The BGS Lexicon of Named Rock Units — Result Details

Fachwen Formation

Computer Code: FA Preferred Map Code: Fa
Status Code: Full
Age range: Ediacaran Period (AD) — Comley Series (EC)
Lithological Description: Conglomerates, sandstones and thin volcanic tuffs at base passing up into siltstones and sandstones.
Definition of Lower Boundary: Unconformity on ash-flow tuffs of the Padarn Tuff Formation (Arfon Group).
Definition of Upper Boundary: Overlain conformably by mudstones of the Llanberis Slates Formation.
Thickness: 40 - 50 m thick.
Geographical Limits: Northern Snowdonia.
Parent Unit: Harlech Grits Group (HGG)
Previous Name(s): Tryfan Grit Group (-4645)
Tryfan Grit Group (A) [Obsolete Name And Code: Use FA] (ACTG)
Tryfan Grit Group (B) [Obsolete Name And Code: Use FA] (ACTV)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Fachwen. 
Reference(s):
Reedman, A J, Leveridge, B E and Evans, R B. 1984. The Arfon Group (Arvonian?) of North Wales. Proceedings of the Geologists' Association, Vol.95, 313-321. 
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:
E119 E106 E118