The BGS Lexicon of Named Rock Units — Result Details

Gamlan Formation

Computer Code: GN Preferred Map Code: Gn
Status Code: Full
Age range: St David's Series (ED) — St David's Series (ED)
Lithological Description: Grey, green and purple colour-banded, locally pyritic mudstones and thin, coarse-grained turbiditic sandstones. Upper part of Formation manganiferous, with thin mudstone beds near the top.
Definition of Lower Boundary: Conformable, sharply overlain by turbiditic siltstones of the Barmouth Formation.
Definition of Upper Boundary: Conformable and transitional with overlying black mudstones of the Clogau Formation (Mawddach Group).
Thickness: 150 to 240m
Geographical Limits: Northern, eastern and southern flanks of Harlech Dome.
Parent Unit: Harlech Grits Group (HGG)
Previous Name(s): Gamlan Shale Group [Obsolete Name And Code: Use GN] (GMSH)
Gamlan Black Shales (-288)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Afon Gain northwest of Mynydd Bach. Tuffitic mudstones near the top of the Formation. Allen, P M and Jackson, A. 1985. 
Partial Type Section  Manganese mine, a short distance north of Frith Farm [SH6123 3047] (manganiferous horizons). Dewey, H and Dines, H G. Tungsten and manganese ores. Memoir of the Geological Survey of Great Britain, 83. 
Reference(s):
Dewey, H and Dines, H G. 1923. Tungsten and manganese ores. Memoir of the Geological Survey of Great Britain. [London: HMSO.] 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E149 E135