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 (-)
Previous Name(s): Carnedd Iago Formation [Obsolete Name And Code: Use ALL] (CRI)
Cwmorthin Formation (-3797)
Graianog Sandstone Formation [Obsolete Name And Code: See Allt Lwyd Formation And Supplementary Information And ] (GRA)
Alternative Name(s): Graianog Sandstone
Type Section  Upper Mawddach Valley. 
Reference Section  Exposures north of Tan-y-Grisiau. Howells and Smith. 1997. 
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. 
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:
E120 E119 E135 E149 E136