Keuper marl is the former name for a layer of mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

s and siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

s from the late Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 period (formerly known as the Keuper period). Typically red, or occasionally
green or grey, it is featureless and contains few fossils. In basin formations, thick halite-bearing layers, or rock salt deposits, are sometimes present at the base of the marl. In modern nomenclature, Keuper marl is included within the Mercia Mudstone Group

Mercia Mudstone Group

The Mercia Mudstone Group is a sequence of sedimentary rocks which occurs widely in the United Kingdom, consisting of beds of various mudstones, siltstones and sandstones....

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In England, Keuper marl is found in Cheshire

Cheshire

Cheshire is a ceremonial county in North West England. Cheshire's county town is the city of Chester, although its largest town is Warrington. Other major towns include Widnes, Congleton, Crewe, Ellesmere Port, Runcorn, Macclesfield, Winsford, Northwich, and Wilmslow...

, Nottinghamshire

Nottinghamshire

Nottinghamshire is a county in the East Midlands of England, bordering South Yorkshire to the north-west, Lincolnshire to the east, Leicestershire to the south, and Derbyshire to the west...

, Devon

Devon

Devon is a large county in southwestern England. The county is sometimes referred to as Devonshire, although the term is rarely used inside the county itself as the county has never been officially "shired", it often indicates a traditional or historical context.The county shares borders with...

, eastern Worcestershire

Worcestershire

Worcestershire is a non-metropolitan county, established in antiquity, located in the West Midlands region of England. For Eurostat purposes it is a NUTS 3 region and is one of three counties that comprise the "Herefordshire, Worcestershire and Warwickshire" NUTS 2 region...

 and northern Yorkshire

Yorkshire

Yorkshire is a historic county of northern England and the largest in the United Kingdom. Because of its great size in comparison to other English counties, functions have been increasingly undertaken over time by its subdivisions, which have also been subject to periodic reform...

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