The BGS Lexicon of Named Rock Units — Result Details

South Wales Coal Measures Group

Computer Code: SWCM Preferred Map Code: notEntered
Status Code: Full
Age range: Langsettian Substage (CA) — Bolsovian Substage (CC)
Lithological Description: Grey (productive) coal-bearing mudstones/siltstones with seatearths and minor sandstones.
Definition of Lower Boundary: The base is placed at the base of the Subcrenatum Marine Band, either in a sequence of mudrocks at the point where a marine fauna makes its first appearance, or where marine mudstone of the Subcrenatum Marine Band overlies quartzites of the underlying Telpyn Point Sandstone Formation of the Marros Group.
Definition of Upper Boundary: The top is taken at the base of the Pennant Sandstone Formation (Warwickshire Group), where grey, productive coal-bearing mudrocks of the South Wales Coal Measures Group are overlain conformably either by the first substantial development of Pennant sandstones or by red beds of the Deri Formation.
Thickness: c.240m on the east crop of the coalfield to c.900m in the Swansea area.
Geographical Limits: South Wales Coalfield; from Pembrokeshire [SM 86 15] in the west eastwards to Abersychan, Monmouthshire [SO 29 02]. It extends from the Black Mountain [SN 670 215] in the north, southwards to Swansea - Port Talbot [SS 80 85] and crops out in a peripheral belt around the coalfield, concealed below the Pennant Sandstone Formation over most of the coalfield. The group is also recorded in the Bristol-Somerset area. The Bristol Coalfield includes the Kingswood Anticline [ST 65 73] and Coalpit Heath Syncline [ST 85 83] to the east and north-east of Bristol, while the Somerset Coalfield comprises the Pensford [ST 65 65] and Radstock synclines [ST 65 55] to the south-east. The Avonmouth [ST 53 80] and Nailsea [ST 45 70] synclines contain coal basins that are structurally distinct, although linked to the major coalfields in the subsurface. The group has also been proved in the subsurface in the Berkshire Coalfield [SU 70 66] and Kent Coalfield [TR 30 40].
Parent Unit: Coal Measures Supergroup (COME)
Previous Name(s): Coal Measures Group (CM)
Coal Measures (-4355)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Type area is the South Wales Coalfield and its western extension in Pembrokeshire [SM 86 15], from where it extends eastwards to Abersychan, Monmouthshire [SO 29 02]. It extends from the Black Mountain [SN 670 215] in the north, southwards to Swansea - Port Talbot [SS 8085] and crops out in a peripheral belt around the coalfield, concealed below the Pennant Sandstone Formation over most of the coalfield. Waters et al., (in prep). 
Reference Section  Stream exposures in Cwm Gwrelych - Nant Llyn Fach - Glyn Neath area, extending south from Pont Walby, 1km east of Glyn Neath, West Glamorgan. Robertson, 1932.; Cleal and Thomas, 1996; Evans et al., 2003. 
Reference(s):
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. 
Cleal, C J and Thomas, B A. 1996. Cwm Gwrelych - Nant Llyn Fach. 83-89 in Cleal, C J and Thomas, B A (editors), British Upper Carboniferous stratigraphy. [London: Chapman and Hall.] 
Evans, B G, Cleal, C J, Thomas, B A and Wimbledon, W A P. 2003. The Westphalian succession of the Glyn-neath area, South Wales; an internationally important geological resource. Proceedings of the Geologists' Association, Vol.114, 291-305. 
Stubblefield, C J and Trotter, F M, 1957. Divisions of the Coal Measures on Geological Survey Maps of England and Wales. Bulletin of the Geological Survey of Great Britain, No.13, p.1-5. 
Woodland, A W, Evans, W B and Stephens, J V. 1957. Classification of the Coal Measures of South Wales with Special reference to the Upper Coal Measures. Bulletin of the Geological Survey of Great Britain, No. 13, 39-60. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable