The Mercia Mudstone Group is an early

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

(a sequence of

rock strata In geology and related fields, a stratum (: strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ...

) which is widespread in Britain, especially in the

English Midlands The Midlands is the central region of England, to the south of Northern England, to the north of southern England, to the east of Wales, and to the west of the North Sea. The Midlands comprises the ceremonial counties of Derbyshire, Herefordshi ...

—the name is derived from the ancient kingdom of

Mercia Mercia (, was one of the principal kingdoms founded at the end of Sub-Roman Britain; the area was settled by Anglo-Saxons in an era called the Heptarchy. It was centred on the River Trent and its tributaries, in a region now known as the Midlan ...

which corresponds to that area. It is frequently encountered in older literature as the Keuper Marl or Keuper Marl Series. The Mercia Mudstone Group is now divided into five formations recognised and mappable across its entire

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

and

subcrop Subcrop is a term in geology. It is a contrast to the term ''outcrop'', if not a perfect antonym. If rocks exposed at the present-day erosion surface are referred to as outcrops, then now-buried rocks that were exposed at ancient erosion surfaces ...

. The formations are a mix of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s and

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, pi ...

s. Historically this sequence of rocks has been subdivided in different ways with different names in each of the

basinal areas A structural basin is a large-scale structural formation of rock strata formed by tectonic warping (folding) of previously flat-lying strata into a syncline fold. They are geological depressions, the inverse of domes. Elongated structural ba ...

in which it is found. Increasing knowledge of the sequences and the more recent development of seamless electronic mapping by the

British Geological Survey The British Geological Survey (BGS) is a partly publicly funded body which aims to advance Earth science, geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. ...

(BGS) necessitated a reappraisal of these divisions. A report published by BGS in 2008 recommended the abandonment of previous divisions and naming schemes in favour of a simpler approach which, having now been adopted, is set out below. *Blue Anchor Formation *Branscombe Mudstone Formation *Arden Sandstone Formation *Sidmouth Mudstone Formation *Tarporley Siltstone Formation Older schemes will remain in maps and literature well into the future,Mercia Mudstone Group
at

Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.org providing a source of potential confusion. An example might be the Arden Sandstone Formation which previously enjoyed lower status as a

member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

and also higher status as a

Distribution

The group crops out widely across England, representing deposition within numerous Triassic basins, some of which are physically connected at depth. From the south there is an almost continuous outcrop from the

Wessex Basin The Wessex Basin is a petroleum-bearing geological area located along the southern coast of England and extending into the English Channel. The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the Englis ...

of east

Devon Devon ( ; historically also known as Devonshire , ) is a ceremonial county in South West England. It is bordered by the Bristol Channel to the north, Somerset and Dorset to the east, the English Channel to the south, and Cornwall to the west ...

Somerset Somerset ( , ), Archaism, archaically Somersetshire ( , , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by the Bristol Channel, Gloucestershire, and Bristol to the north, Wiltshire to the east ...

and

Dorset Dorset ( ; Archaism, archaically: Dorsetshire , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by Somerset to the north-west, Wiltshire to the north and the north-east, Hampshire to the east, t ...

, through the

Bristol Bristol () is a City status in the United Kingdom, cathedral city, unitary authority area and ceremonial county in South West England, the most populous city in the region. Built around the River Avon, Bristol, River Avon, it is bordered by t ...

South Wales South Wales ( ) is a Regions of Wales, loosely defined region of Wales bordered by England to the east and mid Wales to the north. Generally considered to include the Historic counties of Wales, historic counties of Glamorgan and Monmouthshire ( ...

area and the

Worcester Worcester may refer to: Places United Kingdom * Worcester, England, a city and the county town of Worcestershire in England ** Worcester (UK Parliament constituency), an area represented by a Member of Parliament * Worcester Park, London, Engl ...

and Knowle Basins into the English Midlands and including the Needwood Basin of

Staffordshire Staffordshire (; postal abbreviation ''Staffs''.) is a Ceremonial counties of England, ceremonial county in the West Midlands (region), West Midlands of England. It borders Cheshire to the north-west, Derbyshire and Leicestershire to the east, ...

. Northwards the outcrop splits either side of the

Pennines The Pennines (), also known as the Pennine Chain or Pennine Hills, are a range of highland, uplands mainly located in Northern England. Commonly described as the "Vertebral column, backbone of England" because of its length and position, the ra ...

where deposition took place across the East Midlands Shelf of

Nottinghamshire Nottinghamshire (; abbreviated ''Notts.'') is a ceremonial county in the East Midlands of England. The county is bordered by South Yorkshire to the north-west, Lincolnshire to the east, Leicestershire to the south, and Derbyshire to the west. Th ...

and through

Yorkshire Yorkshire ( ) is an area of Northern England which was History of Yorkshire, historically a county. Despite no longer being used for administration, Yorkshire retains a strong regional identity. The county was named after its county town, the ...

to the North Sea coast at

Hartlepool Hartlepool ( ) is a seaside resort, seaside and port town in County Durham, England. It is governed by a unitary authority borough Borough of Hartlepool, named after the town. The borough is part of the devolved Tees Valley area with an estimat ...

