Keuper The Keuper is a lithostratigraphic unit (a sequence of rock strata) in the subsurface of large parts of west and central Europe. The Keuper consists of dolomite, shales or claystones and evaporites that were deposited during the Middle and Late T ...

marl is a former and now deprecated term for multiple layers of mudstone and siltstone of

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

age which occur beneath parts of the

English Midlands The Midlands (also referred to as Central England) are a part of England that broadly correspond to the Kingdom of Mercia of the Early Middle Ages, bordered by Wales, Northern England and Southern England. The Midlands were important in the Ind ...

and neighbouring areas e.g. Cheshire,

Nottinghamshire Nottinghamshire (; abbreviated Notts.) is a landlocked county in the East Midlands region of England, bordering South Yorkshire to the north-west, Lincolnshire to the east, Leicestershire to the south, and Derbyshire to the west. The trad ...

Devon Devon ( , historically known as Devonshire , ) is a ceremonial and non-metropolitan county in South West England. The most populous settlement in Devon is the city of Plymouth, followed by Devon's county town, the city of Exeter. Devo ...

, eastern

Worcestershire Worcestershire ( , ; written abbreviation: Worcs) is a county in the West Midlands of England. The area that is now Worcestershire was absorbed into the unified Kingdom of England in 927, at which time it was constituted as a county (see H ...

and northern

Yorkshire Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other English counties, functions have ...

. Typically red, or occasionally green or grey, these strata are generally featureless and contain few fossils. In basin formations, thick

halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...

-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 The Mercia Mudstone Group is an early Triassic lithostratigraphic group (a sequence of rock strata) which is widespread in Britain, especially in the English Midlands – the name is derived from the ancient kingdom of Mercia which corresponds to t ...

.British Geological Survey: Lexicon of Named Rock Units: Mercia Mudstone Group
(accessed 10 April 2010)Benton M, Cook E, Turner P. ''Permian and Triassic Red Beds and the Penarth Group of Great Britain'', pp. 11, 19. ''Geological Conservation Review Series'', No. 24 (Joint Nature Conservation Committee; 2002)
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References

Siltstone formations Mudstone formations Triassic System of Europe {{UK-geologic-formation-stub