The Cosheston Group is an early

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

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 west

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 the village of Cosheston in south

Pembrokeshire Pembrokeshire ( ; ) is a Principal areas of Wales, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and otherwise by the sea. Haverfordwest is the largest town and ...

. The Group comprises (in ascending order) the Llanstadwell, Burton Cliff, Mill Bay, Lawrenny Cliff and New Shipping formations. 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 exposed in the

Milford Haven Milford Haven ( ) is a town and community (Wales), community in Pembrokeshire, Wales. It is on the north side of the Milford Haven Waterway, an estuary forming a natural harbour that has been used as a port since the Middle Ages. The town was ...

area of southern Pembrokeshire where several partial

type section In geology, a stratotype or type section is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the ...

s are defined. The outcrop extends around the northern and southern shores of the Haven. It is bounded to the north by the Benton Fault between the villages of Rosemarket and Lawrenny, and extends east to New Shipping and west almost to the town of

itself. The rocks of this group have also previously been known as the Cosheston Beds.

Stratigraphy

Llanstadwell, Burton Cliff and Mill Bay formations

This sequence is largely green

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 ...

s with lesser amounts of red sandstone and including intraformational conglomerates and some

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

Lawrenny Cliff and New Shipping formations

These are similar to the underlying formations but are characterised by a coarser grain size. Conglomerates in these two formations have a great diversity of clasts within them.Howells, M.F. 2007 ''British Regional Geology: Wales'' (Keyworth, Nottingham British Geological Survey)

References

{{Reflist Devonian System of Europe Geology of Wales Geological groups of the United Kingdom Sandstone groups Siltstone groups Conglomerate groups