The BGS Lexicon of Named Rock Units — Result Details

South Wales Lower Coal Measures Formation
Computer Code: SWLCM Preferred Map Code: notEntered
Status Code: Full
Age range: Langsettian Sub-age (CA) — Langsettian Sub-age (CA)
Lithological Description: Grey, (productive) coal-bearing mudstones/siltstones, with seatearths and minor sandstones.
Definition of Lower Boundary: The base is taken at the base of the Subcrenatum Marine Band (as defined by Stubblefield and Trotter, 1957), where marine, fossiliferous mudstones at the base of the marine band conformably overlie the topmost mudstones or sandstones of the Marros Group. The Formation rests unconformably on Dinantian or Devonian strata in the Berkshire and Kent coalfields.
Definition of Upper Boundary: The top is taken at the base of the Vanderbeckei (Amman) Marine Band (as defined by Stubblefield and Trotter, 1957), where basal fossiliferous, marine mudstone of the marine band comformably overlies the top of the Amman Rider Coal (and its equivalents).
Thickness: c. 80m on the east crop of the South Wales Coalfield to c.300m in the Swansea area.
Geographical Limits: South Wales Coalfield; from Pembrokeshire [SM 86 15] in the west eastwards to Abersychan, Monmouthshire [SO 29 02]. The Formation 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. Probably also represented in the subsurface in the Berkshire and Kent coalfields.
Parent Unit: South Wales Coal Measures Group (SWCM)
Previous Name(s): Lower Coal Measures Formation [Obsolete Name And Code: Use PLCM, SWLCM, LCMS] (LCM)
New House Sandstone Formation [Obsolete Name And Code: Use Sandstone In SWLCM] (NHSA)
Lower Coal Measures (-1759)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Type area is 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 80 85] and crops out in a peripheral belt around the coalfield. Waters et al., (in prep). 
Type Section  Stream exposures in Cwm Gwrelych - Nant Llyn Fach. The section extends south from near Pont Walby, 1km east of Glyn Neath, West Glamorgan. The entire thickness of the Formation (253m), including base and top is proved in this section. Robertson, 1932; Cleal and Thomas, 1996; Evans et al., 2003. 
Reference(s):
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. 
Waters, C N, Brown, M A E, Dean, M T and Powell, J H. In press. Lithostratigraphical framework for Carboniferous successions of 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.] 
Leitch, D, Owen, T R and Jones, D G. 1958. The basal Coal Measures of the South Wales Coalfield from Llandybie to Brynmawr. Quarterly Journal of the Geological Society of London, Vol.113, 461-486. 
Robertson, T. 1932. Geology of the South Wales Coalfield, Part V, the country around Merthyr Tydfil, 2nd edition. Memoir of the Geological Survey of Great Britain, Sheet 231. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E232
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