. A western arm includes the

Stafford Stafford () is a market town and the county town of Staffordshire, England. It is located about south of Stoke-on-Trent, north of Wolverhampton, and northwest of Birmingham. The town had a population of 71,673 at the 2021–2022 United Kingd ...

and

Cheshire Cheshire ( ) is a Ceremonial counties of England, ceremonial county in North West England. It is bordered by Merseyside to the north-west, Greater Manchester to the north-east, Derbyshire to the east, Staffordshire to the south-east, and Shrop ...

basins, West

Lancashire Lancashire ( , ; abbreviated ''Lancs'') is a ceremonial county in North West England. It is bordered by Cumbria to the north, North Yorkshire and West Yorkshire to the east, Greater Manchester and Merseyside to the south, and the Irish Sea to ...

and the Carlisle Basin—the latter are connected at depth beneath the

Irish Sea The Irish Sea is a body of water that separates the islands of Ireland and Great Britain. It is linked to the Celtic Sea in the south by St George's Channel and to the Inner Seas off the West Coast of Scotland in the north by the North Ch ...

. The group is most thickly developed within the

Cheshire Basin The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of ...

(which also extends southwards into north Shropshire) where it attains a thickness in excess of 1,350 m.

Stratigraphy

Blue Anchor Formation

The formation is named from the village of

Blue Anchor Blue Anchor is a seaside village, in the parish of Old Cleeve, close to Carhampton in Somerset, England. The village takes its name from a 17th-century inn; the bay, Blue Anchor Bay, was previously known as Cleeve Bay. The bay and inn were th ...

on the coast of west Somerset. It consists largely of green to grey mudstones and siltstones (which gave rise to the earlier name of this sequence, the Tea-green Marls) and varies from around 5 m to 67 m in thickness. Though common to all other areas, the Blue Anchor Formation is absent through erosion in the Stafford Basin and in Lancashire. It is of late Norian to early Rhaetian age. It is overlain by the Westbury Formation of the

Penarth Group The Penarth Group is a Rhaetian age (Triassic) lithostratigraphic group (a sequence of rock strata) which is widespread in Britain. It is named from the seaside town of Penarth near Cardiff in south Wales where strata of this age are exposed in ...

which represents widespread inundation of the Triassic basins as global sea levels rose.

Branscombe Mudstone Formation

The formation consists of red-brown mudstones and siltstones in which

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

occurs as beds, veins and nodules. It was formerly known as the 'Brooks Mill Mudstone Formation' in the Cheshire Basin (and named from a location near

Nantwich Nantwich ( ) is a market town and civil parish in the unitary authority of Cheshire East in Cheshire, England. It has among the highest concentrations of listed buildings in England, with notably good examples of Tudor and Georgian architecture ...

), as the 'Cropwell Bishop Formation' on the 'East Midlands Shelf' and as the 'Twyning Mudstone Formation' in the Worcester and Knowle basins. It is equivalent to the 'Triton Formation' of the southern

North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...

. The Branscombe Mudstone Formation, which is named from

Branscombe Branscombe is a village in the East Devon district of the English County of Devon. The parish covers . Its permanent population in 2009 was estimated at 513 by the Family Health Services Authority, reducing to 507 at the 2011 Census. It is ...

on the east Devon coast, is of

Norian The Norian is a division of the Triassic geological period, Period. It has the rank of an age (geology), age (geochronology) or stage (stratigraphy), stage (chronostratigraphy). It lasted from ~227.3 to Mya (unit), million years ago. It was prec ...

age, i.e. 216–204 mya.

Arden Sandstone Formation

The formation is named after the

Forest of Arden The Forest of Arden is a territory and cultural reference point in the English West Midlands, that in antiquity and into the Early Modern Period covered much of that district: 'This great forest once extended across a wide band of Middle Engl ...

in Warwickshire. It consists of a 2 m to 24 m thickness of mudstones, siltstones and sandstones with occasional pebble beds. It generally has a more greenish grey colour than the formations above and below it though it is not readily distinguishable within the sequence in Cheshire. it has locally yielded fossils suggestive of a marine-influenced environment. The Arden Sandstone is also known as North Curry Sandstone which is used as a building material in

Sidmouth Mudstone Formation

Cliffs at the south end of Gunthorpe weir - geograph

The formation includes several members which formerly enjoyed 'formation' status. This sequence was formerly known as the Eldersfield Mudstone Formation in the Worcester and Knowle basins and as the (combined) Edwalton, Gunthorpe and Radcliffe formations on the East Midlands Shelf. In the Cheshire Basin it supersedes the former Bollin Mudstone, Northwich Halite, Wych Mudstone, Byley Mudstone and Wilkesley Halite formations, each of which is now accorded 'member' status. It is of Anisian through Ladinian to

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

age.

