The BGS Lexicon of Named Rock Units — Result Details

South Wales Middle Coal Measures Formation

Computer Code: SWMCM Preferred Map Code: notEntered
Status Code: Full
Age range: Duckmantian Substage (CB) — Bolsovian Substage (CC)
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 Vanderbeckei (Amman) Marine Band (see Stubblefield and Trotter, 1957), where grey, marine, fossiliferous mudstones at the base of the marine band conformably overlie the Amman Rider Coal or the local equivalent where the Amman Rider amalgamates with the underlying coals. In the stream exposures in Cwm Gwrelych - Nant Llyn Fach, the boundary [SN 9010 0533] lies at the top of a 0.18m thick coal, at the base of 0.10m of grey mudstone with lingulids and fish fragments (the Vanderbeckei Marine Band).
Definition of Upper Boundary: The top is taken at the top of the Cambriense (Upper Cwmgorse) Marine Band (Stubblefield and Trotter, 1957), where grey marine mudstones at the top of the marine band are conformably overlain by non-marine mudstones marking the base of the South Wales Upper Coal Measures or by Pennant sandstones marking the base of the Pennant Sandstone Formation.
Thickness: c.120m on east crop [SO 20] of the coalfield to 240m in the Swansea area [SS 88].
Geographical Limits: The Formation crops out in a peripheral belt around the coalfield, extending from south of Little Haven, Pembrokeshire [SM 850 124] in the west to near Pontypool [SO 263 020] in the east; and from near Llandybie [SN 610 149] in the north to Swansea - Port Talbot area [c.SS 780 840] in the south.
Parent Unit: South Wales Coal Measures Group (SWCM)
Previous Name(s): Middle Coal Measures Formation [Obsolete Name And Code: Use PMCM, MCMS, SWMCM] (MCM)
Middle Coal Measures (-3064)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Stream exposures in Cwm Gwrelych - Nant Llyn Fach, about 1km east of Glyn Neath, West Glamorgan; from the base of the Vanderbeckei Marine Band [SN 9010 0533] for about 300m upstream, exposing the basal part of the Formation. Cleal and Thomas, 1996; Evans et al., 2003. 
Type Area  Type area is the South Wales Coalfield. The Formation crops out in a peripheral belt around the coalfield, extending from south of Little Haven, Pembrokeshire in the west to near Pontypool in the east; and from near Llandybie [SN 610 149] in the north to Swansea-Port Talbot area [c.SS 780 840] in the south. Waters et al. (in prep). 
Reference(s):
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. 
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.] 
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. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E232