The Menevian Group is a Cambrian lithostratigraphic group (a sequence of rock strata) in west Wales. The name is derived from Menevia, the Roman name for the St Davids area north of St Brides Bay on Pembrokeshire’s west coast where the strata are well exposed in coastal cliffs. This rock succession has previously been known variously as the Menevian Series and Menevian Beds and largely ascribed to the British regional stratigraphic unit St David’s Epoch, though these terms are now obsolete.[1]
| Menevian Group | |
|---|---|
| Stratigraphic range: St Davids Epoch Cambrian | |
 Mudstones of the Menevia Formation exposed in Porth y Rhaw, west of Solva | |
| Type | Group |
| Sub-units | Upper, Middle, Lower (informal) |
| Underlies | Lingula Flags |
| Overlies | Solva Group |
| Thickness | about 230m |
| Lithology | |
| Primary | mudstones |
| Other | tuffaceous sandstone |
| Location | |
| Region | South West Wales |
| Country | Wales |
| Type section | |
| Named for | St Davids (Roman name for area) |
Outcrops
editThe rock succession is exposed, along the coast southeast of St Davids and in particular at Solva where the outcrop, though largely concealed, continues inland. Both western and eastern cliff sections at Porth-y-Rhaw west of Solva provide the type locality. Further outcrops, though less well exposed, occur northeast of Newgale and again in the Wolf's Castle area.[2]
Lithology and stratigraphy
editThe Group comprises over two hundred metres thickness of mudstones and sandstones together with a basal conglomerate. The sandstones have been laid down as turbidites. Some of the mudstones contain tuff horizons whilst many are pyritous and have been equated with the Clogau Formation which crops out near Harlech, both indicating that the floor of the Welsh Basin was oxygen-poor and the waters were becoming deeper.[3] Trilobite fossils are recorded throughout the sequence whilst the sandstones of the upper Menevian contain brachiopod fossils.[4]
References
edit- ^ "Menevian Group". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved Oct 11, 2013.
- ^ British Geological Survey 1:50,000 scale geological map (England & Wales) sheets 209 St David's, 210 Fishguard
- ^ Howells, M.F. (2007). British regional geology : Wales (1st ed.). Nottingham: British Geological Survey. p. 30. ISBN 978-085272584-9.
- ^ London, P.F. Rawson. The Geological Society (2006). Brenchley, P.J. (ed.). The geology of England and Wales (2. ed.). London: Geological Society Publishing. p. 37. ISBN 9781862392007.