West, Ian M. 2013. Mendip Hills - Geological field guide. Supplement to Geology of the Wessex Coast. Internet site: www.southampton.ac.uk/~imw/Mendip-Hills.htm. By Ian West, School of Ocean and Earth Science, Southampton University. Version: 17th December 2013.

Ian West,

Romsey, Hampshire

Faculty of Natural and Environmental Sciences,
Southampton University,

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Geological Maps

For the eastern Mendips, see the British Geological Survey, Wells Sheet, No. 280, Solid and Drift and the the Frome Sheet, No. 281, Solid and Drift, 1:50,000 of the British Geological Survey. These maps can be purchased from the BGS bookshop, online. A special Geological Sheet ST 45 Cheddar, scale 1:25,000 cover the Cheddar area in detail.

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Many of the exposures in this area are in quarries, either working or abandoned. Please abide strictly by the rules of working quarries. In abandoned quarries take care not approach from above unstable cliff edges. From beneath take care with regard to risk of falling rock. Do not hammer chert, which occurs in the Carboniferous Limestone, and which, when struck, can give off very dangerous splinters that can cause loss of eye sight. Take the usual precautions of using sturdy footware and take suitable clothing when walking on upland areas. Be careful with the steep walls of Cheddar gorge. Do not undertaking caving with proper equipment and without the assistance of experience people and knowledge of the cave systems.

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Whatley Quarry - Introduction

Whatley Quarry, East Mendip Hills, Somerset, an overview

Whatley Quarry in Carboniferous Limestone, East Mendip Hills, Somerset, showing terraces

Whatley Quarry, grid reference ST731479, is a limestone quarry near the village of Whatley on the East Mendip Hills, Somerset. The quarry shows dark grey Carboniferous Limestone, mostly Black Rock Limestone, with a substantial part of lighter Clifton Down Limestone. There is a small area of overlying horizontally bedded buff-coloured Jurassic oolitic limestone forming an angular unconformity with the steeply-dipping Black Rock Limestone. There is much dolomitisation near the top of the Carboniferous limestone section. There are abundant near-vertical fissures and joints near top of limestone with karst weathering and minor pinnacle formation.

There is a visitor centre nearby at Moon's Hill Quarry and visits can be arranged to see Whatley Quarry. A few small parts can be visited directly but most is seen from a minibus driven down the quarry. This is very helpful but it is not easy to see details of the lithology or fauna.

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Whatley Quarry - More Details

East face of Whatley Quarry, with a red unit, Mendip Hills, Somerset

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Whatley Quarry - Lake at the Base

Lake at the bottom of Whatley Quarry, East Mendip Hills, Somerset

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Whatley Quarry - Crushing and Transport

Crushed Carboniferous Limestone being supplied to tips at Whatley Quarry, East Mendip Hills, Somerset

Whatley Quarry, railway terminal, East Mendip Hills, Somerset

The Carboniferous Limestone is crushed and largely transported by rail. There is a special rail terminal at the quarry. The quarry is owned by Hanson plc and is linked by a freight only railway line (used by trains operated by Mendip Rail) to a junction with the Reading to Plymouth line at a junction near Frome station.

The quarry has been the object of protests against its impact on the environment and has had to appeal against planning application decisions because of the claimed derogation of river flows, groundwater abstractions and local springs due to historic dewatering associated with the quarry. Hanson runs a study centre, not far away at Moon's Hill Quarry.

A rather similar and large Carboniferous Limestone quarry is Torr Works quarry, nearby. This is at grid reference ST695446 and is at East Cranmore, near Shepton Mallet. It is also known as Merehead Quarry. Torr Works quarry site covers an area of some 200 hectares, including 60 hectares which have been landscaped to blend with the surrounding countryside. It is operated by the Aggregate Industries Company employing over 200 people and produces 6 million tonnes of limestone annually which is also carried directly from the quarry by Mendip Rail. (Data from Wikipedia.)

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Whatley Quarry - Triassic Fissure Deposits

Calcite and dolomite in a vug that was originally an anhydrite nodule in a Triassic fissure fill, Whatley Quarry, Mendip Hills, Somerset


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Torr Works Quarry - formerly Merehead Quarry, Eastern Mendip Hills


Map showing Merehead Quarry in 1970, eastern Mendip Hills, Somerset

Location map showing the Torr Works Quarry or expanded Merehead Quarry in 2005, eastern Mendip Hills, Somerset

Comparison of the sections of Ordnance Survey maps above show the location and extent of the original Merehead Quarry in about 1970, before it became Torr Works. At that time it occupied only about a quarter of a square kilometre in area. However, it was already served by a railway line that has, no doubt, been of great benefit for regular export of aggregate and stone to southern England. Thus by 2005 it had become very successful and has expanded to area of about one and half kilometres. It is now as "Torr Works", much larger than most other quarries in the region.

Torr Quarry seen from the western side looking eastward, Mendip Hills, Somerset, 2010

Carboniferous Limestone in Torr Works Quarry or Merehead Quarry, eastern Mendip Hills, Somerset, 2010

Monument and viewport above Merehead or Torr Works Quarry, eastern Mendip Hills, Somerset, 2010

Notice at Torr Works Quarry, Mendip Hills, Somerset, showing future plans

It is a very large limestone quarry occupying about one and a half square kilometres. It has been worked into south-dipping Carboniferous Limestone. The entrance to this quarry is very conspicuous from the main A361 Frome to Shepton Mallet road. You approach it, usually quite rapidly, in a dip and curve of the main road. You see a large sign with the words "Torr" and a Union Flag is usually flying from a nearby flagpost. This entrance is just to the east of East Cranmore.

The quarry is not very easy to see on foot but there are footpaths around it. There is quite a long bridleway from Downhead along the northwestern side and round to the southwest of the quarry. The quarry, although very large, is not easily seen, being rather concealed and inaccessible with a combination of wire fencing, a line of small trees and an internal road and bank. Its long term prospects for geological conservation may not necessarily promising because its upper slopes may be smoothed over, rather than preserving bold limestone cliffs. However, details are not known, and the ending of the quarrying is a long time ahead. There is still plenty of limestone; the Black Rock Limestone can be up to 300m thick and is buried beneath the southern end of the quarry. When the quarrying does end, according to a notice, the lower part will be allowed to flood and it will make a large and pleasant lake.

The Carboniferous Limestone is on the south limb of the Beacon Hill Pericline, and dips south at a moderate angle. It is to some extent red-stained by formerly overlying Triassic strata.

With regard to the Wessex coast, the quarry is of interest in that it might be the source of much of the Carboniferous rock armour used for sea defences. This type of rock armour can be seen at Barton-on-Sea and Hurst Spit for example. Information on this rock armour is provided further below, in the present website.

For more information on Torr Works Quarry see the BGS website: Torr Works and Asham Wood.

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Carboniferous Limestone - Details

Black Rock Limestone, Carboniferous, in the northeastern part of Torr Works Quarry, eastern Mendip Hills, Somerset, 2010

A modified geological map of Torr Works Quarry or Merehead Quarry, eastern Mendip Hills, Somerset

For geological information on the area purchase the British Geological Survey, Frome Sheet, No. 281 from the BGS Bookshop online (at present only 12 pounds sterling). The small map above shows a modified and partly redrawn version of just a small part of this map. The quarry is shown by BGS as almost entirely within Carboniferous Limestone, of Mississipian or Lower Carboniferous age. This limestone, which contains many corals and brachiopods, originated in a near-equatorial, warm and shallow sea, about 345 million years ago. The local names such as Black Rock Limestone Formation, Vallis Limestone Formation, Clifton Down Limestone Formation are subdivisions of the Carboniferous Limestone.

The dip is round about 40 degrees towards the south but varies a little over quarry area. Because there is only a moderate dip and because the limestone is thick, it occupies a substantial area. Thus the quarry has been able to expand (unlike Whatley Quarry) to more than one and half kilometres from north to south. It is limited by low ground and change of geological outcrop to the south. It would be restricted in extent by a fault, the Downhead Fault to the west. In theory, but perhaps not in reality for conservation reasons, the quarry could expand to the northeast to take in Asham Wood. There is more Black Rock Limestone in that direction.

There is only one significant fault in the quarry, as shown by the British Geological Survey map, Frome Sheet, 281. This fault is in the southern part, just south of the lake at the base. It crosses the quarry in an east-west direction and downthrows on the north side. The fault is not major and does not change the pattern of outcrop very much. The main part of the quarry seems unfaulted. There is a major fault, the Downhead Fault which is almost parallel to the western margin of the quarry but is beyond it, to the west and near the road to Downhead. This does not affect the quarry. Just south of the quarry, almost on the route of the major road, is the Cranmore Fault, which trends roughly east west (its line is just north of East Cranmore).

Because of this fairly uniform southerly dip the Torr Works Quarry has older Carboniferous Limestone in the northern half. This is of Black Rock Limestone, Courceyan and Chadian in age (i.e. it belongs to the lower part of the Lower Carboniferous). It can attain a thickness of 300 metres and is the thickest of the major subdivisions of the Carboniferous Limestone. The Black Rock Limestone in this part of the Mendips is a dark-coloured carbonate wackestone or packstone, using Dunham's classification (Green, 1992). This means that it usually consists of carbonate allochems ("grains") in a fine grained matrix (either supported by matrix or grains). The microscopic structure of a rock of this type is not likely to be very obvious in the field, even with a handlens. South of that (and just north of the lake) is Vallis Limestone which is Arundian. This is just a narrow belt. The Vallis Limestone is lighter-coloured and is a bioclastic grainstone and packstone (i.e. of shell fragments with or without fine matrix).

Carboniferous Limestone, Clifton Down Limestone,  dipping south and striking east-west, Torr Works Quarry or Merehead Quarry, Mendip Hills, Somerset, 2010

The lake area and the works are on Clifton Down limestone which is Holkerian in age. This can vary from grainstone to calcite-dolomite mudstone. Some sedimentary features of this are shown in the section which follows below.

The Hotwells Limestone (Asbian and Brigantian is beyond the quarry at the hill to the southeast.

More details, including a large scale map of part of the Heale to Downhead area at the margin of the quarry, are available in (Welch, 1933).

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Rock-Armour - Carboniferous Limestone Blocks

Large blocks of limestone from the Merehead or Torr Works Quarry are easily accessible at the shore of Barton-on-Sea, Hampshire. They have been used successfully for sea defence rock armour (armour-rock or armor-rock). The blocks are used in various places in the sea defences but are best observed where they have become scoured clean by the abrasive action of beach material which attacks them during storms.

Fortunately, the blocks have been studied and described, with regard to fossils, in a paper by Lewis et al. (2003). They gave details of the geology of this rock-armour, and in particular the content of fossil echinoderms. The stone was quarried at Merehead Quarry when it was under the ownership of Foster Yeoman, before it became Torr Works. The large limestone blocks were transported from there to Barton, not by train (in spite of the railhead) but by flat-bed lorry, according to Professor Andrew Bradbury, the Coastal Projects Manager of Hampshire County Council.

Crinoid ossicles in Carboniferous Limestone rock armour at Barton-on-Sea, probably Clifton Down Limestone from Merehead or Torr Works Quarry, eastern Mendips

The tabulate coral Syringopora in Carboniferous Limestone rock armour at Barton-on-Sea, Hampshire

Lithostrotion in Clifton Down Limestone, Carboniferous rock armour from Torr Quarry, at Barton-on-Sea, Hampshire

The rock type containing fossils at Barton-on-Sea is Clifton Down Limestone, Devensian, Holkerian (Carboniferous Limestone, Mississipian) according to Lewis, Donovan and Sawford (2003). They reported a rich fauna of echinoderms, corals, bryozoans, trilobites, brachiopods and gastropods. The echinoderms include plates of the tests of the echinoids Palaechinus sp., Archaeocidaris sp. and an indeterminate echinoid. Numerous crinoid ossicles are present and calyces have been found of the crinoids Platycrinitid sp., Actinocrinus sp. aff. A. rotundatus Wright, monobathrid sp. indet., camerate sp. indet. and Taxocrinus sp.

Carboniferous rock armour has been used at Lepe Beach, Hampshire and see this webpage for further information.

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Carboniferous Limestone - Triassic Fissure Fills

A fissure fill with calcite and yellow sediment in a Carboniferous Limestone block at Barton-on-Sea, Hampshire

Red siltstone filling an extensional fissure system in Carboniferous Limestone rock armour at Lepe Beach, Hampshire (neptunean dyke)

Interesting fissure fills ("neptunian dykes") with red and yellow sediment occur in the blocks of Carboniferous Limestone at Barton-on-Sea. See the paper of Wall and Jenkyns (2004) for a discussion of the origin of sediment-filled fissures in the Carboniferous Limestone of the Mendip Hills. Another example from similar Carboniferous rock armour at Lepe Beach is also shown for comparison.

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Rock-Armour - Replaced Evaporites

Calcite pseudomorphs after halite and calcite-replaced anhydrite or gypsum nodules in rock armour, Barton-on-Sea, Hampshire

At Barton-on-Sea, Hampshire, Carboniferous rock armour is of special interest in containing evaporites. This rock armour with evaporites is almost completely unfossiliferous and is darker in colour than the associated fossiliferous limestone from Clifton Down Limestone. It may be dolomite but has not been examined in the laboratory. This unusual limestone is present as rock armour at the shore just east of Hoskin's Gap, Barton-on-Sea. is examined at low tide. It needs to be examined at low tide because high tide or a stormy sea will cause wave wash over these rocks.

Particularly conspicuous in an unfossiliferous dolomite (or limestone) are good calcite pseudomorphs after halite. Some of these are feathery or skeletal. They have all formed in the carbonate sediment when it dried out with a content of very hypersaline (near 350 ppt) brine. Associated with the halite is much nodular calcium sulphate replaced by calcite. This may have originated as gypsum, but in proximity to so much halite it is almost certain that it was changed diagenetically to anhydrite at an early stage.

Calcitised nodular anhydrite or gypsum in a dolomite or dolomitised limestone of sabkha facies, and used in sea defences, Barton-on-Sea, Hampshire

Recent nodules of anhydrite, one with chicken wire structure, Dukhan Sabkha, Qatar

Go to the Qatar Sabkhas webpage to see more on modern analogues.

With regard to the Carboniferous Limestone of Britain and Ireland, evaporites occur in several places. They occur in the Visean (Upper Dinantian) in County Leitrim, Ireland ( West, Brandon and Smith, 1968). These are younger than the Holkerian Clifton Down Limestone, from which the associated fossiliferous rock armour of Barton-on-Sea has come. Evaporite facies have also been found in County Carlow, Ireland ( Nagy et al., 2005). Evaporites have also been found in the Main Limestone of the Lower Carboniferous in South Wales (Bhatt, 1975). A major Carboniferous evaporite deposit, the Hathern Anhydrite, occurs underground in Leicestershire (Llewellyn and Stabbins, 1970). There are traces of evaporites in the Lower Carboniferous elsewhere in Britain, but they have not necessarily all been reported in the literature.

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Moon's Hill Quarry - Introduction - Location

Somerset Earth Science Centre, Moon's Hill Quarry, eastern Mendip Hills, Somerset, 2010

Near the entrance to Moon's Hill Quarry is the Somerset Earth Science Centre (telephone 01749-840156, email info@earthsciencecentre.org.uk). This is an educational centre with rock and mineral specimens, with staff who explain the quarrying and take minibus tours of Moon's Hill and Whatley Quarry. This centre is supported by the quarrying industry. There are, of course, strict Health and Safety regulations and restrictions regarding the working quarries. Access is controlled and guided and limited to certain areas, and appropriate hard hats and high-vis jackets have to be worn. The working quarries discussed below are not places for casual visits and it may not necessarily be possible to obtain any close view of certain stratigraphical units.

New topographic map showing the quarries northeast of Shepton Malley, Mendip Hills, Somerset

An old map of the Silurian Inlier of the Mendip Hills, Somerset, by Reynolds in 1907

Moon's Hill Quarry is deep quarry in the eastern Mendips, near Stoke St. Michael and northeast of Shepton Mallet. The main purpose of the quarry is to produce good aggregate from hard Silurian andesitic rocks. The quality and resistance to wear of the volcanic material is the reason why the quarry is situated on the limited Silurian outcrop of the eastern Mendip Hills.


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Moon's Hill Quarry near Shepton Mallet in the eastern Mendips provides an unusual exposure of Silurian volcanic rocks. The exposures are in a deep quarry, descending in a series of terraces. This webpage provides only a brief introduction to what is seen on a visit to the top of the quarry. Access to the lower parts is not easy because of reasons of Health and Safety regulations in a working quarry. Therefore, the features shown here, and mainly of the volcanic conglomerate, may not be typical of the exposure, and do not include much evidence of the lavas or dykes.

Van De Kemp (1969) commented that there are probably 15 or more rock units in the series, including andesite and rhyodacite lavas, rhyodacite tuffs, "agglomerate", and a dolerite dyke. The predominant rock type is rhyodacite which may be as much as 80 percent of the volcanics. The individual rock units cannot easily be recognised in the overview photographs which follow. The close-up photographs mainly illustrate a volcaniclastic conglomerate. The individual clasts are very rounded and not conspicuously vesicular. Furthermore they are associated with a cross-bedded, reworked tuff showing the action of water.

There is an excellent, educational visitor centre close to the quarry. This facility can arrange visits both to this quarry and to Whatley Quarry, although that direct access to the rock faces is limited to certain areas at the top.

Moon's Hill Quarry in Silurian andesite and volcaniclastic conglomerate, Mendip Hills, Somerset, a general overview, 2010

The bottom of Moon's Hill Quarry in Silurian andesitic volcanics and volcaniclastic conglomerate, Mendip Hills, Somerset, seen from the viewing platform

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Volcaniclastic Conglomerate - Rounded Clasts

Quarry debris, showing rounded clasts, at Moon's Hill Quarry, Silurian volcanics, Mendip Hills, Somerset, 2010

The above photograph shows the location at the start of a guided tour of part of Moon's Hill Quarry. We are at the southeastern corner and looking northwest towards the crushing and sorting machinery of the quarry. Compare the field photograph with the aerial photograph. We are going to go down through the gap in front to a cliff face of volcaniclastic conglomerate at the uppermost terrace.

Vertically orientated conglomerate or agglomerate at Moon's Hill Quarry, Mendip Hills, Somerset, 2010

The view now is of the uppermost cliff and uppermost terrace in the southeastern part of Moon's Hill Quarry. This area is away from the main working face of the quarry so access, under guidance, can be permitted here. The cliff appears at first sight to consist of brown rubble. Closer examination suggests that it is a degraded, partially decomposed conglomerate of volcaniclastic origin.

Volcaniclastic conglomerate block, Silurian, Moon's Hill Quarry, Mendip Hills, Somerset

Recent spheroidal weathing of a rounded clast, Silurian volcanics, Moon's Hill Quarry, eastern Mendip Hills, Somerset

A peculiarity of the volcanic material seen at the top of the quarry is that the clasts are very well-rounded. This was regarded for a long time as a volcaniclastic conglomerate since its description by Professor Sidney Reynolds (1907), or earlier. The clasts were regarded as rounded pebbles of volcanic rock. In modern terminology it is a volcaniclastic conglomerate of epiclastic type (according to the classification given by Dorrik Stow (2005).

However, it should be noted that for a time there was a different view of the origin. Van De Kemp (1969) referred to the rounded clasts as "abundant bombs in ash matrix" and Ponsford (1970) then stated that the deposit should be termed an "agglomerate". Ponsford (1970) drew attention to Dr Doris Reynolds' (1969) suggestion that they were rounded by fluidisation within the vent.

This does not seem a very likely explanation in this case. The association with beds of tuff, apparently reworked by water and apparently fining-upward beds, seems to support a volcaniclastic conglomerate origin. The rounded clasts do not appear to be volcanic bombs.

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Water-reworking of Tuff

Moon's Hill Quarry, Somerset, Silurian volcanics, a finer bed and two fining-upward units

Laminated tuff with some ripple cross-lamination, Moon's Hill Quarry, Mendip Hills, Somerset

Cross-laminated, reworked tuff, Silurian volcanics, Moon's Hill Quarry, Mendip Hills, Somerset

At the top of Moon's Hill quarry there is evidence of reworking of tuffs presumably by water (although possibly by wind). This seem to support the view that the coarser deposits, although of volcanic origin have been reworked by water and that this is the reason for the roundness of the clasts. However, no detailed study has been made and this may not necessarily be correct.

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A block of vesicular andesite, Moon's Hill Quarry, Mendip Hills, Somerset

Shown above is an isolated block of andesite, ex-situ. It has probably been brought up from a lower part of the quarry for display purposes. This andesite is vesicular.

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Mount St. Helens

Mount St  Helens with andesite, dacite and basalt and much tephra, Washington, USA, for comparison with the igneous rocks of Moon's Hill Quarry, eastern Mendips, England

The Silurian igneous rocks of the Mendip Hills are the remains of a vent of a volcano associated with subduction near a plate margin. A broad comparison can be made with the famous volcano, Mount St. Helens, Washington State, USA. This also contains andesites and large quantities of pyroclastics. The general compositional trend of Mount St. Helens has been from rhyodacite to andesite ( Hopson and Melson, 1990) and there is general similarity to these Silurian volcanics.

Of course, the place looks nothing like Mount St. Helens, with the green fields of agricultural land around a subdued but hilly topography. Beneath the surface, though, the record is that of a great volcano, probably associated with subduction. A complication is that the Moon's Hill volcano has been turned on its side by major earth movements and thus the strata are approximately vertical in orientation. It has also been extensively sheared by thrusts and the near-surface part is badly weathered.

