Geology of Wales

Wales is a peninsula in the south-west of the island of Great Britain. The entire area of Wales is about 20,779 km² (8,023 square miles). It is about 274 km (170 miles) north-south and 97 km (60 miles) east-west. Wales is bordered by England to the east and by sea in the other three directions: the Bristol Channel to the south, St George's Channel to the west, and the Irish Sea to the north. Together, Wales has over 1,200km (750 miles) of coastline. There are several islands off the Welsh mainland, the largest being Ynys Môn (Anglesey) in the north west.

Wales is mountainous, particularly in the north and central regions. The mountains were shaped during the last ice age, the Devensian glaciation. The highest mountains in Wales are in Snowdonia ("Eryri"), and include Snowdon ("Yr Wyddfa"), which, at 1085 m (3,560 ft) is the highest peak in Wales. The Brecon Beacons ("Bannau Brycheiniog") are in the south and are joined by the Cambrian Mountains in mid-Wales, the latter name being given to the earliest geological period of the Paleozoic era, the Cambrian.

In the mid-nineteenth 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 much dispute, the next two periods of the Paleozoic era, the Ordovician and Silurian, were named after ancient Celtic tribes from this area.

Tectonics of Avalonia

Avalonia was an ancient microcontinent or terrane whose history formed much of the older rocks of Western Europe. The name is derived from the Avalon Peninsula in Newfoundland. Wales was entirely contained within the Avalonian block, and shares its tectonic chronology.

In the early Cambrian, the supercontinent Pannotia broke up and Avalonia drifted off northwards from Gondwana. This independent movement of Avalonia started from a latitude of about 60° South. The eastern end of Avalonia collided with Baltica, a continental plate occupying the latitudes from about 30°S to 55°S, as the latter slowly rotated anticlockwise towards it. This happened at the end of the Ordovician and during the early Silurian.

In the late Silurian and lower Devonian, the combined Baltica and Avalonia collided progressively, with Laurentia, beginning with the long extremity of Avalonia which is now attached to America. The result of this was the formation of Euramerica. At the completion of this stage, the site of Britain was at 30°S and Nova Scotia at about 45°S. This collision is represented by the Caledonian folding or in North America as an early phase in the Acadian orogeny.

In the Permian, the new continent and another terrane, Armorica which included Iberia, drifted in from Gondwana, trapping Avalonia between it and the continent so adding Iberia/Armorica to Euramerica. This was followed up by the arrival of Gondwana. The effects of these collisions are seen in Europe as the Variscan folding. In North America it shows as later phases of the Acadian orogeny. This was happening at around the Equator during the later Carboniferous, forming Pangaea in such a way that Avalonia was near its centre but partially flooded by shallow sea.

In the Jurassic, Pangaea split into Laurasia and Gondwana, with Avalonia as part of Laurasia. In the Cretaceous, Laurasia broke up into North America and Eurasia with Avalonia split between them.

Regions of Wales

North Wales - Snowdonia may be divided into four areas. The northernmost area includes (west to east): Moel Hebog, Mynydd Mawr and the Nantlle Ridge; the Snowdon massif; the Glyderau, and the Carneddau. These last three groups are the highest mountains in Wales, and include all Wales' 3000-foot mountains.

The second area includes peaks such as Moel Siabod, Cnicht, the Moelwynion, and the mountains around Blaenau Ffestiniog.

The third area includes the Rhinogydd in the west as well as the Arenig and the Migneint (this last being an area of bog).

The southernmost area includes Cadair Idris, the Tarren range, and the Aran group, including Aran Fawddwy, the highest mountain in the United Kingdom south of Snowdon.

Snowdon and its vicinity form a syncline of Ordovician rocks which are partly volcanic. Snowdon itself is largely formed of volcanic ash (tuff) with some sedimentary rock and igneous intrusions. Cadair Idris is also largely formed of Ordovician igneous rocks. The Harlech Dome, which includes the Rhinogydd, is formed of Cambrian gritstones and mudstones.

Elsewhere in North Wales, Pre-Cambrian rocks occur in Anglesey, while the Berwyn range consists mainly of Ordovician sedimentary strata. Carboniferous Limestone occurs north of Llangollen and the Coal Measures near Wrexham.

Ordovician and Silurian sediments folded in the Caledonian Orogeny cover much of Wales, north and south.

