The BGS Lexicon of Named Rock Units — Result Details

Twrch Sandstone Formation

Computer Code: TWR Preferred Map Code: notEntered
Status Code: Full
Age range: Pendleian Substage (CE) — Yeadonian Substage (CY)
Lithological Description: Quartz arenites and quartz conglomerates with minor mudstones.
Definition of Lower Boundary: The lower boundary, which is generally an unconformity, is placed at the base of the major development of quartz arenite and quartz pebble conglomerate. The Twrch Sandstone Formation rests with discordance on the underlying limestones of the Pembroke Limestone Group. In the south of the type area it rests unconformably on rocks of the Oystermouth Formation, in the north on rocks of the Oxwich Head Limestone Formation.
Definition of Upper Boundary: The top is placed at a sharp junction, generally a disconformity, where the lowermost grey mudstones of the Bishopston Mudstone Formation overlie the highest quartz arenites of the Twrch Sandstone Formation. This boundary is exposed at [SN 7719 1508] in the Afon Twrch section.
Thickness: To c.190m
Geographical Limits: On the northern limb of the Pembrokeshire Coalfield westwards from Ragwen Point [SN 220 071] to just east of Druidston [SM 877 170] and eastwards along the north crop of the main South Wales Coalfield to near Blaenavon [SO 270 110], southeast to Machen [ST 225 895].
Parent Unit: Marros Group (MARR)
Previous Name(s): Basal Grits Group (-4943)
Basal Grit (-1247)
Basal Grits (-1880)
Basal Grit (Millstone Grit Of South Wales) [Obsolete Name And Code: Use TWR] (BGM)
Alternative Name(s): none recorded or not applicable
Type Area  Type area is the Twrch Valley, where the Formation is thickest (c.190m) and is well exposed in the Afon Twrch from the top of the Formation [SN 7719 1508] to a fault [SN 7844 1846], where it is faulted against the Avon Group. It is also well exposed on the precipitous slopes (Tyle-Garw) on the east side of the Twrch Valley. Ware, 1939. 
Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
Waters, C N, Barclay, W J, Davies, J R and Waters, R A. In press. Stratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
George, G T. 2000. Characterisation and high resolution sequence stratigraphy of storm-dominated braid delta and shoreface sequences from the Basal Grit Group (Namurian) of the South Wales Variscan peripheral foreland basin. Marine and Petroleum Geology, Vol.17, 445-475. 
Ware, W D. 1939. The Millstone Grit of Carmarthenshire. Proceedings of the Geologists' Association, Vol.50, 168-204. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable