The Aberystwyth Grits Group is a

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

(a sequence of

rock strata In geology and related fields, a stratum (: strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ...

) in mid

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

. The name is derived from

Aberystwyth Aberystwyth (; ) is a University town, university and seaside town and a community (Wales), community in Ceredigion, Wales. It is the largest town in Ceredigion and from Aberaeron, the county's other administrative centre. In 2021, the popula ...

in northern

Ceredigion Ceredigion (), historically Cardiganshire (, ), is a Principal areas of Wales, county in the West Wales, west of Wales. It borders Gwynedd across the River Dyfi, Dyfi estuary to the north, Powys to the east, Carmarthenshire and Pembrokeshire t ...

where the

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

are well exposed in coastal cliffs. The Group comprises the Trefechan Formation and the underlying Mynydd Bach Formation. The rocks of the Aberystwyth Grits Group have also previously been known as the Aberystwyth Grits Formation

Outcrops

These rocks are exposed, along the

Cardigan Bay Cardigan Bay () is a large inlet of the Irish Sea, indenting the west coast of Wales between Bardsey Island, Gwynedd in the north, and Strumble Head, Pembrokeshire at its southern end. It is the largest bay in Wales. Geography Cardigan Bay ha ...

coast both to the east and west of

New Quay New Quay () is a seaside town, community and electoral ward in Ceredigion, Wales; it had a resident population of 1,045 at the 2021 census. Located south-west of Aberystwyth, on Cardigan Bay with a harbour and large sandy beaches, the town ...

and northwards through Aberaeron and Aberystwyth to just short of

Borth Borth () is a village, seaside resort and community in Ceredigion, Mid Wales; it is located 7 miles (11 km) north of Aberystwyth, on the Ceredigion Coast Path. The community includes the settlement of Ynyslas and the population was 1,399 ...

. They extend eastwards, though are usually obscured, in a belt of country between roughly 5 km and 10 km wide reaching from the coast. Much of the eastern boundary is defined by the Bronnant Fault.

Lithology and stratigraphy

The Group comprises several hundred metres thickness of

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.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

and

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

s laid down in the marine

Welsh Basin The Welsh Basin was a northeast-southwest aligned back-arc depositional basin during the Cambrian, Ordovician and Silurian periods during which a considerable thickness of marine sediments was laid down in the area. To the southeast lay the Midlan ...

during the Llandovery epoch of the Silurian period. Palaeocurrents indicating a source in the southwest have been recorded from throughout the group.

References

* {{Refend Silurian System of Europe Geology of Wales Geological groups of the United Kingdom