Geology of Exmoor National Park

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The geology of Exmoor National Park in south-west England contributes significantly to the character of Exmoor, a landscape which was designated as a national park in 1954. The bedrock of the area consists almost wholly of a suite of sedimentary rocks deposited during the Devonian, a period named for the English county of Devon in which the western half of the park sits. The eastern part lies within Somerset and it is within this part of the park that limited outcrops of Triassic and Jurassic age rocks are to be found.

Contents

Devonian

Each of the individual sandstones and mudstones of Devonian age within the park are ascribed to the Exmoor Group.

Lynton Formation

The oldest strata (of Emsian to Eifelian age) are the slates and finely laminated sandstones of the Lynton Formation (traditionally known as the Lynton Slates or Lynton Beds) which are exposed along the coast in the Lynmouth area between Woody Bay and Countisbury Hill. They also extend west in the lowermost part of the cliffs as far as Ramsey Beach, near Heddon's Mouth. The Valley of Rocks is an abandoned valley carved into this formation west of Lynmouth. [1]

Hangman Sandstone Formation

The Lynton Formation sandstones are overlain by those of the Hangman Sandstone Formation (traditionally the Hangman Grits) of Eifelian to Givetian age and which may be up to 2500m thick. This unit also includes some shales, mudstones, siltstones and conglomerates. These rocks are exposed along the entire coast east from Hangman Point to Woody Bay and again from Countisbury Hill to Porlock Weir. They also form the cliffed coast of North Hill, west of Minehead. The highest point on Exmoor, Dunkery Beacon, is formed from the Hangman Sandstone whilst Dunster Castle is built upon a spur of the same rock. [2] In March 2024 scientists reported the discovery of a fossilised forest of calamophyton trees at a coastal site near Minehead, its age being four million years greater than a fossil forest in New York State which had previously held the record as the world's oldest. [3]

Ilfracombe Slates Formation

Next comes the Ilfracombe Slates Formation (earlier known as the Ilfracombe Beds) which is a succession of sandstones, slates and a fossiliferous limestone of Givetian to Frasnian age. [4] The formation is subdivided into several ‘members’ of which the lowest (oldest) is the Wild Pear Slates (named for its occurrence at Wild Pear Beach), overlain by the Lester Slates and Sandstone (named for Lester Point and Cliff where it is exposed), then the Combe Martin Slates and finally the Kentisbury Slates which is the uppermost i.e. youngest part of the formation. The latter two are not exposed on the Exmoor coast but are to found within the park, in a strip of country east from Kentisbury.

Morte Slates Formation

The Ilfracombe Slates are overlain in turn by the Morte Slates Formation of Frasnian to Famennian age. [5] Extending east from Challacombe, the slates provide much of the high ground south of the B3358 road through Simonsbath and Exford including Setta Barrow and form the Brendon Hills before extending to the easternmost extremity of the park at Elworthy.

Pickwell Down Sandstones, Upcott Slates and Baggy Sandstones formations

Sandstones and shales characterise the next unit which is the Pickwell Down Sandstones Formation (earlier known as the Pickwell Down Beds) which is of Famennian age. [6] The Upcott Slates Formation, also of Famennian age, follows. [7] The youngest Devonian rocks within the national park are sandstones, mudstones and siltstones of the overlying Baggy Sandstone Formation (earlier the Baggy and Marwood Beds), a small part of which is to be found within the park boundary just east of Brayford. [8] [9]

Permo-Trias

Unconformably overlying the Hangman Sandstone in the Porlock Basin is the Luccombe Breccia Formation, a presumed Permian age deposit consisting of calcareous breccia, sandstone and conglomerate which may be up to 650m thick. [10]

A further presumed unconformity separates the breccia from the mudstones of the overlying Mercia Mudstone Group which underlie the larger part of the low ground between Exmoor and North Hill. At the top of the group is a 25m thickness of mudstones with gypsum referred to as the Blue Anchor Formation. Above this are around 12m thickness of mudstones and limestone assigned to the Penarth Group. [11]

Lias

A small outlier of largely Jurassic age Lias strata lies at the northern margin of the Porlock Basin up against the southwesterly down-throwing fault which marks the northeastern edge of this half-graben. Perhaps as much as a 100m thickness of the Blue Lias overlies the Penarth Group. The lowermost part of the Lias may be latest Triassic in age. In character it is grey mudstones and shales with thin limestones. [11]

Structure

West of Porlock, the fold axis of the Lynton Anticline traces a curve beneath Culbone Hill, causing the Hangman Sandstones to dip moderately steeply to the north along the coastal strip. A series of broadly east–west oriented folds affect the Hangman Sandstones at North Hill and south of Minehead. [11]

The Lynmouth-East Lyn Fault is a reverse or thrust fault dipping south at about 45 degrees which brings Lynton Slates over the Hangman Sandstones.

Quaternary

Exmoor was to the south of the ice-sheet during the last glacial period though it has been postulated that a small glacier may have occupied the anomalously deep feature on the north side of Winsford Hill known as the Punchbowl. [12] During the earlier Anglian glaciation, the British ice-sheet extended as far south as the present day north coast of Somerset and Devon. Exmoor would have been subject to intense periglacial processes at this time and during other glacial periods.

Tidal flat deposits occupy the low ground at Porlock, landward of which are river terraces and the modern day alluvial deposits associated with the Horner Water and its tributaries. Deposits of fragmentary rock material, the result of weathering and downslope movement of the underlying bedrock are known as ‘head’ and are widely recorded within the valleys of the national park. A number of areas of peat are recorded on the upland surface of Exmoor. Small areas of talus (or scree) are found either side of Heddon Mouth. A variety of marine deposits are to be found along the shoreline including sand and cobbles whilst storm beach deposits are to be seen at Lynmouth. [13]

Mining

Iron ore was formerly mined in the Brendon Hills where mineral-bearing lodes provided lens-shaped bodies of ore. The highest production levels were achieved in 1877 when almost 50,000 tons were mined. Operations reduced thereafter and all work had ceased in 1910. Copper was mined near North Molton where it was found in association with lead, zinc, antimony and manganese ores. These and some gold were emplaced by fluids associated with the emplacement of the Cornubian batholith of which the Dartmoor granite is the largest exposed part. [14]

Silver and lead were worked at Combe Martin during the thirteenth century and again during the Elizabethan era. [14]

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Exmoor is loosely defined as an area of hilly open moorland in west Somerset and north Devon in South West England. It is named after the River Exe, the source of which is situated in the centre of the area, two miles north-west of Simonsbath. Exmoor is more precisely defined as the area of the former ancient royal hunting forest, also called Exmoor, which was officially surveyed 1815–1818 as 18,810 acres (7,610 ha) in extent. The moor has given its name to a National Park, which includes the Brendon Hills, the East Lyn Valley, the Vale of Porlock and 55 km (34 mi) of the Bristol Channel coast. The total area of the Exmoor National Park is 692.8 km2 (267.5 sq mi), of which 71% is in Somerset and 29% in Devon.

<span class="mw-page-title-main">Old Red Sandstone</span> Assemblage of rocks in the North Atlantic region

Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It also extends northwards into Greenland and Svalbard. These areas were a part of the paleocontinent of Euramerica (Laurussia). In Britain it is a lithostratigraphic unit to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology. The presence of Old in the name is to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.

<span class="mw-page-title-main">A39 road</span> Road in south west England

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<span class="mw-page-title-main">Geology of Somerset</span>

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<span class="mw-page-title-main">Oare, Somerset</span> Human settlement in England

Oare is a village and civil parish on Oare Water on Exmoor in the Somerset West and Taunton district of Somerset, England. It is located 6 miles (10 km) east of Lynton and the parish includes the hamlet of Oareford and the village of Culbone which contains its own tiny church.

<span class="mw-page-title-main">Hangman cliffs</span>

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The Lias Group or Lias is a lithostratigraphic unit found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, shales, marls and clays.

<span class="mw-page-title-main">Mercia Mudstone Group</span> Early Triassic lithostratigraphic group

The Mercia Mudstone Group is an early Triassic lithostratigraphic group which is widespread in Britain, especially in the English Midlands—the name is derived from the ancient kingdom of Mercia which corresponds to that area. It is frequently encountered in older literature as the Keuper Marl or Keuper Marl Series.

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For the Skiddaw group of hills, see Skiddaw Group

<span class="mw-page-title-main">Sherwood Sandstone Group</span>

The Sherwood Sandstone Group is a Triassic lithostratigraphic group which is widespread in Britain, especially in the English Midlands. The name is derived from Sherwood Forest in Nottinghamshire which is underlain by rocks of this age. It has economic importance as the reservoir of the Morecambe Bay gas field, the second largest gas field in the UK.

<span class="mw-page-title-main">Exeter Group</span> Group of rock formations

The Exeter Group is a Permian lithostratigraphic group which occurs through East Devon in southwest England. The name is derived from the city of Exeter in Devon which is partly underlain by rocks of this age.

Haffield Breccia, or Clent Breccia, consist of a texturally immature compacted gravel, rich in volcanic clasts with some sedimentary rocks, in a sandy or muddy matrix, which outcrops in the English Midlands, in South Staffordshire, Birmingham and the Malverns. It is thought to have been deposited by during flash floods in rivers that were flowing through a desert, somewhere between 200 and 280 million years ago during the Permian period. The gravel consists of angular fragments, showing that they have not been transported over long distances.

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The Milford Haven Group is a late Silurian to early Devonian lithostratigraphic group in west Wales. The name is derived from the estuary and town of Milford Haven in south Pembrokeshire. The Group comprises calcareous marls with occasional sandstones along with conglomerates and breccias.

<span class="mw-page-title-main">Exmoor Group</span>

The Exmoor Group is a late Devonian to early Carboniferous lithostratigraphic group in southwest England whose outcrop extends from Croyde in north Devon east across Exmoor to Minehead in west Somerset. The group comprises the following formations :

The Tyrone Group is a lithostratigraphical term coined to refer to a particular succession of rock strata which occur in Northern Ireland within the Visean Stage of the Carboniferous Period. It comprises a series of limestones, shales and sandstones which accumulated to a thickness of 2400m in the northwest Carboniferous basin of Ireland. The type areas for the group are the Clogher Valley of County Tyrone and the Fermanagh Highlands of nearby County Fermanagh. The rocks of the group sit unconformably on older rocks of the Shanmullagh Formation of the Fintona Group which are the local representatives of the Lower Old Red Sandstone. The top of the Dartry Limestone, the uppermost part of the group, is a disconformity, above which are the layered sandstones and shales of the Meenymore Formation of the Leitrim Group. The succession continues south and west across the border into the Republic of Ireland, though different names are typically applied.

The geology of Loch Lomond and The Trossachs National Park in the southwestern part of the Scottish Highlands consists largely of Neoproterozoic and Palaeozoic bedrock faulted and folded and subjected to low grade metamorphism during the Caledonian orogeny. These older rocks, assigned to the Dalradian Supergroup, lie to the northwest of the northeast – southwest aligned Highland Boundary Fault which defines the southern edge of the Highlands. A part of this mountainous park extends south of this major geological divide into an area characterised by younger Devonian rocks which are assigned to the Old Red Sandstone.

The geology of the Peak District National Park in England is dominated by a thick succession of faulted and folded sedimentary rocks of Carboniferous age. The Peak District is often divided into a southerly White Peak where Carboniferous Limestone outcrops and a northerly Dark Peak where the overlying succession of sandstones and mudstones dominate the landscape. The scarp and dip slope landscape which characterises the Dark Peak also extends along the eastern and western margins of the park. Although older rocks are present at depth, the oldest rocks which are to be found at the surface in the national park are dolomitic limestones of the Woo Dale Limestone Formation seen where Woo Dale enters Wye Dale east of Buxton.

The geology of the North York Moors National Park in northern England is provided largely by a thick southerly dipping sequence of sedimentary rocks deposited in the Cleveland Basin during the Jurassic Period. A series of ice ages during the Quaternary period has left a variety of glacial deposits, particularly around the margins of the National Park.

References

  1. ↑ British Geological Survey. "Lynton Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  2. ↑ British Geological Survey. "Hangman Sandstone Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  3. ↑ Brosnan, Greg. "World's earliest fossilised forest discovered in Minehead, Somerset". BBC. Retrieved 7 March 2024.
  4. ↑ British Geological Survey. "Ilfracombe Slates Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  5. ↑ British Geological Survey. "Morte Slates Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  6. ↑ British Geological Survey. "Pickwell Down Sandstones Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  7. ↑ British Geological Survey. "Upcott Slates Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  8. ↑ British Geological Survey. "Baggy Sandstones Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  9. ↑ http://www.exmoor-nationalpark.gov.uk/learning?a=116440 p4,5
  10. ↑ British Geological Survey. "Luccombe Breccia Formation". BGS Lexicon of Named Rock Units. Retrieved 2 November 2019.
  11. 1 2 3 British Geological Survey. "Sheet 278 & pt of 294 Minehead (solid & drift geology)". BGS Maps Portal. Retrieved 2 November 2019.
  12. ↑ Harrison,S. and Keen, D.H. in Lewis, C.A. and Richards, A.E. 2005 The glaciations of Wales and adjacent areas Logaston Press, Bristol
  13. ↑ "Geology of Britain viewer | British Geological Survey (BGS)". Archived from the original on 2 December 2016. Retrieved 29 October 2019.
  14. 1 2 "Geology on Exmoor". Exmoor National Park. Exmoor National Park Authority. Retrieved 2 November 2019.
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