The Welsh Basin was a northeast-southwest aligned back-arc depositional basin during the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

, Ordovician and

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...

periods during which a considerable thickness of marine

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

s was laid down in the area. To the southeast lay the Midland Platform (a part of the micro-continent of Avalonia) and to the northwest, within the Iapetus Ocean, through what is now

Ireland Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...

and the Lake District, was an island arc; a northeast-southwest aligned Irish Sea landmass which was associated with volcanic activity. Examination of the sediments and associated fossils allows the deeper centre of the basin to be distinguished from shallower 'platform' areas along its southeastern margins. From the middle Silurian onwards, collision of Avalonia with the more northerly continent of

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

occurred giving rise to the Caledonian Orogeny. The

inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...

of the basin occurred at that time i.e. its uplift and deformation. The southeastern limit of the Welsh Basin can be defined as the Welsh Borderland Fault Zone comprising the Pontesford-Linley Fault and Church Stretton Fault which stretch from

Pembrokeshire Pembrokeshire ( ; cy, Sir Benfro ) is a county in the south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and the rest by sea. The county is home to Pembrokeshire Coast National Park. The Park oc ...

Shropshire Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to ...

and, in the northwest, the Menai Strait Fault Zone. Its extent to the southwest and northeast is uncertain as these areas are cloaked by more recent rocks. Siliciclastic material was derived from the landmass to its southeast

Pretannia The name Britain originates from the Common Brittonic term ''*Pritanī'' and is one of the oldest known names for Great Britain, an island off the north-western coast of continental Europe. The terms Briton and British, similarly derived, refer t ...

, some being deposited in the shallower margins before subsequently being redeposited as turbidites as the sediment mass became unstable from time to time. In addition to the

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s of the Welsh Basin, there are a number of volcanic formations within the overall sequence, particularly within the

Ordovician system The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

. Typical of the sediments of the Welsh Basin are the Llandovery age Aberystwyth Grits which outcrop along the middle coastal section of Cardigan Bay and comprise alternating

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...

s and turbiditic

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

s.Toghill, P. 2000 ''The Geology of Britain: an introduction'' Airlife Publishing.

References

{{reflist Geology of Wales