User:Geopersona/Geology of Wales

The geology of Wales is complex and varied; its study has been of considerable historical significance in the development of geology as a science. All geological periods from the Cryogenian (late Precambrian) to the Jurassic are represented at outcrop whilst younger sedimentary rocks occur beneath the seas immediately off the Welsh coast. The effects of two mountain-building episodes have left their mark in the faulting and folding of much of the Palaeozoic rock sequence. Superficial deposits and landforms created during the present Quaternary period by water and ice are also plentiful and contribute to a remarkably diverse landscape.

Wales' modern character derives in substantial part from the exploitation of its diverse mineral wealth; slate in Snowdonia, coal in the South Wales Valleys and metal ores in Anglesey and mid Wales to name but three. Wales' geology influences farming practices and building stone choices but also planning of developments which must take into account ground stability and liability to flooding - geohazards which an appreciation of the geology can help deal with.

Precambrian
Late Precambrian rocks are widespread on Anglesey, Llŷn and Arfon with other, more restricted occurrences in north Pembrokeshire, Radnorshire and Carmarthenshire. The often intensive metamorphism which these originally volcanic and sedimentary rocks have been subjected to and their generally faulted relationship to neighbouring rocks has meant that geologists’ understanding of them has been limited. The majority lie on or close to the margins of terranes, blocks of the Earth’s crust which have had differing geological histories before assuming their present configuration. However in recent years there has been a progressive elucidation of the way in which Wales’ many terranes came together during the late Precambrian and the Palaeozoic eon.

The Stanner-Hanter Complex on the English border comprises volcanic rocks around 700 million years old which puts them within the Cryogenian period.

Cambrian
Rocks of Cambrian age occur most extensively in an inlier in Merionethshire where the up-arched rocks of the Harlech Dome form the Rhinogs. The Harlech Grits Group comprises sandstones, mudstones and greywackes forms the eroded core of the dome and within this sequence it is specifically the greywackes of the 'Rhinog Formation' which provides the higher hills. Cambrian rocks are also to be found in north Pembrokeshire, Anglesey and Llŷn.

Ordovician
The Ordovician period gave rise to a sequence of sedimentary rocks which stretch from Pembrokeshire eastwards through Carmarthenshire up the Vale of Towy and which are intricately intermixed with those of the succeeding period northwards to the Vale of Conwy. In Snowdonia many Ordovician volcanic rocks give rise to a more rugged landscape than elsewhere in the country. Anglesey and Llŷn are also Ordovician territory. The Ordovician rocks of Wales are typically intensely faulted and folded, having been affected by the earth movements of the Caledonian Orogeny.

Silurian
Rocks arising in the Silurian period are arguably the most significant for Wales’ landscape, if only because a greater percentage of the country’s land area is underlain by rocks of this age than any other. Much of central Wales is formed in Silurian sandstones and mudstones as is the more gentle landscape of central Monmouthshire where the Usk Anticline gives rise to the Usk Inlier. In the north it is the Ludlow age mudstones and sandstones of the Elwy and Nantglyn Formations which form the Clwydian Range, and the Nantglyn Flags and Wenlockian Denbigh Grits which form the Denbigh Moors , Llantysilio Mountain and the Dee Valley around Llangollen. In west Wales, parts of south and central Pembrokeshire around Haverfordwest and Narberth and between Marloes and Daugleddau are formed by Llandovery aged mudstones, sandstones and conglomerates.

Devonian
Devonian age rocks are broadly synonymous with the Old Red Sandstone (commonly referred to as ‘the ORS’) though the lowermost ORS is late Silurian in age. The Anglo-Welsh Basin which stretches from the border with England westwards through the Brecon Beacons National Park into Pembrokeshire includes the larger part of this sequence. It is the sandstones and mudstones of the Lower Devonian Brownstones and Senni Formations, sometimes capped by the hard wearing sandstones of the Plateau Beds which form such striking peaks as Pen y Fan and Sugar Loaf and the dramatic scarps of the Black Mountains and Black Mountain. There are restricted occurrences of Devonian rocks on Anglesey too.

Carboniferous
The Carboniferous period is represented by extensive outcrops across South Wales from the Wye Valley in the east, through the South Wales Coalfield, the Vale of Glamorgan and Gower westwards to southern Pembrokeshire. There are less extensive areas in northeast Wales and along the north coast into Anglesey where similarly aged rocks characterize the landscape. The sequence includes Carboniferous Limestone at its base, followed by coarse sandstones (The ‘Millstone Grit’ of the north and the ‘Twrch Sandstone’ of the south), then mudstones and finally the Coal Measures which comprise a thick succession of mudstones, sandstones and of course coal seams.

Limestone
Though mid Wales lay above sea level during Carboniferous times, shallow tropical seas extended across much of north and south Wales and it was in these environments that a succession of types of limestone were deposited.

The limestone gives rise to impressive cliffed landscapes both on the coast as at the Great Orme in the north and at St Govan’s Head and the Gower Peninsula in the south, and inland at the escarpments of Eglwyseg Mountain near Llangollen and Llangattock hillside in the Usk Valley. Karst landscapes characterize the limestone outcrop and, particularly along the ‘north crop’ of the South Wales Coalfield basin, where the limestone is shallowly buried beneath adjacent sandstones, extensive development of solution hollows has taken place.

The Namurian sequence
The Namurian age mudstones and sandstones which overlie/succeed the limestone give rise to rugged landscapes; typically either poorly drained moorland or else wooded gorges as in the Waterfall Country of southern Powys.

Coal Measures
Landscapes developed over Coal Measures rocks are extensively altered by man, as the coal and iron found within this thick sequence of rocks have long been economically important, particularly since the Industrial Revolution. The former coalfields of Flintshire, Denbighshire and South Wales are witness to this period. In South Wales, the Coal Measures are overlain by the thick sandstones of the Pennant Measures which often provides craggy edges to the plateau which has been deeply dissected to form the South Wales Valleys. The Pennant Sandstone is widely used as a building stone.

Permian
Rocks dating from the Permian period occur in North Wales underlying the Vale of Clwyd and extend into the northeastern fringes of the country from the larger bodies of such rock in Cheshire and Shropshire. The Permo-Triassic rocks in the northeast of Wales are largely concealed by recent material deposited by rivers and the Devensian icesheet. Thicker sequences of Permo-Triassic rock are known to underlie the Bristol Channel, Cardigan Bay and the Irish Sea off the North Wales coast.

Triassic
Triassic sandstones form much of the coast between Barry and Penarth and extend eastwards along the Monmouthshire coast to the English border at Chepstow. They also occur more sporadically further west between Ogmore-by-Sea and Kenfig. There are very localized occurrences of Triassic material in Gower and south Pembrokeshire, much of it breccia-fill of fissures in underlying Carboniferous Limestone. triassic sandstones are also found along the border with Cheshire and Shropshire, though moost often concealed beneath recent fluvial and glacially derived material.

Jurassic
Rocks dating from the Jurassic period occur widely across the Vale of Glamorgan. Much of the Glamorgan coast between Ogmore-by-Sea and Barry is formed by cliffs of layered Jurassic limestone known as the Lias. Extensive and spectacular shore platforms have been developed eg near Southerndown, as these cliffs have retreated inland through frequent rockfall. .

Cretaceous
There are no rocks of Cretaceous age in Wales but they are known to be present within the sub-sea basins off the Welsh coast eg Celtic Sea and the Bristol Channel.

Palaeogene and Neogene
There is scant evidence for rocks of the Palaeogene and Neogene periods in Wales though they are known to occur offshore. The exceptions are the sequence revealed in the Mochras borehole on the coastal strip south of Harlech and the 'pocket deposits' where sediments fill depressions in the Carboniferous Limestone in northeast Wales. In the south, poorly cemented sands occur sporadically in the central and western part of the Brecon Beacons National park; these are thought to be a product of the Cenozoic weathering of the underlying Twrch Sandstone.

Quaternary
The landscape of Wales has assumed its present shape over the last 2.6 million years ie during the Quaternary period which reaches to the present day. Icesheets and valley glaciers which developed during a series of ice ages have significantly altered a landscape which had developed as rivers drained a tilted upland surface which is thought to have emerged from beneath the sea during earlier Cainozoic times.

Glacial legacy
The effects of the last (Devensian) ice age are the most readily understood. An icesheet which at its maximum extent covered virtually all of Wales and reached as far south as Cardiff, Bridgend and Gower left in its wake suites of both erosional and depositional landforms. The glacial cirques of Snowdonia and to a lesser extent of the Cambrian Mountains and the Brecon Beacons are well known. Many pre-existing valleys were further deepened by glacial ice. Cirque moraines in the mountains and terminal and recessional moraines in the major valleys are the most striking depositional legacy of the glaciation. There are too, swarms of drumlins and a widespread plastering of glacial till elsewhere.

Coastal deposits
Following the end of the last ice age, sea levels rose to roughly their current levels by around 6000 years ago. Forests which had become established at or below this level were destroyed though the preserved stumps of trees in growth position can still be seen in the intertidal zone in places, as can the remains of peat desposits which again had originally formed above the high water mark. Redistribution of glacial and fluvial sands has given rise to extenisve dune systems around the Welsh coast, notably at Newborough Warren on Anglesey, Morfa Harlech and Morfa Dyffryn in Gwynedd and at Pendine and Pembrey Burrows in Carmarthenshire and at Merthyr-mawr Warren and Kenfig Burrows in Glamorgan.

Karstic landforms
Within the limestone areas of Wales, there have arisen karstic landscapes during the postglacial period, though elements of these were initiated during and even before successive ice ages. Limestone pavements are best developed along the 'north crop' within the Brecon Beacons National Park and there are numerous sinkholes and shakeholes, together with sinks, sections of dry valley and resurgences. Cave development is extensive and includes systems such as Ogof Ffynnon Ddu (Britain's deepest cave), Dan-yr-ogof (partly showcave) and Wales' most extensive system Ogof Draenen.

Metals
Man has mined metals and metal ores in Wales for millennia. There are copper workings on the Great Orme and at Parys Mountain on Anglesey. Gold has been obtained since pre-Roman times at places like Dolaucothi. Lead and zinc were intensively mined in the Cwmystwyth area of mid Wales.

Slate

 * Main article: Slate industry in Wales

The slate industry of Snowdonia was once of world importance. Purple and green slates of Cambrian age were worked at vast quarries on the flanks of Snowdon and at Bethesda, Dinorwic, Corris and Blaenau Ffestiniog.

Building stone
The abundance of hard rock in Wales means that it has found use in building since the earliest times. The 'bluestones' which form the lintels of Stonehenge were sourced in the Preseli Hills of pembrokeshire. Bronze Age and Iron Age peoples made extensive use of local materials in erecting a variety of cairns, standing stones and defensive works as manifest in Wales' many hill forts.

Sand and gravel
Deposits of glacial and fluvial sands and gravels have been and continue to be worked in numerous areas, principally for the construction industry.