The Mercia Mudstone Group is an early

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

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

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 ei ...

) which is widespread in Britain, especially in 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 ...

– the name is derived from the ancient kingdom of

Mercia la, Merciorum regnum , conventional_long_name=Kingdom of Mercia , common_name=Mercia , status=Kingdom , status_text=Independent kingdom (527–879)Client state of Wessex () , life_span=527–918 , era=Heptarchy , event_start= , date_start= , ye ...

which corresponds to that area. It is frequently encountered in older literature as the

Keuper Marl Keuper marl is a former and now deprecated term for multiple layers of mudstone and siltstone of Triassic age which occur beneath parts of the English Midlands and neighbouring areas e.g. Cheshire, Nottinghamshire, Devon, eastern Worcestershire ...

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. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...

and

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

. 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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

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.Blatt ''et al.'' 1980, p ...

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

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, p ...

s. Historically this sequence of rocks has been subdivided in different ways with different names in each of the basinal areas 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 geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS h ...

(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 is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...

.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 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. Devon is ...

Somerset ( en, All The People of Somerset) , locator_map = , coordinates = , region = South West England , established_date = Ancient , established_by = , preceded_by = , origin = , lord_lieutenant_office =Lord Lieutenant of Somerset , lord_ ...

and

Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the English Channel coast. The ceremonial county comprises the unitary authority areas of Bournemouth, Christchurch and Poole and Dorset (unitary authority), Dors ...

, through the

Bristol Bristol () is a city, ceremonial county and unitary authority in England. Situated on the River Avon, it is bordered by the ceremonial counties of Gloucestershire to the north and Somerset to the south. Bristol is the most populous city in ...

South Wales South Wales ( cy, De Cymru) is a 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 Glamorgan and Monmouthshire, south Wales extends westwards ...

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, Engla ...

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

Staffordshire Staffordshire (; postal abbreviation Staffs.) is a landlocked county in the West Midlands region of England. It borders Cheshire to the northwest, Derbyshire and Leicestershire to the east, Warwickshire to the southeast, the West Midlands Cou ...

. Northwards the outcrop splits either side of the

Pennines The Pennines (), also known as the Pennine Chain or Pennine Hills, are a range of uplands running between three regions of Northern England: North West England on the west, North East England and Yorkshire and the Humber on the east. Commo ...

where deposition took place across the East Midlands Shelf of

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 traditi ...

and through

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

to the North Sea coast at

Hartlepool Hartlepool () is a seaside and port town in County Durham, England. It is the largest settlement and administrative centre of the Borough of Hartlepool. With an estimated population of 90,123, it is the second-largest settlement in County ...

. A western arm includes the

Stafford Stafford () is a market town and the county town of Staffordshire, in the West Midlands region of England. It lies about north of Wolverhampton, south of Stoke-on-Trent and northwest of Birmingham. The town had a population of 70,145 in t ...

and

Cheshire Cheshire ( ) is a ceremonial and historic county in North West England, bordered by Wales to the west, Merseyside and Greater Manchester to the north, Derbyshire to the east, and Staffordshire and Shropshire to the south. Cheshire's county t ...

basins, West

Lancashire Lancashire ( , ; abbreviated Lancs) is the name of a historic county, ceremonial county, and non-metropolitan county in North West England. The boundaries of these three areas differ significantly. The non-metropolitan county of Lancashi ...

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

Irish Sea The Irish Sea or , gv, Y Keayn Yernagh, sco, Erse Sie, gd, Muir Èireann , Ulster-Scots: ''Airish Sea'', cy, Môr Iwerddon . is an extensive body of water that separates the islands of Ireland and Great Britain. It is linked to the Ce ...

. 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 1350m.

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 the Somerset West and Taunton district of Somerset, England. The village takes its name from a 17th-century inn; the bay, Blue Anchor Bay, was previously kno ...

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 5m to 67m 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 ...

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 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, blackboard or sidewalk chalk, and drywall. ...

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, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

. 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 loc ...

on the east Devon coast, is of

Norian The Norian is a division of the Triassic Period. It has the rank of an age (geochronology) or stage (chronostratigraphy). It lasted from ~227 to million years ago. It was preceded by the Carnian and succeeded by the Rhaetian. Stratigraphic defi ...

age, i.e. 216-204 mya.

Arden Sandstone Formation

The formation is named after the

Forest of Arden Arden is an area located mainly in Warwickshire, England, with parts in Staffordshire and Worcestershire, and is traditionally regarded as extending from the River Avon to the River Tame. It was once heavily wooded, giving rise to the name 'F ...

in Warwickshire. It consists of a 2m to 24m 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 of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

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 Byley is a village and civil parish in the unitary authority of Cheshire West and Chester and the ceremonial county of Cheshire, England, about 2½ miles north of Middlewich. According to the 2001 census it had a population of 202, increasing ...

), 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 {{Infobox UK place , static_image_name = Northwich - Town Bridge.jpg , static_image_caption = Town Bridge, the River Weaver and the spire of Holy Trinity Church , official_name = Northwich , country ...

), 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 The River Bollin is a major tributary of the River Mersey in the north-west of England. It rises in Macclesfield Forest at the western end of the Peak District, and can be seen in spring form, from the Buxton to Macclesfield road. The stream t ...

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 vil ...

), 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 Tarporle ...

) is Anisian in age and comprises siltstones, mudstones and sandstones. The thickness of the sequence varies from 20m 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 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 blocks, north and south of the "Beeston Gap". The main mas ...

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 and Tarporley, in the unitary authority of Cheshire West and Chester an ...

together with small tracts of hilly country around

Runcorn Runcorn is an industrial town and cargo port in the Borough of Halton in Cheshire, England. Its population in 2011 was 61,789. The town is in the southeast of the Liverpool City Region, with Liverpool to the northwest across the River Mersey. ...

and

Warrington Warrington () is a town and unparished area in the borough of the same name in the ceremonial county of Cheshire, England, on the banks of the River Mersey. It is east of Liverpool, and west of Manchester. The population in 2019 was estimat ...

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