South Wales has a written record of geological interest going back to the 1100s 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 coal measures near Neath (Gibson late 1600s). Adam Sedgwick and Roderick Murchison did fundamental work on Old Red Sandstone and the underlying rocks.

Several successive periods are represented. Pembrokeshire has outcrops of both Pre-Cambrian and Cambrian rocks. A notable feature of the Ordovician system is a major downwarp known as the Welsh geosyncline. Silurian rocks, largely mudstones and siltstones, are widely distributed in South Wales and are well displayed on the Cardiganshire coast. Carboniferous Limestone outcrops occur in south Pembrokeshire, Gower, the Vale of Glamorgan and the north and east sides of the coalfield.

One of the geological rarities of South Wales is the coastline of Ogmore-by-Sea and Southerndown. Its cliffs are composed of Sutton stone; a very rare freestone that is a banded mixture of lias limestone which contains large elements of Carboniferous limestone. Sutton stone has always been highly regarded: as well as being used in construction throughout the Vale of Glamorgan, it was also shipped over the Bristol Channel to North Devon and North Cornwall which are both deficient in limestone.

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 Geological Series (or epoch) 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.

The Brecon Beacons are a mountain range located in the south-east of Wales, forming the nucleus of the Brecon Beacons National Park (Parc Cenedlaethol Bannau Brycheiniog). The range consists of the mountains to the south of Brecon. The highest of these is Pen y Fan (886 m); other notable summits include Corn Du (873 m), Cribyn (795 m), and Fan y Bîg (719 m). These summits form a long ridge which forms a horseshoe around the head of the Taf Fechan river to the south-east, with long parallel spurs extending to the north-east. The Brecon Beacons are made of Devonian Old Red Sandstone.

Cefn Bryn ( _en. Ridge Hill) is a convert|5|mi|km|sing=on long Old Red Sandstone ridge in the heart of the Gower Peninsula. The highest point on the ridge (186m) is the second highest point in Gower. The grassland on the ridge is known as "Cefn Bryn Common".

Cefn-cerrig Road near Cefn-cerrig Farm, Llandovery, Wales, 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. The GSSP was ratified in 1984.

The boundary is defined as a point immediately above the highest record of the brachiopod "Eocoelia intermedia" and below the first appearance of the succeeding species "Eocoelia curtisi". The boundary also corresponds to the incoming of the acritarchs "Deunffia monospinosa", "Domasia bispinosa" and "Pterospermella". The section, part of the Wormwood Formation, is sandstone and siltstone.

Trefawr Track, a forestry road north of Cwm-coed-aeron Farm, Llandovery, Wales, is the location of the Global Boundary Stratotype Section and Point (GSSP) which marks the boundary between the Rhuddanian and Aeronian stages of the Silurian period on the geologic time scale. The GSSP was ratified in 1984.

The boundary is defined as the first appearance of the graptolite "Monograptus austerus sequens" (the base of the "Monograptus triangulatus" biozone). The section is primarily mudstone, which yields an abundance of shelly faunas. The graptolite "Diplograptus elongatus" occurs immediately below the boundary.

References

* [http://www.eryri-npa.co.uk/ Snowdonia National Park Authority]
* [http://www.breconbeacons.org/ Official site of the Brecon Beacons National Park Authority]
* [http://www.swansea-gower.co.uk/placestovisit/broadpool.htm The Gower Information Center: Broad Pool]
*
*
* John L. Morton, "King of Siluria — How Roderick Murchison Changed the Face of Geology" (Brocken Spectre Publishing, 2004, ISBN 0-9546829-0-4)
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* Martin J. S. Rudwick, "The Great Devonian Controversy: The Shaping of Scientific Knowledge among Gentlemanly Specialists" (University of Chicago Press, 1985) — the rise of Murchison to power
* James A. Secord, "Controversy in Victorian Geology: The Cambrian-Silurian Dispute" (Princeton University Press, 1986) — documents the battle between Murchison and Adam Sedgwick

ee also

* Geology of the British Isles
* Geology of England
* Iceland - a microcontinent

External links

* [http://palaeos.com/Earth/Geography/Avalonia.htm Article about Avalonia at Palaeos.Com] (licensed under Attribution-NonCommercial 1.0 complete with pictures)
* [http://www.dinodata.net/Golonka/phan.pdf The context of Avalonia's movements] (not available without registration)