The BGS Lexicon of Named Rock Units — Result Details

Allt Lwyd Formation

Computer Code: ALL Preferred Map Code: All
Status Code: Full
Age range: Arenig Series (OR) — Arenig Series (OR)
Lithological Description: Interbedded siltstone and coarse- to medium-grained, bioturbated and locally slumped sandstone, locally quartzose or volcaniclastic, subordinate conglomerate and rare mudstones; good sedimentary structures.
Definition of Lower Boundary: Unconformable on rocks of the Mawddach Group and the Rhobell Volcanic Group.
Definition of Upper Boundary: Succeeded chiefly by the Offrwm Volcanic Formation in southern Snowdonia and by mudstones of the Nant Ffrancon Subgroup in the north. Locally unconformable contact in the Snowdonia district.
Thickness: 130 to 330m
Geographical Limits: Northern, central and eastern Snowdonia.
Parent Unit: Not Applicable (-)
Alternative Name(s): GRAIANOG SANDSTONE
Type Section  Upper Mawddach Valley. 
Reference Section  Exposures north of Tan-y-Grisiau. 
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovocian of Snowdonia and the Lleyn Peninsular. Research Report No.RR99/08 British Geological Survey, Keyworth, Nottingham. 40pp. 
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 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E135 E149 E136 E120