Cosheston Group

Cosheston Group Stratigraphic range: Lochkovian to Emsian (Devonian)
Type	Group
Unit of	Old Red Sandstone Supergroup
Sub-units	New Shipping Formation, Lawrenny Cliff Formation, Mill Bay Formation, Burton Cliff Formation, Llanstadwell Formation
Underlies	Avon Group (unconformity)
Overlies	Gelliswick Bay Formation
Thickness	up to 1800m
Lithology
Primary	sandstones, siltstones
Other	conglomerates
Location
Country	Wales
Extent	south Pembrokeshire
Type section
Named for	Cosheston

The Cosheston Group is an early Devonian lithostratigraphic group (a sequence of rock strata) in west Wales. The name is derived from the village of Cosheston 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 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 between the villages of Rosemarket and Lawrenny, and extends east to New Shipping and west almost to the town of Milford Haven itself.^[1] The rocks of this group have also previously been known as the Cosheston Beds.^[2]

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 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 within them.^[3]

References

1. British Geological Survey 1:50,000 scale geological map (England & Wales) sheet 228 Haverfordwest
2. http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=COB (BGS on-line lexicon of rock units)
3. Howells, M.F. 2007 British Regional Geology: Wales (Keyworth, Nottingham British Geological Survey)