of the Coal Measure sediment pattern. To the south of Amroth West you cross the Variscan Orogenic front and at Amroth East the Coal Measure rocks are undisturbed, so the story of coal formation is much easier to work out. Fossil evidence of rootlets and plant fossils can be found.
Start point Begelly SN 119 074

The back roads between Pendine and Carmarthen -
Itinerary 1: The A4066: Pendine to St Clears 9 miles
Pendine Sands were the place to break land speed records. This wonderful beach has sand reworked by the waves from sand generated by the last glaciation. Carboniferous Limestone to the west of the beach, shows evidence for sea-level fluctuations. You find massive beds of broken limestone blocks, formed when sea-level fell and surface waters dissolved the limestone to form caves and sinkholes. The roofs collapsed and the hollows filled with the collapse debris. As sea-level rose, the overlying limestone beds show normal horizontal beds. Carboniferous Limestone forms 10 – 20m thick beds of blue-grey, fossiliferous limestone. Vertical joints keep the cliffs steep but waves undercut the cliffs and rock falls occur, so caution is needed. East of Pendine, ORS shows fine sedimentary structures at Laugharne, on the foreshore and the banks of Afon Taf, where Dylan Thomas wrote ‘Under Milk Wood’ in his boathouse.
Start point Pendine SN 233 080

Itinerary 2: The B4312: Carmarthen to Llansteffan 7 miles
Bedrock exposure is only seen on the shore south of Llansteffan. Grey-green upper ORS mudstones emerge from below red, Irish Sea till. Within the mudstones, which redden as the rocks age, you can find calcrete nodules formed after seasonal rains in semi- to arid soils when Wales was 20 ° south of the Equator, an interesting environmental story.
Start point Carmarthen SN 414 199

Southern Pembrokeshire and Carmarthenshire
The Inland roads: A40, A477 and A478 -
Itinerary 1 The A40: Carmarthen to Fishguard 46 miles
Outcrops are rare. The road follows an Ordovician slate belt, with good colour banding, which can be seen in cuttings at Whitland. To the north of Haverfordwest, Treffgarne Gorge, incised by glacial meltwaters, exposes Precambrian volcanics. Rhyolites form high cliffs above the gorge on which stands Maiden’s Castle, an iron-age hill fort. The disused, roadside aggregate quarries expose ashes and shales, volcanic bombs have been found, as have rippled surfaces indicating these rocks formed under water.
Start point Carmarthen SN 414 199

Itinerary 2 The A477: St Clears to Pembroke and Haverfordwest 33 miles
Road cuttings allow you to see the effects of both the Caledonian and Variscan continental collisions. West of St Clears, lower Palaeozoic grey slates show up in cuttings, with Caledonian folding which developed before the ORS was draped over the eroded surface. West of Red Roses, the soil changes to red showing the presence of ORS and forming good farming land. Further west, brown soils indicate a change to Coal Measures, which are clearly visible in superb road cuttings, weakly folded because they are north of the Variscan front. Pembroke Castle stands on Carboniferous Limestone, but outcrops are rare. Faults thrust Precambrian rocks to the surface at Johnston, where the Variscan Front is crossed, but these diorites can only be seen in a quarry to the west.
Start point St Clears SN 278 161

Itinerary 3 The A478: Tenby to Cardigan 35 miles
The road crosses sandstones of the Carboniferous Coal Measures, the Variscan Front and then red soils indicate ORS, but outcrops are non-existent. After the A40, Ordovician slates form bedrock and again no outcrops occur. A few tors of Ordovician volcanics can be seen.
Start point Tenby SN 125 005

Map of Itineraries