The BGS Lexicon of Named Rock Units — Result Details

Dol-Cyn-Afon Formation

Computer Code: DYN Preferred Map Code: Dyn
Status Code: Full
Age range: Tremadoc Series (O1) — Tremadoc Series (O1)
Lithological Description: Grey mudstone and silty mudstone and siltstone with bioturbated sandstones locally.
Definition of Lower Boundary: Conformable on black mudstones of the Dolgellau Formation.
Definition of Upper Boundary: Unconformably overlain by sandstones or conglomerates of the Allt Lwyd Formation (Nant Ffrancon Subgroup), and locally to the southeast of the Harlech Dome by rocks of the Rhobell Volcanic Group.
Thickness: 340 to 400m, locally thickening to 900m
Geographical Limits: Northern, eastern and southern flanks of the Harlech Dome, Cwm Pennant, Snowdonia.
Parent Unit: Mawddach Group (MWG)
Previous Name(s): Glanypwll Formation (-222)
Dol-Cyn-Afon Member (-4016)
Porthmadog Formation (-4629)
Tremadoc Slates (-4630)
Afon Gam Formation [Obsolete Name And Code: See DYN] (AFO)
Cwm Hesgen Formation (-2162)
Tremadoc Beds (TCB)
Cwmhesgen Formation [Obsolete Name And Code: See DYN, DLU] (CHN)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Basal stratotype, Dol-Cyn-Afon, Upper Mawddach. 
Reference(s):
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 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. 
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 
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E135 E149 E118 E120 E134