The BGS Lexicon of Named Rock Units — Result Details

Llanbedr Formation

Computer Code: LBR Preferred Map Code: Lbr
Status Code: Full
Age range: Comley Series (EC) — Comley Series (EC)
Lithological Description: Cleaved purple, green or bluish-grey mudstone with some siltstone and sandstone beds.
Definition of Lower Boundary: Rests conformably on rocks of the Dolwen Formation; transitional boundary between dominantly sandstone and siltstone sequence of the Dolwen Formation and the mudstone dominated beds above.
Definition of Upper Boundary: Overlain conformably by sandstones of the Rhinog Formation. Boundary taken at the base of the first thick pale green turbiditic sandstone of the Rhinog Formation.
Thickness: 90m in the Dolwen pericline, 180m to the west, near Harlech.
Geographical Limits: Around the Dolwen Pericline; Harlech southwards towards Barmouth, where it is very poorly exposed.
Parent Unit: Harlech Grits Group (HGG)
Previous Name(s): Llanbedr Slates (-356)
Llanbedr Slates And Equivalents [Obsolete Name And Code: Use LBR] (LSL)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Afon Crawcwellt, 115m upstream of Pont-Llyn-y-cafn. Basal stratotype. 
Reference(s):
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. 
Allen, P M, Jackson, A A and Rushton, A W A. 1981. The stratigraphy of the Mawddach Group in the Cambrian succession of North Wales. Proceedings of the Yorkshire Geological Society, Vol.43, 295-329. 
Matley, C A and Wilson, T S. 1946. The Harlech Dome, North of the Barmouth Estuary. Quarterly Journal of the Geological Society of London, Vol.102, 1-40. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E135 E149