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Portishead Formation

Portishead Formation, Devonian, Moon's Hill Quarry, eastern Mendip Hills, Somerset, 2010

At the northern edge of Moon's Hill Quarry there is an exposure of the Portishead Formation. This is Upper Devonian strata, consisting of reddish-brown sandstones (Old Red Sandstone). The photograph above shows it at a distance from the southern side of the quarry. The details were not seen and it is not known whether the basal conglomerates is present here in addition to sandstones. The general red sandstone facies is fluvial, a continental facies (for marine Devonian see Torquay webpage.

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I am very grateful to the members of the Open University Geological Society who participated in a field trip to Moon's Hill and Whatley Quarries in August 2010. I much appreciate the organisation of this field trip by Jeremy Cranmer. Hugh Prudden, the geological expert on the region, kindly sent me a list of references and details of websites. I am much obliged to him for his help. The staff of the visitor centre at Moon's Hill Quarry provided an excellent tour with explanation of features in both Moon's Hill Quarry and Whatley Quarry.

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Alabaster , C.J. 1974. Some Copper, Lead and Manganese minerals from Merehead Quarry, East Mendip. Proceedings of the Bristol Naturalists Society.
BGS - British Geological Survey . 2010? The Rocks of Mendip. Go to website: The Rocks of Mendip.
"Silurian rocks (444 to 416 million years ago) The Silurian rocks of the Mendips are formally known as the Coalbrookdale Formation, and occur as a narrow elongated outcrop in the core of the eroded anticlinal fold that forms Beacon Hill, north-east of Shepton Mallet. The rocks comprise a sequence of fissile mudstones ('Wenlock Shales') around 600 m thick overlain by an interbedded succession of tuffs, agglomerates and andesite lava flows. A vent agglomerate represents a section through an ancient volcanic fissure, fortuitously exposed at the surface because of the almost vertical dip of the strata. Volcanic rocks are rare in the Silurian, and the Mendips are one of the few places in the UK where they can be observed." [continues.. good website!]

BGS (British Geological Survey). BGS website: Torr Works and Asham Wood.

BGS (British Geological Survey). BGS website: History - East Mendip Quarries.
Example extract:
By 1957, Foster Yeoman, having largely completed the upgrading of Dulcote Quarry, near Wells, took over Merehead from Limmer with 150 acres of reserves at a cost of only 15 000 pounds. .... By 1967, output had increased to a quarter of a million tonnes annually. New, larger gyratory crushers were installed in 1969, the Norberg being one of the largest in Europe. These raised output rapidly to 3 million tonnes in 1971 and 5 million tonnes in 1973 (with potential capacity of 7 million tonnes a year), making the quarry the largest single producer of aggregates in Europe. ..
The quarry itself was working through steeply dipping beds 20 degrees to 40 degrees (occasionally with cavities) forming the southern limb of Beacon Hill Pericline. The western border of the quarry is parallel to the Downhead - East Cranmore road immediately to the west of which is the major Downhead Fault which marks the termination of the Carboniferous Limestone in this direction.
Bhatt, J. 1975. Evidence of evaporite deposition in the Lower Carboniferous Main Limestone Series of South Wales, U.K. Sedimentary Geology, vol. 13, issue 1, March 1975, pp. 65-70.
Petrographic examination of the dolomitized Main Limestone Series cropping out in the south, east and northeast corners of the South Wales Coalfield Basin shows evidence of the earlier presence of evaporite minerals. However, it is believed that lack of extensive evaporite deposits in these rocks may be due to the active diagenetic dissolution and oncoming humid coal conditions of post Main Limestone time. The evaporite minerals in the Main Limestone rocks seem to be overwhelmingly early diagenetic in origin in the light of the following observations: (1) calcite or dolomite pseudomorphs after gypsum crystals associated with a fine pelmicrite matrix; (2) association of such pseudomorphs with oolitic pelsparite; and (3) evaporite solution breccia texture.
Green , G.W. and Welch, F.B.A. 1964. The Geology of the Country around Wells and Cheddar. Memoirs of the Geological Survey of Great Britain, No. 280. (Explanation of One-Inch Geological Sheet 280, New Series). With contributions by G.A. Kellaway, D.R.A. Ponsford, M. Brooks, and M. Mitchell. Department of Scientific and Industrial Research, London, Her Majesty's Stationery Office. 225pp.

Green, G.W. 1992. British Regional Geology: Bristol and Gloucester Region. Third Edition, based on previous editions by G.A. Kellaway, F.B.A. Welch and R. Crookall. London, Her Majesty's Stationery Office. 188pp. [a good edition, larger than most British Regional Geology books; this is more in the style of a memoir of the BGS].
Hancock , N.J. 1982. Stratigraphy, palaeogeography and structure of the East Mendips Silurian inlier. Proceedings of the Geologists' Association, London, vol. 93 (3), 247-261.
Abstract: Sedimentary way-up criteria in the 'Wenlock shales' of the East Mendips inlier show that these beds, previously believed to overlie Silurian andesitic lavas and tuffs unconformably, are inverted and underlie the volcanics, probably conformably. A regressive sequence of brachiopod communities occurs in the shale and overlying tuffs. This regression is dated as probably M. riccartonensis zone (Lower Wenlock) by Eocoelia angelina, suggesting that the volcanics are also Wenlock. The regression is not contemporaneous with the widespread late Wenlock shallowing of Wales and the Welsh Borderland, and is probably related to the volcanism. Remapping of the inlier shows that although the Silurian beds are situated in the core of the East Mendips anticline, they exhibit no anticlinal structure within themselves. This, taken together with the discrepant Old Red Sandstone thicknesses north and south of the inlier, demonstrates that the southern boundary of Silurian rocks constitutes and overthrust and not an angular unconformity as hitherto believed. An exposed volcanic neck forms one possible source for the numerous Wenlock bentonites in Wales and the Welsh Borderland.
Hopson , C. A and Melson, W.G. 1990. Compositional Trends and Eruptive Cycles at Mount St. Helens. Geoscience Canada, vol. 17, no. 3, pp. 131-141. By Clifford A. Hopson, William G. Melson
Available online as a pdf file:
Hopson and Melson, Compositional Trends and Eruptive Cycles at Mount St. Helens.
The 40,000-year eruptive history of Mount St. Helens reveals an overall compositional trend from rhyodacite to andesite, with basalt at ~1.9 and ~1.6 ka. A cyclic eruption pattern is superimposed on this trend. Cycles comprised a repose interval, when compositional and thermal gradients developed in the underlying magma body, followed by an eruption interval in which progressive tapping of magma beheaded these gradients. Recovery of gradients varied with duration of the ensuing repose period. Eruption sequences follow the pattern: (1) eruptive progression from Plinian eruptions to dome growth accompanied by pyroclastic flows and tephra, followed (in some cases) by lava flows punctuated by pyroclastic outbursts; (2) a mineralogic progression from hydrous Fe-Mg phenocrysts (hb, cm, bi) toward pyroxenes; (3) a magmatic compositional progression from rhyodacite or dacite to andesite. Progressions 1 and 2 stem mainly from volatile gradients in the magma reservoir whereas progression 3 (and to some extent 2) reflects gradients of melt composition and crystal content. Three eruption cycles within the last 4,000 years follow this pattern. Earlier cycles are probable but only dimly perceived, mainly from the partial record of tephras and pyroclastic flows.
Lewis , D.N., Donovan, S.K. and Sawford, P. 2003. Fossil echinoderms from the Carboniferous Limestone sea defence blocks at Barton-on-Sea, Hampshire, southern England. Proceedings of the Geologists' Association , 114, 307-317. Abstract: The sea defence/coastal protection works at Barton-on-Sea, Hampshire include blocks of Carboniferous Limestone (Clifton Down Limestone Formation, Dinantian, Holkerian) from the Foster Yeoman 'Torr Works' Quarry at Merehead, East Cranmore, Shepton Mallet, Somerset. A rich fauna of echinoderms, corals, bryozoans, trilobites, brachiopods and gastropods is present in these blocks. The echinoderms include plates of the tests of the echinoids Palaechinus sp., Archaeocidaris sp. and an indeterminate echinoid: calyces of the crinoids platycrinitid sp., Actinocrinus sp. aff. A. rotundatus Wright, monobathrid sp. indet., camerate sp. indet. and Taxocrinus sp.; and numerous ossicles, including Cyclothyris (col.) sp. and Pentagonocyclicus (col.) spp. Camerates were important members of early Carboniferous crinoid faunas, although the absence of cladids is notable. Examination of any fossils contained within coastal protection blocks is an important source of information when the place of origin of the blocks is known but is unavailable for study purposes. [by David N. Lewis, Natural History Museum, London, Stephen K. Donovan, Nationaal Natuurhistorisch Museum, Leiden, and Paul Sawford, Ruislip Road, Northolt, Middlesex.]
Llewellyn , P.G. and Stabbins, R. 1970. The Hathern Anhydrite Series, Lower Carboniferous, Leicestershire, England, Institute of Mining and Mineralogy, 79 (1970), pp. B1-B15.
