Geology of Somerset

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Somerset

geological sections from the Silurian through the Devonian and Carboniferous to the Permian
which influence the landscape, together with water-related features.

The low-lying areas of the

Sites of Special Scientific Interest for geological reasons, and support a range of flora and fauna as can be seen from the List of Sites of Special Scientific Interest in Somerset
.

Rock ages

The oldest rocks are of

mudstones in a narrow outcrop to the northeast of Shepton Mallet, in the eastern Mendip Hills.[1]

Rocks from the Devonian (419–359 million years ago) are found across much of Exmoor,[2] the Quantocks (including Hangman Sandstone and Cockercombe tuff), and in the cores of the folded masses of the Mendip Hills.

Somerset Levels and Moors. The limestones are very fossiliferous, and contain evidence of the abundant marine life that existed at the time of their creation, including fossil crinoids (sea-lilies), corals and brachiopods
.

At the end of the Permian (299–252 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 (299–252 million years ago) red sandstones and breccia outcrop, although rocks of Triassic age (252–201 million years ago) underlie much of Somerset and form the solid geology of the Somerset Moors and Levels.[3] There are no glacial deposits.

Carhampton

The Triassic rocks consist of red marls, sandstones, breccias and conglomerates which spread over the older rocks. The Dolomitic 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. Blue Lias was burnt locally to provide a source of lime for making lime mortar. It is still used as a decorative building stone. Blue Lias is believed to have been quarried on the Polden Hills as early as the 15th century and was quarried in Puriton from the early 19th century until 1973, when the local cement works closed.[4]

Above the Lias is the Lower

Alluvial flats and peat bogs occupy much of the centre of Somerset.[7]

Coastline

A rocky island with steep sides rises sharply out of the water.
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 several hundred feet lower than today so that the Bristol Channel was almost non-existent. The Bristol Channel has one of the largest tidal ranges in the world, up to 12 metres (39 ft) at Burnham-on-Sea for example,[8] behind only the Bay of Fundy and Ungava Bay[9] in Canada.[10][11] Normal high tide may be enhanced by between 3 metres (10 ft) and 4 metres (13 ft) during storm surges.[12] This feature has meant that large areas of the county have been liable to flooding by the sea. Thus the present coastline is partly due to a belt of marine clay at the coast and partly due to seawalls built to reclaim areas previously flooded at high tide.[7] 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 Parrett estuary[13] forming cliffs along the coastline near Clevedon and near Minehead,[14] with low sandhills near Burnham-on-Sea.[15] There are sandy beaches mainly at Burnham-on-Sea, Brean and Weston-super-Mare.[16] There are also storm ridges, salt marsh, and sand dunes.[17]

Main river valleys

The main valleys between the hills are filled with alluvial deposits from the hills or sea. The county has many small

Cale flows into Dorset through the Blackmore Vale, while the Exe flows into Devon. The (Dorset) Axe, the Culm and the Otter rise in Somerset but flow into Devon.[21]

River flowing between grassy banks surrounded by trees.
River Chew between Stanton Drew and Pensford

The courses of the rivers Parrett, Somerset Axe, Brue and Cary run across the Somerset Levels and have generally been changed to improve the flow.[22] The River Axe rises from Wookey Hole Caves, due to water draining into the ground at 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) on Bridgwater Bay.[23]

The River Brue rises at Brewham, close to the county border with Wiltshire. It flows through Bruton and is joined by the rivers Pitt and Alham. The river then flows past East and West Lydford to Baltonsborough and then turns north to 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.[23] It is joined by the North Drain and the Hartlake river. In Huntspill Moor the Brue is linked to the man-made Huntspill river by the artificial Cripps river.[24] The Brue reaches the sea near Burnham-on-Sea.

A straight water filled channel surrounded by an avenue of trees and grassy banks.
River Brue near Glastonbury

The

Kings Sedgemoor in an artificial channel, the King's Sedgemoor Drain, joining the Parrett at Dunball north of Bridgwater.[24]

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 Yeo (Ivel) rivers.[21] The Parrett flows through Langport and then through Middle Moor, 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.[23]

The River Tone originates at Beverton Pond on the

West Moor, Curry and Hay Moors and Stan Moor to Burrowbridge where it meets the Parrett.[26]

The River Exe rises at Exehead on Exmoor[27] and flows south-east to Exton where it is joined by the River Quarme. It then flows south to Exebridge where it meets the Barle and passes into Devon.[28]

Levels and moors

Main article: Somerset Levels
View over flat landscape with a patchwork of fields.
The view towards Brent Knoll from Glastonbury Tor

The

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.[15] The latter is separated by the Polden Hills from the main wetland of the Parrett/Tone/Cary valleys.[15] The Poldens are a low narrow ridge of Blue Lias
with alternating bands of limestone and clay. Because of the nature of the Levels and Moors, the Poldens have a significant visual impact.

The Somerset Levels run from the coast up to 30 kilometres (19 mi) inland. These wetlands cover 600 square kilometres (232 sq mi), most of which is no higher than 8 metres (26 ft) above sea level.[29] There are coastal marine clay deposits, and further inland there are many peaty areas.[7] Dotted within this wetland landscape are slightly raised inter-glacial "islands" called burtles.[30] These have been settled from the Mesolithic onward, with wooden causeways linking them to higher ground.[31] There are also rocky outcrops, such as Brent Knoll and Glastonbury Tor which have also housed ancient settlements.[32]

The water levels in the moors and levels are controlled by a series of small narrow canals called

hedges.[33] Some parts are allowed to flood in winter. The area is mainly used for grazing but some peat extraction is carried out.[34]

Northern uplands

Main articles: Mendip Hills and Somerset Coalfield

This is the area between the

area of outstanding natural beauty, stretching from Frome in the east to 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 at 324 metres (1,063 ft).[35] There is an escarpment facing south to the Levels and Moors while the dip slope to the north is broken up.[20]

Tyre tracks run through flat grassland toward a large mound on the distant horizon.
A view of Black Down from near Charterhouse Wireless Station in the Mendip Hills

To the north of

sodium sulphates.[36]

The Old Red Sandstone is a series of red sandstones, marls and conglomerates. It rises as an anticline in the Mendips and appears in the Avon Gorge and at Portishead. Carboniferous Limestone, of marine origin, covers the sandstone and appears in the Avon Gorge and at Weston-super-Mare where it contains volcanic rocks.[37] 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. It represents the remnants of a much higher range of hills that existed hundreds of millions years ago.

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.[39]

Pennant Sandstone. Locally the beds are folded and faulted. There were mines in the Radstock and Nailsea areas but these have closed. This was one of the first areas in the world to undergo systematic geological study and mapping by John Strachey and William Smith in the 18th century.[41][42] They observed the rock layers, or strata, which led Smith to the creation of a testable hypothesis, which he termed the Principle of Faunal Succession.[43]

The Mendips were mined for lead, silver, coal, ochre, fuller's earth and zinc but this has finished.[38][39][44] They were also quarried for stone, notably at Bath and Doulting.[5] Today the Mendips are a major source of aggregates.[45]

Southern uplands

Triangular shaped building made of corrugated iron and stone.
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

Ham Hill (also known as Hamdon Hill) is particularly important to geologists because of the assemblages of fossils which it contains, the sedimentary features which it displays and the way it relates to other rocks of equivalent age in the close vicinity.[48] It has been quarried since Roman times at least.[49]

The Blackdowns are on the south-west border of Somerset, extending into Devon. They are composed of

Upper Greensand.[47] 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.[21]

The

sandstones of Devonian age that were deposited by large deltas that built out into a shallow sea.[50]

Gently sloping rock slab beach. In the distance are cliffs showing lines of striation.
Beach at Quantoxhead. The wave cut platform at low tide. The short 'cliff' shows the rock strata

The Quantock Hills are largely formed by rocks of the

heathland with woods on the slopes.[25]

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 Site of Special Scientific Interest (SSSI) and is considered to be of international geological importance.[54] 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.[55] At Blue Anchor the coloured alabaster found in the cliffs gave rise to the name of the colour "Watchet Blue".[56]

Exmoor

Main article: Exmoor
Rounded hills, some wooded, and a river valley are covered in snow.
Exmoor in winter

palynological record of a mid to late Flandrian vegetation history on Exmoor. The pollen sequence in the peat is calibrated by radiocarbon dating.[58] The Glenthorne area demonstrates the Trentishoe Formation of the Hangman Sandstone Group. The Hangman Sandstone represents the Middle Devonian sequence of North Devon and Somerset.[59] These unusual freshwater deposits in the Hangman Grits, were mainly formed in desert conditions.[60]

As this area of Britain was not subject to

glaciation, the plateau remains as a remarkably old landform.[61][62]
Dunkery Beacon; at 519 metres (1,703 ft) it is also the highest point in Somerset.[64]

Exmoor has 55 kilometres (34 mi) of coastline, including the highest cliffs in England, which reach a height of 1,350 feet (411 m) at Culbone Hill.[65] However, the crest of this coastal ridge of hills is more than 1.6 kilometres (1.0 mi) from the sea. If a cliff is defined as having a slope greater than 60 degrees, the highest cliff on mainland Britain is Great Hangman near Combe Martin at 318 metres (1,043 ft) high, with a cliff face of 214 metres (702 ft).[64] Its sister cliff is the 218 metres (715 ft) Little Hangman,[66] which marks the edge of Exmoor.

Exmoor's woodlands sometimes reach the shoreline,[67] especially between Porlock and The Foreland, where they form the single longest stretch of coastal woodland in England and Wales.[68] The Exmoor Coastal Heaths have been recognised as a Site of Special Scientific Interest due to the diversity of plant species present.[69]

Small stream gushes over rocks, surrounded by woodlands.
The East Lyn River

The high ground forms the

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, although 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.[73]

Because Exmoor was a

Medieval times. The first house on the moor was only built at Simonsbath in 1654.[74] It was not until the 19th century that farms
were built around the moor.

The

Lype Hill.[75] Iron ore mining[76] was carried out from Roman times up to the early 20th century.[75]

See also

References

  1. ^ Roche, David (2004). "Moons Hill Quarry, Stoke St Michael, Shepton Mallet" (PDF). Geodiversity Audit of Active Aggregate Quarries. Somerset County Council. Retrieved 21 August 2008.
  2. ^ "Geology of Exmoor". Everything Exmoor. Retrieved 21 August 2008.
  3. ^ a b c "Somerset". English Nature, Special Sites, Somerset Geology. Retrieved 30 October 2006.
  4. ISBN 0-19-722780-5
    . Page 183.
  5. ^
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  6. ^
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  7. ^
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  8. ^ "Severn Estuary Barrage". UK Environment Agency. 31 May 2006. Archived from the original (PDF) on 30 September 2007. Retrieved 3 September 2007.
  9. ^ Charles T. O'reilly, Ron Solvason, and Christian Solomon. "Resolving the World's largest tides", in J.A Percy, A.J. Evans, P.G. Wells, and S.J. Rolston (Editors) 2005: The Changing Bay of Fundy-Beyond 400 years, Proceedings of the 6th Bay of Fundy Workshop, Cornwallis, Nova Scotia, Sept. 29, 2004 to October 2, 2004. Environment Canada-Atlantic Region, Occasional Report no. 23. Dartmouth, NS and Sackville, NB.
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  11. ^ "Coast: Bristol Channel". BBC. Retrieved 27 August 2007.
  12. ISBN 0-7185-0069-5
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  13. ^ a b "Somerset Geology" (PDF). Good rock guide. Archived from the original (PDF) on 8 June 2013. Retrieved 6 June 2012.
  14. ^ "Somerset Coast". Visit Somerset. Retrieved 20 August 2008.
  15. ^
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  16. ^ "Burnham-on-Sea, Berrow and Brean Sands". Sedgemoor District Council. Retrieved 20 August 2008.
  17. doi:10.1111/j.1502-3885.1978.tb00280.x. Archived from the original
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  18. ^ "FOCUS on Industrial Archaeology No. 68, June 2007". Hampshire Industrial Archaeology Society website. Archived from the original on 10 November 2007. Retrieved 30 October 2007.
  19. ^ "River Parrett". Waterscape.com. Retrieved 21 August 2008.
  20. ^ a b Landranger Map 172: Bristol & Bath. Published in 2006 by the Ordnance Survey
  21. ^ a b c Landranger Map 193: Taunton & Lyme Regis. Published in 2007 by the Ordnance Survey
  22. ISBN 978-0-19-921715-1
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  23. ^ a b c d Landranger Map 182: Weston-super-Mare. Published in 2005 by the Ordnance Survey
  24. ^
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  25. ^ a b c Landranger Map 181: Minehead & Brendon Hills. Published in 2006 by the Ordnance Survey
  26. ^ Landranger Map 183: Yeovil & Frome. Published in 2005 by the Ordnance Survey
  27. ^ Landranger Map 180: Barnstaple & Ilfracombe. Published in 2008 by the Ordnance Survey
  28. ^ Landranger Map 192: Exeter & Sidmouth. Published in 2007 by the Ordnance Survey
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  32. ISBN 0-86183-215-9
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  33. ISBN 0-521-07486-X
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  43. ^ As recounted in Simon Winchester, The Map that Changed the World (New York: HarperCollins, 2001), pp. 59-91.
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  66. JSTOR 1789584
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  67. ^ "Woodland and Trees". Exmoor National Park. Archived from the original on 31 May 2012. Retrieved 6 June 2012.
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  69. ^ "Exmoor Coastal Heaths" (PDF). English Nature. Archived from the original (PDF) on 13 October 2006. Retrieved 12 August 2006.
  70. ^ "Water on Exmoor — Filex 7" (PDF). Exmoor National Park. Archived from the original (PDF) on 30 May 2008. Retrieved 3 December 2007.
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  73. ^ Gathercole, Clare. "Minehead" (PDF). Somerset Urban Archaeological Survey. Somerset County Council. Archived from the original on 17 July 2011. Retrieved 20 August 2008.
  74. ^ Historic England. "Simonsbath House Hotel (1058031)". National Heritage List for England. Retrieved 27 November 2007.
  75. ^ a b "Brendon Hills". Everything Exmoor. Archived from the original on 14 January 2009. Retrieved 21 August 2008.
  76. ^ "Brendon Hills NMP". National Mapping Programme. English Heritage. Retrieved 21 August 2008.

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