The BGS Lexicon of Named Rock Units — Result Details

Pwll-y-Cwm Oolite Formation

Computer Code: PWCO Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Courceyan Substage (CF)
Lithological Description: Pale grey ooidal grainstone.
Definition of Lower Boundary: The base is placed at an irregular, palaeokarstic surface cut in underlying dolostones of the Sychnant Dolomite Formation, which is overlain by ooidal grainstones of the Pwll-y-Cwm Oolite Formation.
Definition of Upper Boundary: The top is a sharp, planar boundary at which the topmost ooildal limestones of the Pwll-y-Cwm Oolite Formation are overlain by buff dolostones of the Pantydarren Formation.
Thickness: c.7m; 7.6m at Type Section.
Geographical Limits: Northeast crop of the South Wales Coalfield, correlation at Cwar-yr-Hendre [SO 090 146] is uncertain, but the basal 2-4m of ooidal grainstone of the sequence there is the Pwll-y-Cwm Oolite, it extends eastwards from there to Craig-yr-Hafod [SO 2720 1001] where it is dolomitized.
Parent Unit: Clydach Valley Subgroup (CLD)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  River gorge section at Pwll-y-Cwm in the Clydach River near Brynmawr, south Wales. Here, at the eponymous type locality, the Pwll-y-Cwm Oolite Formation is 7.6m thick and forms a waterfall. It is sharply overlain by fine-grained dolostone of the Pantydarren Formation. Barclay, 1989. 
Reference Section  Roadside section on the Heads of the Valleys road (A456) in the Clydach valley, 2km east of Brynmawr, South Wales provides a more accessible section than the type section. The palaeokarstic surface forming the base of the Formation is exposed. Barclay, 1989. 
George, T N. 1954. Pre-Seminulan Main Limestone of the Avonian Series in Breconshire. Quarterly Journal of the Geological Society of London, Vol.110, 282-322. 
Barclay, W J, Jackson, D I, Mitchell, M, Owen, B, Riley, N J, White, D E, Strong, G E and Monkhouse, R A. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Memoir of the British Geological Survey. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: