| Exmoor Group | |
|---|---|
| Stratigraphic range:Emsian (Devonian) toTournaisian (Carboniferous) | |
 Hangman Sandstone Formation at Glenthorne Beach, 6 km W ofPorlock Weir | |
| Type | Group |
| Sub-units | Lynton Formation, Hangman Sandstone Formation, Ilfracombe Slates (Formation), Morte Slates (Formation), Pickwell Down Sandstones (Formation), Upcott Slates (Formation), Baggy Sandstone Formation, Pilton Mudstone Formation |
| Underlies | Doddiscombe Formation ofTeign Valley Group |
| Thickness | about 7000 m |
| Lithology | |
| Primary | mudstones, |
| Other | siltstones,sandstones,limestones,conglomerates |
| Location | |
| Region | England |
| Country | United Kingdom |
| Extent | northDevon to westSomerset |
| Type section | |
| Named for | Exmoor |
TheExmoor Group is a lateDevonian to earlyCarboniferouslithostratigraphicgroup (a sequence ofrock strata) in southwestEngland whose outcrop extends fromCroyde in northDevon east acrossExmoor toMinehead in westSomerset. The group comprises the followingformations (in stratigraphic order, i.e., the oldest at the base and the youngest at the top):
Each of these divisions has been given different names by different authors in the past including those shown in brackets above. Some that had been classed as 'formations' (or even in one case as a 'group') are now 'members'.[1][2]
Extending east fromWoody Bay, the Lynton Formation underlies the village from which the sequence is named and continues in a thin strip of country as far east as the hamlet ofOare. There is a small inlier of the Lynton Slates at the foot of theQuantock scarp atTriscombe. The formation is composed of slates and siltstones together with some sandstone. Though its base is not seen, it is believed to be between 3–400 m thick. There are fossils ofbrachiopods,bivalves andbryozoans present, consistent with a shallow marine depositional environment.[3][4] One of Devon and Exmoor's better known natural tourist attractions isValley of the Rocks, a dry valley developed in this formation just to the west ofLynton.[5]
The Hangman Sandstone forms the coastal outcrop east fromHangman Point as far asHeddon's Mouth and indeed the larger part of the cliffs eastwards again to Woody Bay. It again forms the coastal cliffs fromLynmouth Bay east toPorlock Weir, includingForeland Point andCulbone Hill and also fromHurlstone Point toMinehead. ThePorlock Ridge and SaltmarshSite of Special Scientific Interest has a shingle ridge deposited during theHolocene epoch.[6] Inland it forms the high ground east toDunkery Beacon and much ofCroydon Hill.[7] The larger part of theQuantock Hills are formed from the same strata.[8][9] The Hangman Sandstone consist of several members which once enjoyed the status of 'formations'. These are (oldest base, youngest at top):
Names in brackets indicate earlier names for the current members. The 'Stringocephalus Beds' and 'Sherrycombe Beds' were formerly grouped as the 'Upper Hangman Grits'.[10]
The thickness of the strata is estimated at between 1660 m and 2500 m; intense faulting and folding precludes an exact measurement being made. Consisting largely of sandstones but with some alternating shales and slates and some conglomerates, most of the formation is non-marine in origin. Plant and shell remains have been found though fossils are generally scarce within the formation. The sequence east of Lynmouth used to be referred to as the "Foreland Grits", and was thought to be somewhat older, and hence lower in the sequence, than the Hangman Grits[11]
In March 2024 scientists reported the discovery of a fossilised forest ofCalamophyton trees at a site near Minehead, its age being four million years greater than a fossil forest inNew York State which had previously held the record as the world's oldest.[12]
The Trentishoe Member exhibits unusual deposition at theGlenthorneSite of Special Scientific Interest.[13]
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The formation comprises slates and limestones of marine origin and sandstones and slates with a shallow marine ordeltaic origin.[14] In the west, the Ilfracombe Slates are subdivided into (oldest at base, youngest at top):
The Wild Pear Slates are named from Wild Pear Beach atCombe Martin Bay where this sequence outcrops on the coast. The lower boundary of this sequence is athrust fault which carries the slates up over the Hangman Grits. The overlying Lester Slates-and-Sandstones run from Widmouth Head, beneath the village of Combe Martin and on in a SSE direction to nearParracombe. The Combe Martin Slates in turn extend from Beacon Point atHele in a narrowing outcrop toPinkworthy Pond and beyond. The Kentisbury Slates form the coastal outcrop atIlfracombe itself and extend inland viaKentisbury,Challacombe andSimonsbath towardsExford.
In the east, the following subdivisions are identified. Some earlier (now formally obsolete) names are given in brackets afterwards:
In the Quantock Hills, the Avill Slates form the high ground of Lydeard Hill aboveWest Bagborough, together with the lower northeastern slopes of the range betweenAisholt andHolford. Atuff occurs within the Avill member on these eastern slopes at Keeper's Combe. The Cutcombe Slates form the ground immediately east of the Avill outcrop in this area whilst the Leighland Slates make up a band of higher ground betweenCothelstone andEnmore with scattered outcrops extending north toNether Stowey. Several limestone beds are named within the two upper members; oldest/lowermost first - the Rodhuish, Roadwater, Aisholt, Holwell and Leigh Barton limestones.[15][16][17][18]
The Morte Slates are a series of silvery-grey to green coloured slates with some subordinate sandstone and siltstone. A well-developedcleavage has removed what fossils may have earlier been present in these one-time submarine muds. These rocks extend fromMorte Point, where they form the coast betweenWoolacombe and Flat Point, south-southeastwards throughWest Down andArlington then east toWithypool. Further east the outcrop broadens to form theBrendon Hills and the rolling country aroundClatworthy Reservoir. The eastern extent of the main outcrop reaches the villages ofStogumber andTolland. An outlier forms the southeastern part of the Quantock Hills betweenGoathurst andWest Monkton.
Over the centuries the Brendon Hills have been mined for minerals, notablyironstone from which iron is extracted for makingsteel. During the 19th century this activity reached a peak with theWest Somerset Mineral Railway, including an 800 feet (244 m) incline, being built to take the ore toWatchet from where it was sent toEbbw Vale forsmelting.[19] The main mining operations ended when the mines were worked out towards the end of the 19th century.[20]
The Pickwell Down Sandstones are red and brown sandstones with shales which extend from the hills of Pickwell Down and Woolacombe Down overlookingMorte Bay east-southeastwards toMuddiford andBratton Fleming. The outcrop pattern is shaped by a major east–west aligned syncline/anticline pair betweenBrayford, where there is a stone quarry, andDulverton. East of Dulverton the formation forms the prominentHaddon Hill and extends via Heydon Hill as far east as the village ofWiveliscombe. There are a number of former workings for iron,copper andmanganese across the western part of its outcrop.[21] A geomorphological survey at Anstey's Coombe showed that mining had taken place at the site during both theRomano-British period and the 16th to 17th century.[22]
The Upcott Slates provide the coastal cliffs on the south side of Morte Bay and their narrow faulted outcrop continues east-southeastwards beneathGeorgeham and to Winsham andMarwood. The outcrop continues east viaShirwell to Little Bray, north of Brayford. Slightly offset by faulting to the south, it continues east to North Radworthy. A major east–west syncline in this area gives rise to an outcrop stretching southwest to East Buckland and then by virtue of a parallel anticline, stretching southeast then east toNorth Molton. Subject to numerous northwest–southeast aligned faults, the outcrop can be followed viaMolland to east of Dulverton. The formation consists of slates varying in colour from buff through grey and green to purple. These rocks originated as muds laid down in swamps and freshwater lakes.[23][24]
The Baggy Sandstones form the headland ofBaggy Point and the faulted outcrop stretches away to the east north of Croyde, throughKnowle toStoke Rivers andBrayford. Its outcrop wraps around a syncline/anticline fold pair in a reverse-S shape via East Buckland and parallels the outcrop of the Upcott Slates east to Dulverton. It is mapped as a distinguishable unit of the Exmoor Group as far as theBatherm valley, east ofSkilgate. The formation consists of sandstones, siltstones and shales including somefeldspar andmica rich units, variously of marine through brackish to freshwater origin. The outcrop forms a small yet prominent ridge along its outcrop.[25]
The Pilton Mudstones form the coast at Croyde Bay and form the headland at Saunton Down. The formation extends eastwards through Barnstaple to theBuckland area where due to a syncline/anticline pair, the surface outcrop divides. A thinner band of these strata continues east coincident with the valley of theRiver Yeo and theformer railway toBrushford. The outcrop broadens once again forming the country aroundClayhanger andWaterrow before reaching its furthest east atKittisford and at Nunnington Park, south of Wiveliscombe. The formation consists of shales and siltstones along with bands of calcareous sandstone. Fossils include brachiopods and bivalves in the lower part of the formation,[26] suggestive of a shallow marine depositional environment and, in its upper part,trilobites andgoniatites indicating deeper water conditions. The sandstones thicken to the east and have been worked in quarries betweenCharles and Brayford. Limestones occurring towards the top of the formation have also been worked in places. Whilst most of it is Devonian, the uppermost part of the formation is Carboniferous in age though the precise location of the boundary is difficult to determine.[27]