The BGS Lexicon of Named Rock Units — Result Details

Crafnant Volcanic Group [Obsolete Name And Code: See SV]

Computer Code: CV Preferred Map Code: notEntered
Status Code: Index Level
Age range: Longvillian Substage (OL) — Woolstonian Substage (OS)
Lithological Description: Ashflow tuffs with slates, siltstones, limestones and hyaloclastites, alternating thin acid tuffs, tuffites, siltstones and mudstones, and ill sorted pyroclastic deposits with argillaceous debris.
Definition of Lower Boundary: Drawn at the base of the Lower Crafnant Volcanic Formation where ashflow tuff conformably rests upon coarse often pebbly greywackes, regularly bedded and commonly crossbedded of the Carneddau Group.
Definition of Upper Boundary: Drawn at the conformable upward passage of ashflow tuffs with slates, siltstones, limestones and hyaloclastites and conformably overlain by black and grey slates and mudstones of the Llan Rhychwyn Slates.
Thickness: Up to 270 metres.
Geographical Limits: Occupying high ground south of the Afon Llugwy, between Clogwyn-Mawr in the west and Llyn-y-Parc in the east.
Parent Unit: Not Entered (?)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Llyn Crafnant district, near Trefriw, Caernarvonshire. 
Reference(s):
Howells, M F, Francis, E H, Leveridge, B E and Evans, C D R, 1978. Classical areas of British Geology. Capel Curig and Betws-y-Coed. Description of 1:25 000 Sheet SH75. London: HMSO for Institute of Geological Sciences. 
Davies, D A B. 1936. The Ordovician rocks of the Trefriw district (North Wales). Quarterly Journal of the Geological Society. Vol.92, 64-65,70-77. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable