Geology of Somerset
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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 Britain. These comprise a sequence of lavas, tuffs (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 red sandstones and breccia outcrop while rocks of Triassic age underlie much of Somerset and form the solid geology to the Somerset Moors and Levels.[1]
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
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[edit] Coastline
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 with the Bristol Channel was almost non-existent. The Bristol Channel has one of the largest tidal ranges 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 flooding 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 Parrett estuary [2] 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 and Weston-Super-Mare. There are also sandy beaches, storm ridges, salt marsh, and sand dunes.
[edit] Main River Valleys
The main valleys between the hills are filled with alluvial deposits from the hills or sea. The county has many small rivers, 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 Chew and the Frome flow into the Avon which forms most of the northern county boundary with Gloucestershire. The 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 Dorset.
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. The River Axe rises from Wookey Hole, due to water draining into the ground at swallett holes on top of the Mendips. The river passes through Panborough Moor, Wedmoor 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 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. 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. 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 Moor 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 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.
[edit] Levels and Moors
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. The latter is separated by the Polden Hills from the main wetland of the Parrett/Tone/Cary valleys. The Polden 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.
[edit] Northern Uplands
This is the area between the 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 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 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 are Landsdown, Langridge and Solsbury hills. These are outliers of the Cotswolds. Bath is noted for its thermal waters (48 ºC) that are rich in calcium and sodium sulphates.
The Old Red Sandstone is a series of red Sandstone, marls and conglomerates. It rises as an anticline in the Mendips and appears in the Avon Gorge and at Portishead. Carboniferous limestone, of maritime origin, appears in the Avon Gorge and at Weston-Super-Mare where it contains volcanic rocks. There are no glacial deposits. 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.[3] This has allowed the formation of features such as Cheddar Gorge, Ebbor Gorge and Burrington Combe. A major feature of the Mendips is caves and swallett holes caused by dissolution of the rock by water. Further east there are silurian volcanos, carboniferous limestone outcrops, variscan thrust tectonics, Permo-Triassic conglomerates, sediment-filled fissures, a classic unconformity, Jurassic clays and limestones, Cretaceous Greensand and chalk topped with Tertiary remnants including Sarsen Stones. 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 shales separated by Pennant sandstone. Locally the beds are folded and faulted.
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.
[edit] Southern Uplands
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 (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.
[edit] 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. All of these hill ranges are surrounded by mudstones and sandstones of Triassic age, which represent the deposits of large river systems that crossed a desert plain.[4] 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. The Quantocks have few caves though there is a notably one at Holwell.
[edit] Exmoor
Exmoor is a disected 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 is the wildest in Somerset. 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 farms were built around the moor.
[edit] 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 times up to the early 20th century.
[edit] References
- ^ Somerset. English Nature, Special Sites, Somerset Geology. Retrieved on 2006-10-30.
- ^ Somerset Geology. Good rock guide. Retrieved on 2006-10-30.
- ^ Somerset. English Nature, Special Sites, Somerset Geology. Retrieved on 2006-10-30.
- ^ Somerset. English Nature, Special Sites, Somerset Geology. Retrieved on 2006-10-30.
