Mercia Mudstone Group
Bristol and Gloucester Regional Guide

Keuper Marl is a now deprecated term for multiple layers of mudstone and siltstone of Triassic age which occur beneath parts of the English Midlands and neighbouring areas. In modern nomenclature it is included within the Mercia Mudstone Group.[1]

Typically red, occasionally green or grey, these strata can range up to thicknesses of 1350 metres (4,429 ft), as found underlying the Cheshire PlainRelatively flat expanse of lowland in North West England, the surface expression of the Cheshire Basin, an area of sedimentary rocks overlain by Mercia Mudstones laid down about 250 million years ago..[1]