The BGS Lexicon of Named Rock Units — Result Details

Foel Grach Basalt Formation

Computer Code: FGB Preferred Map Code: FgB
Status Code: Full
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Basalt and basalt breccias.
Definition of Lower Boundary: Rests on part of the Nant Ffrancon Subgroup and locally on the Braich Tu Du Volcanic Formation.
Definition of Upper Boundary: Overlain by outflow of the Foel Fras Volcanic Complex and by part of the Nant Ffrancon Subgroup.
Thickness: 400m in north; c.180m to the south.
Geographical Limits: Carnedd Llewelyn to Carnedd Dafydd.
Parent Unit: Llewelyn Volcanic Group (LLV)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Carnedd Llewelyn. 
Reference(s):
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:
E106