The BGS Lexicon of Named Rock Units — Result Details

Cwm Eigiau Formation

Computer Code: CEI Preferred Map Code: CEi
Status Code: Pending Upgrade
Age range: Soudleyan Substage (OY) — Longvillian Substage (OL)
Lithological Description: Sandstones with siltstones and silty mudstones, tuffs and tuffites.
Definition of Lower Boundary: Conformably overlies the rhyolitic ash-flow tuffs of the Capel Curig Volcanic Formation (Llewelyn Volcanic Group) or, where the latter are not present, it overlies mudstones of the Nant Ffrancon Subgroup.
Definition of Upper Boundary: Overlain by ash-flow tuffs of the Lower Rhyolitic Tuff Formation (Snowdon Volcanic Group) and (on the Lleyn Peninsular) the Allt Fawr Rhyolitic Tuff Formation.
Thickness: 450 - 1300m
Geographical Limits: Not yet provided.
Parent Unit: Not Applicable (-)
Previous Name(s): Carn-Y-Parc Formation (-2680)
Carn Y Parc Formation [Obsolete Name And Code: Use CEI] (CYP)
Blaen Eidda Member [Obsolete Name And Code: Part Of CEI] (BEI)
Eidda Formation [Obsolete Name And Code: Use CEI] (EI)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Pont y Gromlech. Howells, M F, Leveridge, B E, Evans, C D R and Nutt, M J C. 1981. 
Type Section  Nantmor Valley, southeast side. Orton, G J. 1988. 
Reference Section  Northeast of Capel Curig. 
Reference(s):
Orton, G J. 1988. A spectrum of Middle Ordovician fan deltas and braidplain deltas, North Wales: a consequence of varying fluvial clastic input. 23-49 in Nemac, W and Steel, R J (editors), Fan Deltas: Sedimentology and Tectonic Settings. [Glasgow: Blackie.] 
Howells, M F, Leveridge, B E, Evans, C D R and Nutt, M J C, 1981. Classical areas of British Geology. Dolgarrog. Description of 1:25 000 Sheet SH76. London: HMSO for Institute of Geological Sciences. 
British Geological Survey. 1993. Corwen, provisional. England and Wales sheet 120. Solid and drift, 1:50 000 (Southampton: Ordnance Survey for British Geological Survey.) 
Howells, M F and Smith, M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales). (London: The Stationery Office). 104pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E106 E094 E120 E107 E134