The BGS Lexicon of Named Rock Units — Result Details

Cromhall Sandstone Formation

Computer Code: CHSA Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Arundian Substage (CJ) — Brigantian Substage (CX)
Lithological Description: Includes up to three tongues. The lower tongue comprises brown and red fine- to coarse-grained quartzitic sandstone with subordinate mudstones and sparse thin limestones. Base is locally conglomeratic. The middle tongue is similar but contains units of dolomitised limestone. The upper tongue comprises grey and red coarse-grained quartzitic sandstones, sandy crinoidal and oolitic limestones, mudstones, siltstones and grey seatearths arranged in cyclic sequences. Units of crinoidal and oolitic limestone are locally developed. The formation was deposited mainly in a fluvial environment on a marine influenced coastal plain.
Definition of Lower Boundary: Lower boundary of the Cromhall Sandstone Formation is taken at the base of the lower tongue, which is taken at the base of the first sandstone overlying the calcite and dolomite mudstones and limestones of the underlying Clifton Down Mudstone Formation in the Bristol area and the Llanelly Formation in the Monmouth and Chepstow area. Base of middle tongue is taken at the base of the first sandstone in the upper part of the Clifton Down Limestone Formation. Base of the upper tongue is taken at the base of the first sandstone or mudstone above the skeletal and oolitic limestones of the Oxwich Head Limestone Formation.
Definition of Upper Boundary: Lower tongue: Top taken at the incoming of calcite mudstones and oolitic and skeletal limestones of the Clifton Down Limestone in the Bristol area and the Hunts Bay Oolite Subgroup in the Monmouth and Chepstow area above the sandstones and mudstones of the lower tongue. Middle tongue: Top taken at the base of the entrance of the skeletal limestones of the Oxwich Head Limestone Formation above the sandstones and mudstones of the middle tongue. Upper boundary of the Cromhall Sandstone Formation taken at top of upper tongue: Top taken at entrance of limestones of the overlying uppermost Oxwich Head Limestone Formation or cherts and cherty marine mudstones of the overlying undivided Tenby Group above the sandstones and mudstones of the upper tongue.
Thickness: Lower tongue: 6m at Chipping Sodbury to 43m at Cromhall. Middle tongue: 27m at Chipping Sodbury to 30m at Tytherington. Upper tongue: 18m on Broadfield Down thickening north to 240m at Cromhall.
Geographical Limits: Bristol, [ST 60 74] Monmouth and Chepstow area [ST 53 930]. Passes laterally southwards into formations of the Pembroke Limestone Group [Llanelly Formation, Clifton Down Mudstone, Clifton Down Limestone and Oxwich Head Limestone formations].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Middle Cromhall Sandstone Formation [Obsolete Name And Code Obsolete: Use CHSA] (MCS)
Drybrook Sandstone Group [Obsolete Name And Code: Use CHSA] (DS)
Upper Cromhall Sandstone [Obsolete Name And Code: Use CHSA] (UCS)
Upper Drybrook Sandstone [Obsolete Name And Code: Part Of CHSA] (UDS)
Lower Cromhall Sandstone Formation [Obsolete Name And Code: Use CHSA] (LCS)
Upper Cromhall Sandstone Formation (-3296)
Lower Drybrook Sandstone [Obsolete Name And Code: Part Of CHSA] (LDS)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Cromwell area, north of Bristol. Three tongues mapped in this area: Lower tongue [43m] overlies Clifton Down Mudstone Formation and is overlain by Clifton Down Limestone Formation. Middle tongue [30m] overlies Clifton Down Limestone Formation and is overlain by the Oxwich Head Limestone Formation. Upper tongue (240m) overlies and is overlain by he Oxwich head Limestone Formation. [Kellaway and Welch, 1993.] 
Reference(s):
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, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
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. 
Kellaway, G A and Welch, F B A, 1955. The Upper Old Red Sandstone and Lower Carboniferous rocks of the Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No.9, p.1-21. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable