The Cosheston Group is an early

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

lithostratigraphic group (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 ei ...

) in west Wales. The name is derived from the village of

Cosheston Cosheston is a village, parish and community in Pembrokeshire, Wales. It is situated on an inlet of the Daugleddau estuary, 3 km north-east of Pembroke. The parish includes the settlement of Bateman's Hill. The northern part of the communit ...

in south

Pembrokeshire Pembrokeshire ( ; cy, Sir Benfro ) is a Local government in Wales#Principal areas, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and the rest by sea. The count ...

. 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 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 ei ...

are exposed in the

Milford Haven Milford Haven ( cy, Aberdaugleddau, meaning "mouth of the two Rivers Cleddau") is both a town and a community in Pembrokeshire, Wales. It is situated on the north side of the Milford Haven Waterway, an estuary forming a natural harbour that has ...

area of southern Pembrokeshire where several partial type sections are defined. The outcrop extends around the northern and southern shores of the Haven. It is bounded to the north by the

Benton Fault The Benton Fault is a geological fault in southern Pembrokeshire in Wales that was active as a normal fault during the deposition of the Old Red Sandstone during the Devonian period, forming a thick half graben. During the Carboniferous The ...

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 sandstones 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.Blatt ''et al.'' 1980, p ...

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