The BGS Lexicon of Named Rock Units — Result Details

Bishopston Mudstone Formation

Computer Code: BISHM Preferred Map Code: notEntered
Status Code: Full
Age range: Pendleian Substage (CE) — Yeadonian Substage (CY)
Lithological Description: Mid to dark grey mudstones, commonly fossiliferous, with some beds of mid grey siltstone and beds of interbedded siltstone and mudstone. Sporadic, minor pale grey quartzitic sandstones and rare thin coals.
Definition of Lower Boundary: In the type area the lower boundary is placed at the base of thick, dark grey fossiliferous mudstones, where they overlie cherts and dark grey fissile mudstones of the Aberkenfig Formation conformably. The contact is not exposed there. Elsewhere, the lower boundary is placed at the base of thick grey, sporadically fossiliferous mudstones, which overlie the quartzitic sandstones of the Twrch Sandstone Formation disconformably.
Definition of Upper Boundary: The top is placed at the top of the highest dark grey mudstone, where it is overlain disconformably by pale grey quartzitic sandstones of the Telpyn Sandstone Formation.
Thickness: To c.750m in the type area of Barland Common, Bishopston.
Geographical Limits: The Formation crops out in a narrow belt around the periphery of the South Wales Coalfield and its extension in Pembrokeshire. The outcrop around the main coalfield extends from near Kidwelly [SN 360 050] on the north crop eastnortheast to near Brynmawr [SO 20 11] and south to near Caerphilly [ST 20 86] and west to Swansea Bay [SS 79 84]; in Gower from Bishopston to Mumbles; and in the Pembroke Peninsula from Druidston [SM 860 169] to Marros [SN 198 076]; and on the southern limb of the syncline from Tenby [SN 135 010] northwest to near the Cleddau River [c.SN 012 085].
Parent Unit: Marros Group (MARR)
Previous Name(s): Middle Shale(s) (Group) (-2611)
Bishopston Formation (-59)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Sea shore sections near Marros Sands, from Telpyn Point eastwards, Carmarthenshire, where 107.9m (354 feet) of the Formation are exposed, the lowermost 15-18m being unexposed. 
Type Area  Type area is Bishopston, Gower, where the formation is at the thickest and of most basinal facies. A section through the lower half of the succession along the mill-race and stream at Barland Common, Bishopston, comprising 490m of strata, was first recorded by De La Beche (1846). At least 20 goniatite-bearing marine bands were recorded during the 1945-54 survey by J V Stephens (Barclay, in prep). The strata dip steeply (c.75 degress to the north). De La Beche, 1846., Barclay (in prep). 
Reference(s):
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. 
Archer, A A. 1995. Notes on the Millstone Grit of the north crop of the Pembrokeshire Coalfield. Proceedings of the Geologists' Association, Vol.76, 137-150. 
Barclay, W J, in prep. Geology of the country around Swansea. Sheet Description of the British Geological Survey. 1:50 000 Sheet 247 Swansea (England and Wales). 
De La Beche, H. 1846. On the formation of rocks of south Wales and south-west England. Memoir of the Geological Survey of Great Britain, No.1. 
George, G T and Kelling, G. 1982. Stratigraphy and sedimentology of Upper Carboniferous sequences in the coalfield of southwest Dyfed. 175-201 in Bassett, M G (Editor), Geological excursions in Dyfed, south-west Wales. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E247 E248 E231 E232