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
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. 
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. 
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. 
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. 
Whittard, W F and Simpson, S, (Editors), 1958. Lexique Stratigraphique International, Europe, Part 3a III, Cambrian. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E135 E149