The BGS Lexicon of Named Rock Units — Result Details

Winstow Chert Formation

Computer Code: WCH Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Brigantian Substage (CX)
Lithological Description: Thin- to medium-bedded green, grey-green, black and purple radiolarian cherts with interbeds of siliceous mudstones and "shales". In the Chudleigh area, there is a thin (to 14m) basal unit of black siliceous and sooty mudstones (the Winstow Mudstone Member). The uppermost part of the Formation, the Posidonia Beds, comprises 10m of thin-bedded cherts, shales and limestones with an abundant fauna of Posidoniid bivalves and spirally striated goniatites.
Definition of Lower Boundary: In the Chudleigh area the lower boundary is taken at the incoming of black sooty and siliceous mudstones of the Winstow Mudstone Member above the purple, green and grey mudstones of the underlying Whiteway Mudstone Formation. The contact is generally sharp. In the Plymouth area it is taken at the incoming of dark grey cherts and siliceous mudstones of the Winstow Chert Formation above the mid grey silty mudstones of the Saltash Formation.
Definition of Upper Boundary: In the Chudleigh area the upper boundary is taken at the incoming of dark grey and greenish grey mudstones with thin siltstones and fine- to coarse-grained sandstones of the overlying Ugbrooke Sandstone Formation above the thin-bedded cherts, shales and limestones (Posidonia Beds) of the Winstow Chert Formation. In the Plymouth area the top of the formation is not exposed.
Thickness: 45m in the Chudleigh area; about 30m+ in the Plymouth area.
Geographical Limits: Between Chudleigh (SX 870 784) and Kingsteignton (SX 88 74), south Devon and in the middle Tamar valley (SX 44 61) in the Plymouth area.
Parent Unit: Chudleigh Group (CHUD)
Previous Name(s): Winstow Cherts (-3120)
Saltash Formation (SAH)
Winstow Chert [Obsolete Name And Code: See WCH] (WNC)
Lower Culm Measures (-77)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Quarry near Winstow Cottages, 1.5km southwest of Chudleigh, south Devon. Quarry exposes 1.5m of cherts with thin mudstone interbeds in the middle part of the Formation. Neither the basal nor upper contact of the Formation is exposed. House and Butcher, 1973. 
Reference(s):
House, M R and Butcher, N E, 1973. Excavations in the Devonian and Carboniferous Rocks near Chudleigh, south Devon. Transactions of the Royal Geological Society of Cornwall, Vol.20, p.199-220. 
Ussher, W A E. 1882. The British Culm Measures. Proceedings of the Somerset Archeological and Natural History Society, Vol.38, 111-219. 
Leveridge B E, Holder, M T, Goode, A J J, Scrivener R C, Jones N S and Merriman, R J. 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales). 
Ussher, W A E. 1887. The Culm of Devonshire. Geological Magazine, Vol.24, 10-17. 
Selwood, E B, Edwards, R A, Simpson, S, Chesher, J A, Hamblin, R J O, Henson, M R, Riddolls, B W and Waters, R A. 1984. Geology of the countryside around Newton Abbot. Memoir of the British Geological Survey, Sheet 339 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E339