The BGS Lexicon of Named Rock Units — Result Details

Offrwm Volcanic Formation

Computer Code: OF Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Llanvirn Series (OE) — Llanvirn Series (OE)
Lithological Description: Thick- to thin-bedded, locally welded, felsic ash-flow tuffs, subordinate rhyolitic lavas and dark grey mudstones.
Definition of Lower Boundary: Conformably overlies the siltstones and sandstones of the Allt Lwyd Formation or the conglomerates of its highest subdivision, the Aran Boulder Bed Member in the Harlech area and western part of the Cadair Idris district, but is disconformable on the Dol-cyn-afon, Dolgellau and Ffestiniog Flags formations in the south and east.
Definition of Upper Boundary: Conformably overlain by the Cregennen Formation (locally the Bryn Brith Member) over much of the Cadair Idris district, but unconformably overlain by the Ty'r Gawen Mudstone Formation (Fron Newydd Member) in the southwest. In the Harlech district it is variably overlain by the Cregennen, Melau and Benglog formations, with possible unconformity/disconformity.
Thickness: 0 to 290m
Geographical Limits: Cadair Idris and Harlech districts, North Wales.
Parent Unit: Aran Volcanic Group (AVG)
Previous Name(s): Mynydd-Y-Gader Group (-1064)
Beacon Hill Acid Group (-3593)
Mynydd Gader Volcanic Series (-4822)
Rhyolitic Series And Ashy Series (-4823)
Lower Acid Group (-432)
Upper Volcanic Series (-1101)
Lower Acid Volcanic Group (LAV)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Crags on the slopes around Foel Offrwm, North Wales. Allen, P M and Jackson, A A. 1985. 
Reference(s):
Pratt, W T, Woodhall, D G and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149. (England and Wales). (London: HMSO), 111pp 
Allen, P M and Jackson, A A. 1985. Geology of the country around Harlech. Memoir of the British Geological Survey, Sheet 135 with part of Sheet 149 [England and Wales]. [London: HMSO.] 112pp. 
Jones, B, 1933. The geology of the Fairbourne-Llwyngwril district, Merioneth. Quarterly Journal of the Geological Society, Vol.89, 145-171. 
Lake, P and Reynolds, S H. 1896. The Lingula Flags and igneous rocks of the neighbourhood of Dolgelly. Quarterly Journal of the Geological Society of London, Vol.52, 511-527. 
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. 
Cox, A H and Wells, A K, 1916. The Ordovician sequence in the Cader Idris district (Merioneth). British Association for the Advancement of Science Report for 1915 (Manchester), p.424. 
Cox, A H and Wells, A K, 1927. The geology of the Dolgelley district, Merionethshire. Proceedings of the Geologists' Association, Vol.38, 265-318. 
Davies, R G, 1959. The Cader Idris Granophyre and its associated rocks. Quarterly Journal of the Geological Society, London. Vol.115, 189-216. 
Lake, P and Reynolds, S H, 1912. The geology of Mynydd-y-Gader, Dolgelly. Quarterly Journal of the Geological Society of London, Vol.68, 345-363. 
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:
E149 E135