Wilkesley Halite member

Formerly known as the Upper Keuper Saliferous Beds and as the Wilkesley Halite Formation (named from the Cheshire hamlet of Wilkesley), this member is Ladinian to Carnian in age.

Wych Mudstone member

Formerly known as (the upper part of) the Middle Keuper Marl and as the Wych Mudstone Formation (named from the

Wych Brook The Wych Brook, Worthenbury Brook and Red Brook, formerly known as the River Elfe, is a tributary of the River Dee in England and Wales, forming part of both the historic and present-day border between the two countries. The stream forms part ...

on the Cheshire/Flintshire/Shropshire border), this member is Anisian to Ladinian in age.

Byley Mudstone member

Formerly known as (the lower part of) the Middle Keuper Marl and as the Byley Mudstone Formation (named from the Cheshire village of Byley), this member is Anisian in age.

Northwich Halite member

Formerly known as the Lower Keuper Saliferous Beds and as the Northwich Halite Formation (named from the town of

Northwich Northwich is a market town and civil parish in the Cheshire West and Chester borough of Cheshire, England. It lies on the Cheshire Plain, at the confluence of the rivers Weaver and Dane, east of Chester, south of Warrington and south of Ma ...

), this member is Anisian in age.

Bollin Mudstone member

Formerly known as the Lower Keuper Marl, lower mudstone and also as the Bollin Mudstone Formation (named from the River Bollin in east

), this member is (?Scythian to) Anisian in age. In West Cumbria, the Sidmouth Mudstone Formation is divided thus:

Singleton Mudstone member

Formerly known as the Singleton Mudstone Formation (named from the Lancashire village of

Singleton Singleton may refer to: Sciences, technology Mathematics * Singleton (mathematics), a set with exactly one element * Singleton field, used in conformal field theory Computing * Singleton pattern, a design pattern that allows only one instance ...

), this member is Early Triassic to Anisian in age.

Kirkham Mudstone member

Formerly known as the Kirkham Mudstone Formation (named from the small Lancashire town of Kirkham), this member is Anisian to Ladinian in age.

Preesall Halite member

Formerly known as the Preesall Salt (named from the Lancashire village of

Preesall Preesall is a town, civil parish and electoral ward in Lancashire, England. The parish (until 1910 known as Preesall with Hackensall) covers the eastern bank of the estuary of the River Wyre, including Knott End-on-Sea, Pilling Lane and the v ...

), this member is Anisian to Ladinian in age.

Tarporley Siltstone Formation

The Tarporley Siltstone Formation (named from the Cheshire village of

Tarporley Tarporley is a large village and civil parish in Cheshire, England. The civil parish also contains the village of Rhuddall Heath. Tarporley is bypassed by the A49 and A51 roads. At the 2011 census, the population was 2,614. History Tarporl ...

) is Anisian in age and comprises siltstones, mudstones and sandstones. The thickness of the sequence varies from 20 m in parts of the East Midlands to around 220m in the Cheshire Basin. It is encountered in older literature under various guises but commonly as the Keuper Waterstones or simply Waterstones. These rocks feature prominently along the

Mid Cheshire Ridge The Mid Cheshire Ridge, sometimes referred to as the Cheshire Sandstone Ridge, is a range of low sandstone hills which stretch north to south through Cheshire in North West England. The ridge is discontinuous, with the hills forming two main bloc ...

where they form the summits of such hills as Eddisbury Hill and the Old Pale and the high ground around Tarporley and

Utkinton 200px, Map of civil parish of Utkinton within the former borough of Vale Royal Utkinton is a village and former civil parish, now in the parishes of Utkinton and Cotebrook, in the unitary authority area of Cheshire West and Chester and the ce ...

together with small tracts of hilly country around

Runcorn Runcorn is an industrial town and Runcorn Docks, cargo port in the Borough of Halton, Cheshire, England. Runcorn is on the south bank of the River Mersey, where the estuary narrows to form the Runcorn Gap. It is upstream from the port of Live ...

and

Warrington Warrington () is an industrial town in the Borough of Warrington, borough of the same name in Cheshire, England. The town sits on the banks of the River Mersey and was Historic counties of England, historically part of Lancashire. It is east o ...

.British Geological Survey 1:50,000 scale geological map sheets (England and Wales series) 97 ''Runcorn'' and 109 ''Chester'' The Malpas Sandstone Member is distinguished within this formation.

References

{{reflist Geologic formations of Wales Geological groups of the United Kingdom Geologic formations of England Geologic formations of Scotland Triassic System of Europe Triassic England Triassic Scotland Triassic Wales Mudstone groups Sandstone groups Siltstone groups Fossiliferous stratigraphic units in Wales Evaporite deposits Ichnofossiliferous groups Paleontology in England Paleontology in Scotland Geology of Cheshire