The Aberystwyth Grits Group is a

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 Paleozoi ...

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 ide ...

(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 e ...

) in mid

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

. The name is derived from Aberystwyth in northern

Ceredigion Ceredigion ( , , ) is a county in the west of Wales, corresponding to the historic county of Cardiganshire. During the second half of the first millennium Ceredigion was a minor kingdom. It has been administered as a county since 1282. C ...

where the

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 e ...

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 ( cy, Bae Ceredigion) 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. Geo ...

coast both to the east and west of

New Quay New Quay ( cy, Cei Newydd) is a seaside town (and electoral ward) in Ceredigion, Wales, with a resident population of around 1,200 people, reducing to 1,082 at the 2011 census. Located south-west of Aberystwyth on Cardigan Bay with a harbour ...

and northwards through

Aberaeron Aberaeron, previously anglicised as Aberayron, is a town, community, and electoral ward between Aberystwyth and Cardigan, in Ceredigion, Wales. Ceredigion County Council offices are in Aberaeron. The name of the town is Welsh for ''mouth of th ...

and Aberystwyth to just short of

Borth Borth ( cy, Y Borth) is a village and seaside resort in Ceredigion, Mid Wales, 7 miles (11 km) north of Aberystwyth on the Ceredigion Coast Path. The community includes the settlement of Ynyslas. The population was 1,399 in 2011. From be ...

. 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 and

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 silicates ...

turbidite A turbidite is the geologic 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 Turbidites w ...

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 Midla ...

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