The BGS Lexicon of Named Rock Units — Result Details

Porth Meudwy Formation

Computer Code: PMU Preferred Map Code: PMu
Status Code: Pending Upgrade
Age range: Fennian Stage (OF) — Fennian Stage (OF)
Lithological Description: Coarse-grained turbiditic sandstones interbedded with very coarse matrix-supported conglomerates, the latter representing mass-flow deposits containing boulders up to 1m of phyllitic and schistose mylonites comparable to those of the Llyn Shear Zone. Subordinate mudstone beds.
Definition of Lower Boundary: Conformable on the siltstones of the Wig Bach Formation, the boundary being taken at the base of the first coarse-grained turbidite sandstone or slumped unit of the Porth Meudwy Formation.
Definition of Upper Boundary: Conformably overlain by the ferruginous siltstones, sandstones and ironstones of the Trygarn Formation, the boundary being taken at the top of the highest coarse clastic bed.
Thickness: 25 to 35m
Geographical Limits: Not yet entered.
Parent Unit: Nant Ffrancon Subgroup (NFR)
Previous Name(s): Porth Meudwy Beds (-1122)
Porth Meudwy Member (-459)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  North side of Porth Meudwy and along precipitous coastal cliff sections for nearly 2km as far as Bau Ogof-eiral.[SH 1563 2404]. Gibbons, W and McCarroll, D. 1993. 
Reference(s):
Gibbons W and McCarroll D. 1993. Geology of the country around Aberdaron, including Bardsey Island. Memoir of the British Geological Survey, Sheet 133 (England and Wales). (London: H.M.S.O.). 88pp. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E133