The Cosheston Group is an early Devonian

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 igneou ...

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 west

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

. 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 communi ...

in south Pembrokeshire. The Group comprises (in ascending order) the Llanstadwell, Burton Cliff, Mill Bay, Lawrenny Cliff and New Shipping formations. The strata are exposed in the Milford Haven area of southern Pembrokeshire where several partial

type section A stratotype or type section in geology 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 stan ...

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 Rosemarket is a village, parish and community in Pembrokeshire, Wales, north of Milford Haven. Name The name does not refer to flowers but to the hundred of Roose, the former Welsh cantref of Rhos. History The village was a marcher borough ...

and Lawrenny, and extends east to New Shipping and west almost to the town of Milford Haven 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 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 ...

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

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 Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

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