The BGS Lexicon of Named Rock Units — Result Details

Dwyfach Formation

Computer Code: DWF Preferred Map Code: Dwf
Status Code: Pending Upgrade
Age range: Longvillian Substage (OL) — Woolstonian Substage (OS)
Lithological Description: Medium- to coarse-grained sandstones and subordinate interbedded mudstones (Bodwrog Member), passing upwards into grey calcareous mudstones and siltstones with subordinate sandstones (Plastirion Member).
Definition of Lower Boundary: Underlain by rhyolitic ash-flow tuff of the Allt Fawr Rhyolitic Tuff Formation. Probably also unconformable on Nant Ffrancon Subgroup mudstones and siltstones, and Upper Lodge Formation lavas and tuffs.
Definition of Upper Boundary: Overlain by, and partly interbedded laterally with a sequence of lavas and tuffs with some volcaniclastic sediments of the Llanbedrog Volcanic Group.
Thickness: 150 to 600m
Geographical Limits: Pwllheli area, northwest Wales.
Parent Unit: Ogwen Group (OGW)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Partial Type Section  Within the Dwyfach Valley at Plas Talhenbont, northwest of Llanystumdwy. No further details. Young, T P, Gibbons, W and McCarroll, D. 2002. 
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. 
Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used: