The BGS Lexicon of Named Rock Units — Result Details

Maentwrog Formation

Computer Code: MW Preferred Map Code: Mw
Status Code: Full
Age range: St David's Series (ED) — Merioneth Series (EM)
Lithological Description: Interbedded mudstones, coarse-grained sandstones and fine-grained turbiditic sandstones. The upper part of the Formation is dominated by mudstones.
Definition of Lower Boundary: Underlain by black mudstones of the Clogau Formation. Sharp contact, possible a non-sequence or disconformity.
Definition of Upper Boundary: Conformable and gradational up into sandstones and mudstones of the Ffestiniog Flags Formation, with change in mudstone colour from pale to dark grey.
Thickness: Unknown in detail, 500 to1200m
Geographical Limits: Northern, eastern and southern flanks of the Harlech Dome.
Parent Unit: Mawddach Group (MWG)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Afon Hirgwm just upstream of Vigra Bridge, Bontddu. 
Reference(s):
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 
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:
E135 E119 E149 E134