The BGS Lexicon of Named Rock Units — Result Details

Craig Y Llam Formation

Computer Code: CYLF Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Massive, ash-flow tuffs, locally lapilli-rich, with subordinate rhyolites, mudstone debris flows and tuffaceous sediments.
Definition of Lower Boundary: A well exposed transition from disturbed or bedded mustones of the Tyr Gawen Formation to massive, locally nodular ash-flow tuffs. Contact disrupted in places by loading and fluidisation of underlying mudstone.
Definition of Upper Boundary: Well exposed. Transition from massive tuff, rhyolite or tuffaceous siltstones into mudstone. Contact placed where mudstone becomes dominant.
Thickness: 0-300m
Geographical Limits: Extends westwards from Cadair Idris as far as Ffridd Cocyn (6250 0450) where it dies out.
Parent Unit: Aran Volcanic Group (AVG)
Previous Name(s): Craig Y Llam Group (-3344)
Upper Acid Group (-1470)
Craig-Y-Llam Group [Obsolete Name And Code: See CYLF] (CYLL)
Upper Acid Volcanic Group [Obsolete Name And Code: See CYLF] (UAV)
Alternative Name(s): none recorded or not applicable
Stratotypes:
none recorded or not applicable
Reference(s):
Cox, A H, and Wells, A K, 1921. The Lower Palaeozoic rocks of the Arthog-Dolgellau district (Merionethshire). Quarterly Journal of the Geological Society, London, Vol.76, p. 254-324. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable