The BGS Lexicon of Named Rock Units — Result Details

Dolgellau Formation

Computer Code: DLU Preferred Map Code: Dlu
Status Code: Full
Age range: Merioneth Series (EM) — Merioneth Series (EM)
Lithological Description: Black to dark grey laminated pyritous mudstones and laminated siltstones, carbonaceous.
Definition of Lower Boundary: Conformably overlies sandstones of the Ffestiniog Flags Formation with a sharp to gradational contact.
Definition of Upper Boundary: Conformably and transitionally overlain by sandstones and siltstones of the Dol-Cyn-Afon Formation. At Ogof-Ddu marked by phosphatic sandstones with graptolites.
Thickness: 35 to 150m
Geographical Limits: Northern, eastern and southern flanks of the Harlech Dome, Snowdonia.
Parent Unit: Mawddach Group (MWG)
Previous Name(s): Cwmhesgen Formation [Obsolete Name And Code: See DYN, DLU] (CHN)
Dolgellau Member (-4610)
Dolgelly Beds [Obsolete Name And Code: Use DLU] (DGB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Nant Derbyniad Quarry, 8km east of Llan Ffestiniog. Howells, M F and Smith, M. 1997. 
Reference Section  Ogof Ddu. 900m east of Criccieth Howells, M F and Smith, M. 1997. 
Type Section  Upper Mawddach, near junction with Afon Cwm Hesgen. 
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. 
Fearnsides, W G. 1905. On the geology of Arenig and Moel Llyfnant. Quarterly Journal of the Geological Society, Vol.61, 608-640. 
Belt, T. 1867. On the "Lingula Flags" or "Ffestiniog Group" of the Dolgelly District. Parts I and II. Gelogical Magazine, Vol.4, 493-495, 536-543. 
Howells, M F and Smith, M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales). (London: The Stationery Office). 104pp. 
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. 
Fearnsides, W G. 1910. The Tremadoc slates and associated rocks of south-east Caernarfonshire. Quarterly Journal of the Geological Society of London, Vol.66, 142-188. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E135 E149