The BGS Lexicon of Named Rock Units — Result Details

Garn Caws Sandstone Formation

Computer Code: GACA Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Arundian Substage (CJ)
Lithological Description: White, silica-cemented ("quartzitic") sandstones and conglomerates with some thin intercalated plant-bearing green-grey mudstones and seatearth.
Definition of Lower Boundary: The base is placed at the base of silica-cemented ("quartzitic") sandstone or conglomerate where it rests with sharp base on an erosion surface cut in the underlying Gilwern Clay Member of the Llanelly Formation.
Definition of Upper Boundary: The top is placed at the top of the silica-cemented ("quartzitic") sandstone, where it is sharply and unconformably overlain by the basal limestone of the Dowlais Limestone Formation.
Thickness: To 12m (maximum at Blaen Onneu Quarry [SO 155 169].
Geographical Limits: Northeast crop of the South Wales Coalfield, from the area of Cwar yr Hendre [SO 099 149] eastwards to the type area of Garn Caws [SO 120 164], and farther eastwards to the Clydach Valley [c.SO 215 127].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Dowlais Limestone (-1551)
Gilwern Clay Member (GWCL)
Garn Caws Sandstone (-271)
Alternative Name(s): none recorded or not applicable
Type Area  Extensive exposures on the hill of Garn Caws, Powys. Barclay, 1989. 
Reference Section  Excellent exposures at Blaen Onneu Quarry, Powys, show 12m of quartzitic (silica-cemented) sandstone with some thin green-grey, plant-bearing mudstone beds. Wright, 1981; 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. 
Barclay, W J and Jackson, D I. 1982. The geology of Carno adit, Ebbw Vale, South Wales. Report of the Institute of Geological Sciences, No.82/1, 41-44. 
Dickson, J A D and Wright, V P. 1993. Carboniferous Limestone of the north crop of the South Wales Coalfield. 331-349 in Woodcock, N H and Bassett, M G (editors), Geological excursions in Powys, central Wales. [Cardiff: University of Wales Press, National Museum of Wales.] 
Wright, V P. 1981. Llanelly Formation. 36-72 in Wright, V P, Raven, M and Burchette, T P (editors), A field guide to the Carboniferous Limestone around Abergavenny. [Cardiff: Department of Geology, University College of Wales.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used: