The BGS Lexicon of Named Rock Units — Result Details

Plynlimon Glacigenic Formation

Computer Code: PLYNT Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: Generally stiff, blue-grey, stony, sandy and silty clay and clayey gravel, characteristically containing clasts of Lower Palaeozoic rocks (derived from the Cambrian Mountains). Subordinate stratified sand and gravel are locally present. Colour and dominant clast composition are variable and generally reflect the local bedrock geology; in central and west Wales it is generally grey and the clasts are exclusively Lower Palaeozoic. In Broughton Bay it contains shell fragments (Bowen, 1999).
Definition of Lower Boundary: Generally unconformable on bedrock or regolith/head (e.g. Davies et al., 1997). At Broughton Bay it unconformably overlies Ipswichian raised beach gravels and head (Campbell and Bowen, 1989, 52-54) assigned to the Pennard Formation by Bowen (1999)
Definition of Upper Boundary: Generally the ground surface or contact with overlying peat, head (Severn and Avon and West Wales catchments subgroups), Kenfig Formation or Ynyslas Formation (British Coastal Deposits Group). Near Llanrhystud it is overlain by the Lleyn Till Member of the St Asaph Glacigenic Formation (McMillan et al., in prep.)(Lleyn Formation of Campbell and Bowen, 1999; Davies et al., 1997) and in the Shropshire lowlands by the Shrewsbury Glacigenic Formation (Thomas, 1989).
Thickness: Highly variable, locally in excess of 30m.
Geographical Limits: Denbighshire, Mid and west Wales, southwest Wales approximately north of Black Mountain and east of St Clears and Llandysul
Parent Unit: Wales Glacigenic Subgroup (WALES)
Previous Name(s): Welsh Till (-3232)
Central Wales Drift (-4636)
Alternative Name(s): Merion Formation
Elenid Formation
Llanmadoc Member
Stratotypes:
Type Section  Coastal section between Morfa Bychan and Mynachdy'r-graig in Cardigan Bay (Ceredigion) exposing up to 40m of drift overlying and blanketed against, a pre-Devensian "fossil cliff". A basal Head deposit is sharply overlain by up to 30m of Plynlimon Glacigenic Formation exhibiting evidence of solifluction, itself sharply overlain and overstepped by Head and Loess. Davies et al. 1997. 
Reference(s):
Campbell, S and Bowen, D Q. 1989. Quaternary of Wales. Geological Conservation Review. (Peterborough: Nature Conservancy Council.) 
Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G and Zalasiewicz, J A, 1997. Geology of the country around Llanilar and Rhayader. Memoir for 1:50 000 Geological Sheets 178 and 179. 
Bowen, D Q, 1999. Wales. Chapter 7 in Bowen, D Q (Editor), A revised correlation of Quaternary deposits in the British Isles. Geological Society of London Special Report, No.23. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable