Geology of Somerset
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/* Bibliography */ Geology of Somerset ISBN etc
[[Image:EnglandSomerset.png|thumb|[[Somerset]]]]
[[Somerset]] is a rural county in the southwest of [[England]], covering 4171 km². It is bounded on the north-west by the [[Bristol Channel]], on the north by [[Bristol]] and [[Gloucestershire]], on the north-east by [[Wiltshire]], on the south-east by [[Dorset]], and on the south west and west by [[Devon]]. It has broad central plains with several ranges of low hills. The landscape divides into four main [[geological]] sections together with water-related features.
The oldest rocks are of [[Silurian]] age (443-417 million years ago) and are the most southerly known outcrop of rocks of this age in [[Great Britain|Britain]]. These comprise a sequence of [[lavas]], [[tuff]]s (volcanic ash), [[shales]] and [[mudstones]] which form a narrow outcrop to the northeast of [[Shepton Mallet]] in the eastern [[Mendip Hills]]. Rocks from the [[Devonian]] (417-354 million years ago) can be found in much of [[Exmoor]], the [[Quantocks]] and in the cores to the folded masses of the Mendip Hills. Rocks of the [[Carboniferous]] Period (354-290 million years ago) are represented by the [[Carboniferous Limestone]] which forms the landform of the Mendip Hills, rising abruptly out of the flat landscape of the [[Somerset Levels and Moors]]. The limestones are very fossiliferous and yield evidence in the form of [[fossil]] [[crinoids]] (sea-lilies), [[corals]] and [[brachiopods]] of the abundant marine life of the time. At the end of the [[Permian]] (290-248 million years ago) and [[Triassic]] periods the [[Variscan orogeny]] resulted in the formation of several mountainous areas including [[Dartmoor]] in the south, Exmoor and the Quantocks and the Mendips. In the [[Taunton]] area Permian (295-250 million years ago) red sandstones and [[breccia]] outcrop, while rocks of Triassic age (248-204 million years ago) underlie much of Somerset and form the solid geology to the Somerset Moors and Levels.<ref>{{cite web | title=Somerset | work=English Nature, Special Sites, Somerset Geology | url=http://www.english-nature.org.uk/special/geological/sites/area_ID30.asp | accessdate=2006-10-30}}</ref> There are no glacial deposits.
The Triassic rocks consist of red marls, sandstones, breccias and conglomerates which spread over the older rocks. The Dolometic Conglomerate is an old shingle beach of Keuper Marl age. The Rhaetic Beds are full of fossils due to invasion of the Jurassic Sea. The Lias consists of clays and limestones, the latter being quarried and are famous for their fossils. Above the Lias is the Lower Oolite Series which are chiefly clays and oolile limestone. The famous Bath Stone is obtained from the Great Oolite bed. Oxford Clay is the chief member of the Middle Oolite Series and above this are the Upper Cretaceous rocks with Gault, Upper Greensand and Chalk. Alluvial flats and peat bogs occupy much of the centre of Somerset.
The wide variety of landscapes has led to several areas being designated as [[Sites of Special Scientific Interest]] for geological reasons, as can be seen from the [[List of Sites of Special Scientific Interest in Somerset]]
==Coastline==
[[Image:Stholm2.jpg|thumb|[[Steep Holm]]]]
In [[prehistoric]] times the coastline of Somerset was very different from the present one, the sea level at the last glacial maximum being some 30 m lower than today so that the [[Bristol Channel]] was almost non-existent. The Bristol Channel has one of the largest [[tidal range]]s in the world, up to 12 m at [[Burnham-on-Sea]] for example. This feature has meant that large areas of the county have been liable to [[flood]]ing by the sea. Thus the present coastline is partly due to seawalls built to reclaim areas previously flooded at high tide. The coastline contains exposures of Devonian sediments and tectonics west of Minehead adjoining the classic exposures of Mesozoic sediments and structural features which extend eastward to the [[River Parrett|Parrett]] estuary <ref>{{cite web | title=Somerset Geology | work=Good rock guide | url=http://mysite.wanadoo-members.co.uk/hughprudden/sgg.htm | accessdate=2006-10-30}}</ref> forming cliffs along the coastline near [[Clevedon]] and near [[Minehead]], with low sandhills near Burnham-on-Sea. There are sandy beaches mainly at Burnham-on-Sea, [[Brean Down|Brean]] and [[Weston-super-Mare]]. There are also storm ridges, salt marsh, and sand dunes.
==Main River Valleys==
[[Image:Smriver chew.JPG|thumb|[[River Chew]] between [[Stanton Drew]] and [[Pensford]].]]
[[Image:uk_som_brue.jpg|thumb|[[River Brue]] near [[Glastonbury]].]]
The main valleys between the hills are filled with alluvial deposits from the hills or sea. The county has many small [[river]]s, most of which flow into the Bristol Channel. Many of the latter rivers now have clysts on them to control the sea, but formerly they were tidal for some way inland. The main exception to this is the [[River Parrett]], which still has a [[tidal bore]]. However the [[River Chew|Chew]] and the [[River Frome|Frome]] flow into the [[River Avon, Bristol|Avon]] which forms most of the northern county boundary with [[Gloucestershire]]. The [[River Cale|Cale]] flows into Dorset through the [[Blackmore Vale]], while the [[River Exe|Exe]] flows into Devon. The (Dorset) [[River Axe|Axe]], the [[River Culm|Culm]] and the [[River Otter|Otter]] rise in Somerset but flow into [[Dorset]].
The courses of the rivers [[River Parrett|Parrett]], [[River Axe, Somerset|Somerset Axe]], [[River Brue|Brue]] and [[River Cary|Cary]] run across the [[Somerset Levels]] and have generally been changed to improve the flow. The River Axe rises from [[Wookey Hole Caves]], due to water draining into the ground at [[Sinkhole|swallet holes]] on top of the [[Mendips]]. The river passes through [[Panborough Moor]], [[Wedmore]] Moor, [[Ox Moor]], [[Stoke Moor]] and [[Mark Moor]] and reaches the sea at [[Uphill]] (near [[Weston-super-Mare]]).
The [[River Brue]] rises at [[Brewham]], close to the county border with [[Wiltshire]]. It flows through [[Bruton]] and is joined by the rivers [[River Pitt|Pitt]] and [[River Alham|Alham]]. The river then flows past East and [[West Lydford]] to ''Baltonsborough'' and then turns north to [[Street, Somerset|Street]] across [[Butts Moor]], [[South Moor]] and [[Kennard Moor]]. Originally it then joined the Axe but now it flows west across [[Westhay Moor]], [[Tealham and Tadham Moors]], [[Chilton Moor]], [[Mark Moor]] and [[Huntspill]] Moor. It is joined by the North Drain and the Hartlake river. In [[Huntspill Moor]] the Brue is linked to the man-made [[River Huntspill|Huntspill river]] by the artificial Cripps river. The Brue reaches the sea near [[Burnham-on-Sea]].
The [[River Cary]] originates in [[Castle Cary]] in the east of Somerset. It flows south-west through [[Cary Moor]] to [[Cary Fitzpaine]]. The river then turns north-west to the north of [[Somerton]]. It then used to turn south to join the Parrett but now passes through [[Somerton Moo]]r and crosses [[Kings Sedgemoor]] in an artificial channel, joining the Parrett at [[Dunball]] north of [[Bridgwater]].
The river Parrett originates at [[Cheddington]], Dorset, just over the border with Somerset. It enters Somerset at [[Haselbury Plucknett]] where it is joined by the Broad river. It passes to the east of [[South Petherton]] and flows north through [[Thorney Moor]] and [[Muchelney Level]] and it is then joined by the Isle and (Ivel) Yeo rivers. The Parrett flows through [[Langport]] and then through [[Middle Moor]], [[Aller Hill|Aller Moor]] to [[Burrowbridge]] where it is joined by the [[River Tone]]. It then passes through [[Earlake Moor]], [[Hartlake Moor]], [[Weston Level]] and [[South Moor]]. It continues north through [[Bridgwater]], [[Horsey Level]], past [[Pawlett Ham]] and [[Pawlett Level]] to the coast near Burnham-on-Sea.
The River Tone originates at Beverton Pond on the [[Brendon Hills]] in the west of Somerset. It flows south into Clatworthy reservoir and then to Greenham where it changes course to go north-east to [[Taunton]]. It continues east through [[West Moor]], [[Curry and Hay Moors]] and [[Stan Moor]] to [[Burrowbridge]] where it meets the Parrett.
The River Exe rises at [[Exehead]] on [[Exmoor]] and flows south-east to Exton where it is joined by the [[River Quarme]]. It then flows south to [[Exbridge]] where it meets the Barle and passes into Devon.
==Levels and Moors==
[[Image:uk_som_levels.jpg|thumb|The view towards [[Brent Knoll]] from [[Glastonbury Tor]].]]
The [[Somerset Levels and Moors|North Somerset Levels]] are to the east of Weston-super-Mare, while the Axe valley is separated by the Isle of Wedmore from the Brue valley. The latter is separated by the [[Polden Hills]] from the main wetland of the Parrett/Tone/Cary valleys. The Poldens are a low narrow ridge of [[Blue Lias]] with alternating bands of limesone and clay. Because of the nature of the Levels and Moors, the Poldens have a significant visual impact.
The Levels run from the coast up to 30 km inland. These wetlands cover 600 km², most being no higher than 8 m above sea level. There are coastal marine clay deposits while further inland there are areas that are often peaty. Within this wetland there are slightly raised inter-glacial "islands", called burtles. These have been settled from the [[Mesolithic]] onward, with wooden causeways linking them to higher ground. There are also rocky outcrops, such as [[Brent Knoll]] and [[Glastonbury Tor]] which similarly have had ancient settlements.
The water levels in the moors and levels are controlled by a series of small rhines (a narrow canal) (pronounced reens) and larger drains, together with gates and pumping stations. The area is mainly used for grazing but some [[peat]] extraction is carried out. Some parts are allowed to flood in winter.
==Northern Uplands==
[[Image:Beaconbatchmay2005.jpg|thumb|A view of Black Down from near [[Charterhouse]] Wireless Station in the [[Mendip Hills]]]]
This is the area between the [[River Avon, Bristol|River Avon]] to the north and the Axe valley. The north of Somerset is dominated by the tableland of the [[Mendip Hills]], stretching from [[Frome]] in the east to [[Crook Peak to Shute Shelve Hill|Crook Peak]] in the west, with outliers of [[Bleadon Hill]] and [[Brean Down]] as well as [[Steep Holm]] in the Bristol Channel. The highest point is [[Black Down, Somerset|Black down]] at 324m. There is an escarpment facing south to the Levels and Moors while the dip slope to the north is broken up.
To the north of [[Bath, Somerset|Bath]] are Landsdown, Langridge and [[Solsbury Hill|Solsbury]] hills. These are outliers of the [[Cotswolds]]. Bath is noted for its thermal waters (48 ºC) that are rich in [[calcium]] and [[sodium sulphate]]s.
The [[Old Red Sandstone]] is a series of red [[Sandstone]], [[marl]]s and [[Conglomerate (geology)|conglomerate]]s. It rises as an [[anticline]] in the Mendips and appears in the [[Avon Gorge]] and at [[Portishead, Somerset|Portishead]]. [[Carboniferous limestone]], of maritime origin, covers the sandstone and appears in the Avon Gorge and at Weston-super-Mare where it contains [[volcano|volcanic]] rocks. There are outlying hills at Worlebury, Middle Hope, the Failand Ridge, Broadfields Down, Portishead Down and Wrington Hill.
