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 of mountains, hills and coastal plains.
Wales's 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's 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.
History of geological study
South Wales has a written record of geological interest going back to the 12th century when Giraldus Cambrensis noted pyritous shales near Newport. George Owen in 1603 correctly identified the stratigraphic relationship between the Carboniferous Limestone and the Coal Measures. Some of the first published representations of fossils were those of fossil plants taken from the coal measures near Neath (Gibson late 17th century).
In the mid-19th century, two prominent geologists, Roderick Murchison and Adam Sedgwick used their studies of the geology of Wales to establish certain principles of stratigraphy and palaeontology. They did fundamental work on the Old Red Sandstone but are remembered more for their work on the lower Palaeozoic sequence. It was Sedgwick who established the Cambrian system and Murchison first described the Silurian, naming it for the ancient Silures tribe which occupied mid Wales. An overlap between the two systems as mapped led eventually to protracted dispute between the two erstwhile collaborators. After their deaths, Charles Lapworth erected the Ordovician system (again named for an ancient tribe of northwest Wales, the Ordovices), to account for the sequence of rocks at the heart of the controversy.
More recently two locations in mid Wales have been selected to globally define stages of the Silurian period. Cefn-cerig Road near Cefn-cerig Farm, Llandovery, is the location of the Global Boundary Stratotype Section and Point (GSSP) which marks the boundary between the Aeronian and Telychian stages of the Silurian period on the geologic time scale. Similarly Trefawr Track, a forestry road north of Cwm-coed-aeron Farm, Llandovery, is the location of the GSSP marking the boundary between the Rhuddanian and Aeronian stages. Both GSSPs were ratified in 1984. :See main articles on Cefn-cerig Road and Trefawr Track.
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's many terranes came together during the late Precambrian and the Palaeozoic era.
The Stanner-Hanter Complex on the English border comprises volcanic rocks around 700 million years old which puts them within the Cryogenian period.
Rocks found in a quarry near to the village Llangynog, Carmarthenshire, in 1977 contain some of the Earth's oldest fossils which date from the Ediacaran period, 564 million years ago, when Wales was part of the micro-continent Avalonia.
Palaeozoic
Cambrian Rocks of Cambrian age occur most extensively in an inlier in Merionethshire where the up-arched rocks of the Harlech Dome form the Rhinogydd. 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. Snowdon itself is largely formed of volcanic ash (tuff) with some sedimentary rock and igneous intrusions folded into a syncline. Cadair Idris is also largely formed of Ordovician igneous rocks. 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. A notable feature of the Ordovician system is a major downwarp known as the Welsh geosyncline.
Silurian Rocks dating from the Silurian period are, by one measure, the most significant for Wales's landscape; a greater percentage of the country's land area is directly 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,British Geological Survey 1999 Flint England and Wales sheet 108, 1:50,000 scale geological map and the Nantglyn Flags and Wenlockian Denbigh Grits which form the Denbigh Moors,British Geological Survey 1993 Corwen England and Wales sheet 120, 1:50,000 scale geological map Llantysilio Mountain and the Dee Valley around Llangollen.British Geological Survey 1994 Wrexham England and Wales sheet 121, 1:50,000 scale geological map (provisional) 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.British Geological Survey 1978/1976 Milford Haven & Haverfordwest England and Wales sheets 226/227 & 228, 1:50,000 scale geological map
The Caledonian orogeny A long and complex series of continental collisions known collectively as the Caledonian orogeny began in the Ordovician and continued through the Silurian into Devonian times. The effect in this area was to cause folding and faulting of the existing rock sequence, most particularly within the Welsh Basin, a process which intensified northwards. Parts of the sequence were subjected to low grade metamorphism, the most significant of which, from an economic point of view, would be the Cambrian and Ordovician slates of North Wales. The northeast to southwest 'grain' of much of the country was imparted at this time with a series of major fault zones persisting from that time to the present, some of which, particularly in the northwest, are believed to represent terrane boundaries. The siliciclastic deposits of the succeeding Devonian period represent in large part the rapid attrition of the extensive Caledonian Mountain belt created by the Caledonian collision.
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.
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 humans, 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.
A major geological feature of the Upper Carboniferous sub-period in South Wales is the South Wales Coalfield syncline. The rocks comprising this important area were laid down during the Westphalian Age approximately 314-308 million years ago (Ma), when climatic conditions were equatorial. This Westphalian succession includes a sequence with a thickness of more than 1800 m in the west. The Coal Measures were laid down on a low-lying waterlogged plain with peat mires immediately south of an ancient geological feature known as the Wales-London-Brabant High.
Variscan Orogeny