The BGS Lexicon of Named Rock Units — Result Details

Craig-Y-Llam Group [Obsolete Name And Code: See CYLF]

Computer Code: CYLL Preferred Map Code: notEntered
Status Code: Index Level
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Rhyolitic and acid-keratophyric lavas and tuffs with sporadic slates. (Correlative of Craig-y-Llam Group = Upper Acid Volcanic Group).
Definition of Lower Boundary: Drawn at base of lavas and tuffs of Craig-y-Llam Group where they rest conformably on the Llyn Cau Mudstones of Llandeilo age.
Definition of Upper Boundary: Drawn at conformable upward passage from ash beds of the Craig-y-Llam Group to the Talyllyn Mudstones of Caradoc age.
Thickness: Approx. 270-450 metres.
Geographical Limits: Mid-Wales.
Parent Unit: Not Entered (?)
Previous Name(s): none recorded or not applicable
Alternative Name(s): Upper Acid Volcanic Group
Stratotypes:
Type Area  Craig-y-Llam, East of Cader Idris, Gwynedd. 
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. 
Wells, A K. 1925. The geology of the Rhobell Fawr district (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, 463-538. 
Pugh, W J. 1923. The geology of the district around Corris and Aberllefenni, (Merionethshire). Quarterly Journal of the Geological Society, London, Vol.79, 508-541. 
Cox, A H and Wells, A K, 1916. The Ordovician sequence in the Cader Idris district (Merioneth). British Association for the Advancement of Science Report for 1915 (Manchester), p.424. 
Pugh, W J. 1928. The geology of the district around Dinas Mawddwy (Merioneth). Quarterly Journal of the Geological Society of London. Vol.84, 345-381. 
Davies, R G, 1959. The Cader Idris Granophyre and its associated rocks. Quarterly Journal of the Geological Society, London. Vol.115, 189-216. 
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. 
Pugh, W J, 1929. The geology of the district between Llanymawddwy and Llanuwchllyn (Merioneth). Quarterly Journal of the Geological Society, Vol.85, p.248. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable