The BGS Lexicon of Named Rock Units — Result Details

Harlech Grits Group

Computer Code: HGG Preferred Map Code: HGG
Status Code: Full
Age range: Comley Series (EC) — St David's Series (ED)
Lithological Description: Sandstones, mudstones and turbiditic siltstones, conglomerates and manganese ore beds. See component formations (in descending order: Gamlan, Barmouth, Hafotty, Rhinog, Llanbedr, Dolwen) for detailed account of lithologies.
Definition of Lower Boundary: Not exposed in core of Harlech Dome but proved resting on Bryn Teg Volcanic rocks in Bryn Teg borehole.
Definition of Upper Boundary: Base of Clogau Formation (Mawddach Group).
Thickness: Approximately 2000m
Geographical Limits: Exposed in the central area of the Harlech Dome.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Harlech Dome. 
Type Area  Exposed over the area between Harlech, Barmouth and Trawsfynydd. See entries for Gamlan, Barmouth, Hafotty, Rhinog, Lanbedr and Dolwen formations. 
Reference(s):
Whittard, W F and Simpson, S, (Editors), 1958. Lexique Stratigraphique International, Europe, Part 3a III, Cambrian. 
Cowie, J W, Rushton, A W A and Stubblefield, C J, 1972. A correlation of Cambrian rocks in the British Isles. Special Report of the Geological Society of London, No.2, p.26. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E135 E149