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): Garn Caws Sandstone (-271)
Dowlais Limestone (-1551)
Gilwern Clay Member (GWCL)
Alternative Name(s): none recorded or not applicable
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. 
Type Area  Extensive exposures on the hill of Garn Caws, Powys. 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. 
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.] 
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: