The BGS Lexicon of Named Rock Units — Result Details

Bryncroes Formation

Computer Code: BRCS Preferred Map Code: Br
Status Code: Full
Age range: Arenig Series (OR) — Arenig Series (OR)
Lithological Description: A conglomeratic basal division grading upwards into thinly interbedded and interlaminated sandstones and mudstones.
Definition of Lower Boundary: Unconformable on the metamorphosed and igneous rocks of the Sarn Complex.
Definition of Upper Boundary: Overlain by siltstones, mudstones and oolitic ironstones of the Trygarn Formation. The boundary, which is possibly a disconformity or non-sequence, is defined by the first oolitic ironstone bed within the Trygarn Formation.
Thickness: 200m
Geographical Limits: Pwllheli area, northwest Wales.
Parent Unit: Nant Ffrancon Subgroup (NFR)
Previous Name(s): Sarn Formation [Obsolete Name and Code: Use BRCS] (-4490)
Wig Bach Formation (WGB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Coastal cliff exposure ar Porth Cadlan, Llyn Peninsula, showing the middle part of the formation. Beckley, A J. 1988. 
Partial Type Section  Basal conglomeratic stratotype and unconformity with the Sarn Complex at Mountain Quarry near Coed-y-Gromlech. Young, T P, Gibbons, W and McCarroll, D. 2002. 
Reference(s):
Beckly, A J, 1988. The Stratigraphy of the Arenig Series in the Aberdaron to Sarn Area, Western Llyn, North Wales. Geological Journal, Vol.23, 321-337. 
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:
E134