The BGS Lexicon of Named Rock Units — Result Details

Craig Cau Formation

Computer Code: CGU Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Felsic ash-flow tuffs interbedded with subordinate tuffites and mudstones in the lower part of the formation, but becoming massive and welded in the upper part. Associated intrusive and extrusive rhyolites.
Definition of Lower Boundary: Conformably overlies and laterally inter-tongues with the Ty'r Gawen Mudstone Formation in the south and west of the Cadair Idris district, and conformably overlies crystal tuffs of the Benglog Formation in the northeast, where its base is less clearly defined.
Definition of Upper Boundary: Conformably overlain by mudstones of the Ceiswyn Formation.
Thickness: 0 to 400m
Geographical Limits: Cadair Idris area, North Wales.
Parent Unit: Aran Volcanic Group (AVG)
Previous Name(s): Upper Acid Volcanic Group [Obsolete Name And Code: See CYLF] (UAV)
Craig Y Lam Group (-2076)
Perfeddnant Ashes (-2077)
Craig Y Lam Formation (-773)
Alternative Name(s): Craig Y Ffynnon Formation
Aran Fawddwy Formation
Llyn Conwy Formation
Pistyllion Formation
Stratotypes:
Type Section  Cliff section immediately north and east of the southern Minfordd Path, Cadair Idris, south and west of Llyn Cau. Pratt, W T, Woodhall, D G and Howells, M F. 1995. 
Partial Type Section  The crags forming Bird's Rock (Craig yr Aderyn). Pratt, W T, Woodhall, D G and Howells, M F. 1995. 
Reference(s):
Cox, A H and Wells, A K, 1927. The geology of the Dolgelley district, Merionethshire. Proceedings of the Geologists' Association, Vol.38, 265-318. 
Cox, A H. 1925. The geology of the Cadair Idris Range, (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, p.539-592. 
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. 
Pratt, W T, Woodhall, D G and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149. (England and Wales). (London: HMSO), 111pp 
Ridgeway, J. 1975. The stratigraphy of Ordovician volcanic rocks around the south and east of the Harlech Dome in Merionethshire, North Wales. Geological Journal, Vol.10, 87-106. 
Zalasiewicz, J A. 1992. Graptolite dating of Ordovician vulcanicity in the Arenig area, North Wales. Geological Journal, Vol.27, 379-389. 
Jehu, R M. 1926. The geology of the district around Towyn and Abergynolwyn (Merioneth). Quarterly Journal of the Geological Society of London, Vol.82, 465-489. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149 E136 E135 E150