The BGS Lexicon of Named Rock Units — Result Details

Penstrowed Grits Formation

Computer Code: PEG Preferred Map Code: PEG
Status Code: Full
Age range: Sheinwoodian Age (SS) — Sheinwoodian Age (SS)
Lithological Description: Mudstones with greywacke sandstones, medium- to coarse-grained in thick beds, many isolated, but commonly in packets, forming 20%-100% of the packet.
Definition of Lower Boundary: Diachronous within the lower Wenlock; closes with the upper boundary near the lateral margins of the turbidite basin. Underlying formation: Nant-ysgollon Shales.
Definition of Upper Boundary: Diachronous within the lower Wenlock (see lower boundary). Overlying formation : Nantglyn Flags.
Thickness: Various, 0- >100m
Geographical Limits: Erected in 1969 in isolation, for the Newtown area only. Subsequent mapping has joined the outcrop to that of the Denbigh Grits in Clwyd via the Llangadfan Syncline, Potter (1971), Cummins (1967), BGS 1:50k sheet 150 (Bala), Alder (1976), Campbell (1983), BGS 1:50k sheet 120 (Corwen), and GS 1" Old Series Sheet 60 (Montgomery).
Parent Unit: Not Available (!)
Previous Name(s): Denbighshire Grits And Shales (-4248)
Alternative Name(s): Denbigh Grits (Now Formation)
Stratotypes:
Reference Section  Penstrowed Quarry. 
Type Section  Not designated (better sections probably on adjacent ground to west). Representative locality : Penstrowed Quarry [SO 0676 9088], west of Newtown, Powys. 
Reference(s):
Cummins, W A. 1963. The geology of the northern part of the Llangadfan Syncline, Montgomeryshire. Unpublished PhD thesis, University of Liverpool. 
Campbell, S D G. 1983. The geology of an area between Bala and Betws-y-Coed, North Wales. Unpublished PhD Thesis, University of Cambridge. 
Cummins, W A. 1957. The Denbigh Grits: Wenlock greywackes in Wales. Geological Magazine, Vol.94, 437-451. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E165 E179 E151 E164 E150 E180 E120 E137