The main geological component of the Mendips is [[carboniferous limestone]] and represent the remnants of a much higher range of hills that existed hundreds of millions years ago.<ref>{{cite web | title=Somerset | work=English Nature, Special Sites, Somerset Geology | url=http://www.english-nature.org.uk/special/geological/sites/area_ID30.asp | accessdate=2006-10-30}}</ref> This has allowed the formation of features such as [[Cheddar Gorge]], [[Ebbor Gorge]] and [[Burrington Combe]]. There are a wide variety of [[Caves of the Mendip Hills]] and [[Sinkhole|swallett holes]] caused by dissolution of the rock by water. Further east there are [[silurian]] [[volcanos]], Carboniferous limestone outcrops, variscan thrust tectonics, [[Permian-Triassic extinction event|Permo-Triassic]] conglomerates, sediment-filled fissures, a classic unconformity, [[Jurassic]] clays and limestones, [[Cretaceous]] Greensand and chalk topped with Tertiary remnants including [[Sarsen|Sarsen Stone]]s. These sediments have yielded a fairly rich fossil fauna of [[brachiopods]] and [[trilobites]] indicating that they were deposited in a shallow marine sea into which the lavas were extruded. The rocks are quarried at [[Moons Hill]] near [[Stoke St Michael]] for aggregate.
[[Coal measures]] appear in the [[Radstock]] district, and surrounding [[Somerset coalfield]] (largely concealed by [[Triassic]] and newer rocks). There are two series of coal-bearing sandstones and [[shale]]s separated by Pennant sandstone. Locally the beds are folded and faulted. There were mines in the Radstock and Nailsea areas but these have closed.
The Mendips were mined for [[lead]], [[silver]], [[coal]], [[ochre]], [[Fuller's earth]] and [[zinc]] but this has finished. They were also quarried for stone, notably at Bath and Doulting. Today the Mendips are a major source of aggregates.
==Southern Uplands==
[[Image:Stoke_under_ham_hill.jpg|thumb|Ham Hill summit and war memorial seen from Stoke-sub-Hamdon]]
To the south of Somerset there is an upland with a series of rolling valleys and scarps, from Penselwood in the east to the [[Blackdown Hills]] in the west. This is a geologically complex area of clays, limstone and marl. The honey-coloured limestone has been quarried at [[Ham Hill Country Park|Ham Hill]] (also known as Hamdon Hill) since Roman times at least.
The Blackdowns are on the south-west border of Somerset, extending into Devon. They are composed of [[Upper Greensand]]. The scarp faces north and is steep and wooded, with a south facing dip slope. There is an open plateau, which is not as high as the Mendips.
==Quantocks==
The [[Quantock Hills]] are a 20 km long broad ridge from the coast near [[Watchet]] in the north to near Taunton in the south. They reach 384 m high at [[Wills Neck]] and are separated from Exmoor and the Brendons by a [[rift valley]]. The Quantocks and the Brendon Hills at the eastern end of Exmoor are formed by thick sequences of [[slates]] and [[sandstones]] of [[Devonian]] age that were deposited by large deltas that built out into a shallow sea.
[[Image:Quantocksbeach.jpg|thumb|Beach at Quantock's Head. The wave cut platform at low tide. The short 'cliff' shows the rock strata]]
The Quantock Hills are largely formed by rocks of the [[Devonian]] Period, which consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock. In the higher north western areas older [[Early Devonian]] rocks, known as Hangman Grits, predominate, and can be seen in the exposed rock at [[West Quantoxhead]] quarry, which were worked for road building.<ref name="waite">{{cite book |title=Portrait of the Quantocks |last=Waite |first=Vincent |authorlink= |coauthors= |year=1964 |publisher=Robert Hale |location=London |isbn=0709111584 |pages= }}</ref> Further south there are newer [[Middle Devonian|Middle]] and [[Late Devonian]] rocks, known as Ilfracombe beds and Morte Slates. These include sandstone and limestone, which have been quarried near [[Aisholt]]. At Great Holwell, south of Aisholt, there is a limestone cave, which is the only one in the Devonian limestone of North [[Devon]] and West Somerset.<ref name="waite"/> The lower fringes around the hills are composed of younger rocks of the [[Triassic]] period,<ref>{{cite web |url=http://www.somerset.gov.uk/countryside/quantockhills/quantockhillsgeology.htm |title=Quantock Hills Geology |accessdate=2008-03-03 |format= |work=Quantock Hills AONB }}</ref> these are known as [[New Red Sandstone]] rocks which which represent the deposits of large river systems that crossed a desert plain,<ref>{{cite web | title=Somerset | work=English Nature, Special Sites, Somerset Geology | url=http://www.english-nature.org.uk/special/geological/sites/area_ID30.asp | accessdate=2006-10-30}}</ref>, and often contain irregular masses or veins of [[gypsum]], which was worked on the foreshore at [[Watchet]].<ref name="waite"/> The scarp is to the west with a dip slope to the east. The west side is cut by combes with broad valleys on the east. The hill tops are open [[heathland]] with woods on the slopes.
