THE ROADSIDE GEOLOGY OF WALES

 Chapter 1. INTRODUCTION

Fig. 1. Simplified Geologic Map of Wales

The geology of the British Isles is a 3000 million-year story of moving and colliding continents, stretched and broken crust, and rivers and glaciers shaping the landscape.

                The rocks of North Wales are famous among geologists. It was here that almost two centuries ago geologists recognized and named the three subdivisions of the early Palaeozoic Era, the period from 570 to 410 million years ago. They are: Cambrian, after the Cambrian mountains that form the backbone of Wales; and the Ordovician and Silurian periods, named after ancient Celtic tribes of the region.                    

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).


Fig. 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.           

GEOLOGIC HISTORY OF WALES

The geologic history of the British Isles begins in Precambrian time, nearly three thousand million years ago. Rocks almost this old are at the surface in northwestern Scotland and elsewhere are the basement beneath the younger rocks. The oldest rocks in Wales are nearly 1000 million years old. They poke through younger rocks in a few places in Anglesey, St. David's Peninsula, and the Welsh Borderland. 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 sixty degrees south of the equator. You can best see these Precambrian rocks 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, part of a small continent called Eastern Avalon that included much of England, Wales, southern Ireland and Belgium. Eastern Avalon was close to the supercontinent Gondwana, which contained all the present southern continents.

                Near the end of Precambrian time, Eastern Avalon began to drift slowly away from Gondwana and the Iapetus Ocean began to close. The ocean floor slab plunged beneath the Eastern Avalon continent through a trench near the present 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 Avalon towards it, as a sinking ship drags survivors towards it, and the Avalon crust stretched.

                The stretching crust cracked and blocks sank between the fractures, creating small basins that were invaded by the sea. The basins extended from the Irish Sea platform in the north to the Welsh Borderlands in the south. Sand and mud poured into the basins in Cambrian time. As the basins deepened, mud settled into their deepest parts; the mud was later deeply buried and squeezed to form slate that is now mined in Snowdonia.

                Eastern Avalon's drift speeded up in Ordovician time, narrowing the Iapetus Ocean, but opening another called the Rheic Ocean to the south. The Iapetus Ocean floor continued to sink through the oceanic trench, and volcanoes continued to erupt violently above the sinking slab, accumulating thick piles of volcanic rocks in the Lake District of England and in the spectacular mountains of Snowdonia.

                Continued stretching of the crust caused a coalescing of the isolated basins of Cambrian time to create a single Welsh Basin of central and north Wales. Sand and silt collected near the edges of the basin, but most of it collected mud black with organic matter. In some parts of the basin, 4000 metres of mud and silt collected in the seventy million year span of Ordovician time. That is about a half millimetre per year. The violent volcanic eruptions probably occurred sporadically, but they dominate the modern landscape of Snowdonia because the volcanic rocks resist erosion more than the surrounding slates.

Fig. 3. The Welsh Basin in Ordovician to Silurian times.

About 450 million years ago, in early Silurian time, the North American plate began to impinge on the Avalon plate. The ocean floor stopped sinking, and the volcanoes died. However, sediments continued to pour into the Welsh Basin. In contrast to the fine-grained mud and silt of Ordovician age, the Silurian rocks are mostly sandstone. The sea cliffs between Newquay and Aberystwyth expose fine examples of these sandstones. 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. By the end of Silurian time the Welsh Basin was full of sediment.

The Iapetus Ocean closed by the end of Silurian time, about 425 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, and 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 time to collect in valleys and the lowland of what is now south Wales. Most Devonian 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 Carboniferous times, 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 that had separated the North American and northern European supercontinent from the southern Gondwana supercontinent closed during Carboniferous time as the oceanic crust sank beneath Gondwana. The collision between these continents began in Late Carboniferous time, 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, depositing thick layers of peat. As more sediment buried the peat, it gradually squeezed out the moisture to convert it into the coal of South Wales.

                The collision of Gondwana with the northern continents continued and they merged to form the supercontinent Pangaea that included all of the present 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 time, 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 twenty 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, 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 time. 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 six hundred metres above sea level. Remnants 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 so it rose. 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 time, 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.

Fig. 4. The Ice Ages of Wales.

 

Glaciers covered Scandinavia and most of the British Isles at the peak of glaciation, 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 300,000 years ago. Snowdonia had its own ice cap, and ice spreading from centres in the Irish Sea ice flowed around the Welsh ice to cover Anglesey, the Lleyn, most of South-West Wales and filled the Bristol Channel. Glaciers covered Wales again about 150,000 years ago, but later glaciers obscure 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 Late Devensian that reached a maximum from about 20,000 to 15,000 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.

When the ice melted, huge torrents of water flowed beneath the ice and channeled 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. 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.

                During the latest episode of glaciation, the Loch Lomond advance between 11,000 and 10,000 years ago, 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 gouged out valleys. These are particularly impressive in Nant Ffrancon along the A5 in Snowdonia. In other parts of Wales, small glaciers formed at higher elevations as far south as the Welsh Coalfield.             

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 you can see head 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 4000 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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