Cromhall Sandstone

Carboniferous (Arundian-Holkerian)

Lithology
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. (Lower Drybrook Sandstone). The middle tongue is similar but contains units of oolitic dolomitised limestone. (Drybrook 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. (Upper Drybrook Sandstone). The formation was deposited mainly in a fluvial environment on a marine influenced coastal plain with a marine transition in the southern part of the area responsible for the central limestone unit.

Upper Boundary
Taken at the unconfomity between the sandstones and the coarse, cobble conglomerate of the Trenchard Formation.

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.

Representative Sites
Shakematle Sand Quarries
Tintern Quarry
Edgehills Quarry

Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units
and BGS Memoirs.