Several areas have outcrops of slates and between [[St Audries]] and [[Kilve]], younger rocks of the [[Jurassic]] Period can be found. This area falls within the [[Blue Anchor to Lilstock Coast SSSI|Blue Anchor to Lilstock]] [[Site of Special Scientific Interest]] (SSSI) and is considered to be of international geological importance. At Kilve are the remains of a red brick retort, built in 1924, when it was discovered that the shale found in the cliffs was rich in oil.<ref>{{cite web | title=Oil retort house | work=Images of England | url=http://www.imagesofengland.org.uk/search/details.aspx?id=265166 | accessdate=2007-10-07}}</ref> At [[Blue Anchor]] the coloured alabaster found in the cliffs gave rise to the name of the colour "Watchet Blue".<ref name="curio">{{cite book |title=Curiosities of Somerset |last=Leete-Hodge |first=Lornie |authorlink= |coauthors= |year=1985 |publisher=Bossiney Books |location=Bodmin |isbn=0906456983 |pages=41 }}</ref>
==Exmoor==
[[Image:SnowyExmoor.jpg|thumb|[[Exmoor]] in winter]]
[[Exmoor]] is a dissected plateau of [[Devonian]] [[sedimentary]] rock, rising to 517 m at [[Dunkery Beacon]]. It extends into Devon but the majority of the area is in Somerset. Much of the area is a [[National Park]]. The landscape is one of rounded hills, with hogs-back cliffs at the coast due to geological movements. Because of high rainfall there are boggy areas and the part by the [[Chains (geological site)|Chains]] is the wildest in Somerset.
As this area of Britain was not subject to [[glaciation]], the plateau remains as a remarkably old landform.<ref>{{cite web |url=http://www.exmoor-nationalpark.gov.uk/index/looking_after/looking_after_landscape/geology.htm |title=Geology |accessdate=2007-11-28 |format= |work=Exmoor National Park }}</ref><ref>{{cite web |url=http://www.english-nature.org.uk/science/natural/NA_HAbDetails.asp?Name=Exmoor+and+the+Quantocks&N=87&H=65 |title=Exmoor and the Quantocks |accessdate=2007-11-28 |format= |work=Natural England }}</ref>
[[Quartz]] and [[iron]] mineralisation can be detected in outcrops and subsoil.<ref>{{cite web |url=http://mysite.wanadoo-members.co.uk/hughprudden/sgg.htm |title=Somerset Geology |accessdate=2007-11-28 |format= |work=Good Rock Guide }}</ref> The underlying rocks are covered by moors are supported by wet, acid soil.<ref name="eelandscape">{{cite web |url=http://www.everythingexmoor.org.uk/_L/Landscape.php |title=Landscape of Exmoor National Park |accessdate=2007-12-03 |format= |work=Everything Exmoor }}</ref> The highest point on Exmoor is [[Dunkery Beacon]]; at {{convert|1704|ft|m|0}} it is also the highest point in Somerset.<ref name="moorfacts"/>
Exmoor has {{convert|34|mi|km|0}} of coastline, including the highest cliffs in England, which reach a height of {{convert|1350|ft|m|0|lk=on}} at [[Culbone]] Hill. However, the crest of this coastal ridge of hills is more than a mile (1.6 km) from the sea. If a cliff is defined as having a slope greater than 60 degrees, the highest cliff on mainland Britain is [[Hangman cliffs|Great Hangman]] near [[Combe Martin]] at {{convert|1043|ft|m|0}} high, with a cliff face of {{convert|800|ft|m|0}}.<ref name="moorfacts">{{cite web |url=http://www.exmoor-nationalpark.gov.uk/index/learning_about/moor_facts.htm |title=Moor Facts |accessdate=2007-11-28 |format= |work=Exmoor National Park }}</ref> Its sister cliff is the {{convert|716|ft|m|0}} Little Hangman, which marks the edge of Exmoor.
Exmoor's woodlands sometimes reach the shoreline,<ref>{{cite web|url=http://www.exmoor-nationalpark.gov.uk/index/looking_after/looking_after_landscape/woodland_and_trees.htm|title=Woodland and Trees|accessdate=2007-12-08}}</ref> especially between [[Porlock]] and ''The Foreland'', where they form the single longest stretch of coastal woodland in England and Wales.<ref>{{cite web|url=http://www.exmoor-nationalpark.gov.uk/index/looking_after/looking_after_landscape/rivers_coast_lines.htm|title=Landscape / Rivers and the coast|publisher=Exmoor Park|accessdate=2007-12-08}}</ref> The [[Exmoor Coastal Heaths]] have been recognised as a [[Site of Special Scientific Interest]] due to the diversity of plant species present.<ref>{{cite web | title=Exmoor Coastal Heaths | work=English Nature | url=http://www.english-nature.org.uk/citation/citation_photo/1004152.pdf | accessdate=2006-08-12}}</ref>
[[Image:LowerEastLynRiver.jpg|thumb|left|The [[East Lyn River]]]]
The high ground forms the [[Drainage basin|catchment area]] for numerous rivers and streams. There are about {{convert|300|mi|km|0}} of named rivers on Exmoor.<ref>{{cite web |url=http://www.exmoor-nationalpark.gov.uk/filex_07.pdf |title=Water on Exmoor - Filex 7 |accessdate=2007-12-03 |format=PDF |work=Exmoor National Park }}</ref> The [[River Exe]], from which Exmoor takes its name,<ref>{{cite web |url=http://www.visit-exmoor.co.uk/site/about-exmoor/countryside |title=Countryside |accessdate=2007-12-07 |format= |work=Vsit Exmoor }}</ref><ref>{{cite web |url=http://www.equinetourism.co.uk/editorial/exmoorridinginfo.asp |title=Exmoor National Park |accessdate=2007-12-07 |format= |work=Equine Tourism }}</ref> [[source (river)|rises]] at [[Exe Head]] near the village of [[Simonsbath]], close to the [[Bristol Channel]] coast, but flows more or less directly due south, so that most of its length lies in [[Devon]]. The river and the [[Barle Valley]] are both designated as [[biological Site of Special Scientific Interest|biological sites of Special Scientific Interest]]. Another tributary, the [[River Haddeo]], flows from the [[Wimbleball Lake]].
The action of streams has cut combes through the hills down to the sea, which are now wooded. Much of Exmoor is open heathland. There is an outlier of Exmoor at North Hill near Minehead. Iron working was formerly carried out, probably from the Roman period onward.
Because Exmoor was a [[royal forest]], ie a hunting reserve, it was unpopulated in [[Medieval]] times. The first house on the moor was only built at [[Simonsbath]] in [[1654]]. It was not until the 19th century that [[farm]]s were built around the moor.
==Brendons==
The [[Brendon Hills]] are to the east of Exmoor and are an outlier of it. They are separated from Exmoor by the valley of the [[River Aville]]. They have the same undulating landscape. The Brendons reach a height of 422 m at [[Lype Hill]]. [[Iron ore]] [[mining]] was carried out from [[Roman Empire|Roman]] times up to the early 20th century.
== References ==
{{reflist|2}}
==Bibliography==
* {{cite book|last=Hardy|first=Peter|title=The Geology of Somerset|publisher=Ex Libris Press|location=Bradford on Avon|date=1999|isbn=0948578424}}
* British Geological Survey 1:250,000 Sheets 50N04W and 51N04W.
* British Regional Geology, Bristol and Gloucester Region.
* British Regional Geology, South West England.
==See also==
*[[Somerset]]
*[[Geology of the UK]]
*[[History of Somerset]]
*[[Geology of England]]
== External links ==
* [http://www.somerset.gov.uk/media/087/B7/Report.pdf The Aggregate Landscape of Somerset: Predicting the Archaeological Resource]
{{Somerset}}
[[Category:Geology of England]]
[[Category:Geography of Somerset]]