The BGS Lexicon of Named Rock Units — Result Details

Foel Fras Volcanic Complex

Computer Code: FFV Preferred Map Code: FFV
Status Code: Full
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Trachyandesite lavas, tuffs and intrusions. Tuffs may be reworked, massive and welded, or bedded and unwelded.
Definition of Lower Boundary: Overlies mudstones of the Nant Ffrancon Subgroup and locally on the Foel Grach Basalt and Braich Tu Du Formation. Interdigitates with the Conwy Rhyolite Formation.
Definition of Upper Boundary: Underlies siltstones of the Nant Ffrancon Subgroup.
Thickness: About 1000m. Outflow much thinner, of the order 100m.
Geographical Limits: Foel Fras (8km west-north-west of Dolgarrog), south-west to Nant Ffrancon.
Parent Unit: Llewelyn Volcanic Group (LLV)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Reference Section  Foel Fras hill and Bera Bach. 
Howells, M F, Reedman, A J and Campbell, S D G, 1991. Ordovician (Caradoc) marginal basin volcanism in Snowdonia (north-west Wales). (London: HMSO for the British Geological Survey). 
Howells, M F, Leveridge, B E, Addison, R and Reedman, A J. 1983. The lithostratigraphical division of the Ordovician underlying the Snowdon and Crafnant volcanic groups, North Wales. Report of the Institute of Geological Sciences, No. 83/1. 
Howells M F, Reedman A J and Leveridge B E. 1985. Geology of the country around Bangor: Explanation for 1:50 000 geological sheet 106 (England and Wales). British Geological Survey Sheet Explanation. [London: HMSO.] 34pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: