Geology of Wales
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.
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.
South Wales has a written record of geological interest going back to the 12th century when Giraldus Cambrensis noted pyritous shales near Newport.
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.
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.
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.
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.
[1] 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.
[5] 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.
[6] 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.
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 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.
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.
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.
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.
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.
The most intensely affected rocks are those of south and central Pembrokeshire where steeply dipping and vertical strata are commonplace and multiple folding, faulting and overthrusting are well seen in coastal sections.
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.
An ice sheet 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.
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 deposits which again had originally formed above the high-water mark.
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.
Purple and green slates of Cambrian age were worked at vast quarries on the flanks of Snowdon and at Bethesda, Dinorwig, Corris and Blaenau Ffestiniog.
The contamination of land, groundwater and watercourses is a risk in areas where mineral exploitation has taken place, notably in the Central Wales Orefield and in former coal mining districts.
In addition, two extensive areas are designated as Geoparks; the entire island of Anglesey as 'GeoMôn' in the north and Fforest Fawr within the Brecon Beacons National Park in the south.
Local authorities and other agencies in the public sector continue to be involved in the acquisition of significant earth heritage sites, their conservation and interpretation.
