The BGS Lexicon of Named Rock Units — Result Details

Craig-Y-Gaer Coral Bed

Computer Code: CYGC Preferred Map Code: notEntered
Status Code: Full
Age range: Chadian Substage (CI) — Chadian Substage (CI)
Lithological Description: Pale grey, shelly, fossiliferous, crinoid-rich coarse grainstone.
Definition of Lower Boundary: The base is placed at a sharp, irregular, partly stylolitized erosion surface where the coarse grainstone at the base of the Craig-y-Gaer Coral Bed rests, locally with angular discordance, on micritic lime mudstones at the top of the underlying Darren Ddu Member of the Coed Ffyddlwn Formation.
Definition of Upper Boundary: The upper boundary is locally gradational, but mainly a sharp, stylolitized irregular surface above which are the ooidal grainstones that form most of the Gilwern Oolite Formation.
Thickness: 2 to 3m
Geographical Limits: Clydach Valley [SO 215 126], on the northeast crop of the South Wales Coalfield.
Parent Unit: Gilwern Oolite Formation (GWO)
Previous Name(s): Coral Bed (CORA)
Alternative Name(s): none recorded or not applicable
Type Section  A pinnacle left during quarrying at Craig-y-Gaer [SO 2238 1330] on north side of the Clydach Valley, 2km east of Brynmawr, Blaenau Gwent, South Wales, exposes the bed, here a crinoidal grainstone 2m thick, and the underlying lime mudstones at the top of the Darren Ddu Member of the Coed Ffyddlwn Formation. Barclay, 1989. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: