The BGS Lexicon of Named Rock Units — Result Details

Trenchard Formation

Computer Code: TRGP Preferred Map Code: notEntered
Status Code: Full
Age range: Asturian Substage (CAS) — Asturian Substage (CAS)
Lithological Description: Grey or pinkish grey quartzose sandstone. Quartzose conglomerate beds are common at the base of the formation, increasing in thickness and clast size towards the north.
Definition of Lower Boundary: The basal conglomerate of the Trenchard Formation rests unconformably upon the quartzitic sandstone of the Cromhall Sandstone Formation.
Definition of Upper Boundary: Taken at the base of the lowermost sandstone of "Pennant"- type (lithic arenite) of the Pennant Sandstone Formation, resting conformably above coarse-grained sandstone of the Trenchard Formation. This is a significant change to the Trotter (1942), who included all strata below the Coleford High Delf as part of his Trenchard Group.
Thickness: Up to 120 m thick in the north of the Forest of Dean Coalfield.
Geographical Limits: Restricted to the northern part of the Forest of Dean coalfield, approximately north of a line from Mailscott Wood, Coleford (SO 562 142) to Ruspidge, Cinderford (SO 649 119).
Parent Unit: Warwickshire Group (WAWK)
Previous Name(s): Trenchard Group (*336)
Alternative Name(s): none recorded or not applicable
Partial Type Section  The disused Puddlebrook Quarry, 1 km north of Drybrook: Exposure of the basal conglomerate of the Trenchard Formation. Cleal and Thomas, 1996. 
Trotter, F M, 1942. Geology of the Forest of Dean and Iron-ore Field. Memoir of the Geological Survey of Great Britain. 
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. 
Jones, P C. 1972. Quartzarenite and litharenite facies in the fluvial foreland deposits of the Trenchard Group (Westphalian), Forest of Dean, England. Sedimentary Geology, Vol. 8, 177-198. 
Cleal, C and Thomas, B A. 1996. British Upper Carboniferous stratigraphy. Geological conservation review series No. 11. [London: Chapman and Hall]. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E233 E215