Nagy , Z.R., Somerville, I.D., Gregg, J.M, Becker, S.D. and Shelton, K.L. 2005. Lower Carboniferous peritidal carbonates and associated evaporites adjacent to the Leinster Massif, southeast Irish Midlands. Geological Journal, vol. 40, Issue 2, pp. 173-192. By Zsolt R. Nagy, Ian D. Somerville, Jay M. Gregg, Stephen P. Becker, and Kevin L. Shelton.
Analysis of a 275 m-thick section in the Milford Borehole, GSI-91-25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?-late Chadian to late Holkerian (Viséan, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68 m-thick, well-developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian–Holkerian strata, transgressive–regressive carbonate units are recognized. These comprise high-energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite-bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif.
The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base-metal sulphide deposits.
Ponsford, D.R.A. 1970. Silurian volcanic rocks of the Mendip Hills, Somerset. Geological Magazine, vol. .
Available online: Ponsford, 1970, Silurian volcanic rocks.
The recent paper by Dr. P.C. van de Kamp on the above topic (Geol. Mag., 106, 1969, 542-553) is of considerable interest. The author claims that the main rock is rhyodacite whereas all previous publications and our own researches have suggested that the main lava flows are of andesitic composition. The main reason for the new opinion would seem to be aa high quartz content in the rocks based on chemical analyses and it is pertinent to enquire whether this could be due to secondary enrichment.
[.. continues ...]
A further interesting point in connection with the Silurian volcanic rocks of the Mendips concerns the origin of the rounded lava masses found in the tuff beds and referred to by Dr van de Kemp as "abundant bombs in ash matrix". Since their first discovery by Reynolds (1907) they have been called conglomerates but I agree that they should be termed agglomerates. However, like Reynolds, I could not accept that they are bombs and could offer no alternative until a a recent paper by Dr Doris Reynolds (1969) which has supplied, in my opinion, the true explanation, namely gas-fluidization whereby tuff fragments, moved by gas under pressure, becomes an erosive agent in the volcanic vent. The agglomerate was reported to occur in strength 0.8Km. (half a mile) east of Moons Hill and interpreted by Reynolds (1907) as marking the site of a vent. His ideas would now seem to be confirmed but many details remain to be resolved. [references follow]
Reynolds , Sidney H. 1907. A Silurian inlier in the eastern Mendips. Quarterly Journal of the Geological Society, London, Vol. 63, pp. 217-240. By Professor Sidney, Hugh Reynolds, M.A. F.G.S. Read March 13th 1907.
The Mendip Hills consist of four periclinal upfolds of Carboniferous Limestone, arranged en echelon from north-west to south-east. Each pericline includes a core of Old Red Sandstone [Devonian], and the igneous rocks. It is with these that the present paper deals. The existence of igneous rocks in the Eastern Mendips was first noted by Charles Moore [Quarterly Journal of the Geological Society, 1867, vol. 23.] who described them as:
"a basaltic dyke of considerable thickness emerging from the beneath the Old Red Sandstone at East End near Stoke Lane" [the rocks are actually of andesite not basalt]
He considered that, from the general physical character of the Mendips, it was not improbable that the dyke might be co-extensive with their range. He not only attributed the upheaval of whole Mendip range to the intrusion of this igneous mass, but also considered that it was responsible for the remarkable inverted character of the Carboniferous beds at Luckington, where the Coal-Measures are worked under the Carboniferous Limestone.
John Morris refers to the rock at Stoke Lane, as
"a dyke of considerable thickness, emerging from beneath the Old Red Sandstone, occurring as bosses in the field, but, traced for some distance over the district, it is conglomeratic in places, and pronounced by Mr. D. Forbes to be dolerite"
The igneous rocks are not shown in Sander's map of the Bristol Coalfield (published in 1864), but appear in the map of the Geological Survey (1884), as a series of isolated patches extending from Downhead on the east to Beacon Plantation, southwest of Stoke Lane, on the west, a distance of about 3 miles. [....continues].


Reynolds, Doris L. 1954. Fluidization as a geological process, and its bearing on the problem of intrusive granites, American Journal of Science, 252 (1954), pp. 577-613. By Dr. Doris Reynolds.

Reynolds, Doris L. 1969. Fluidization as a volcanological agent. Proceedings of the Geological Society, London, No. 1655, pp. 110-115.
Stow , D.A.V. 2005. Sedimentary Rocks in the Field: A Colour Guide. By Dorrik A.V. Stow. Manson Publishing, 320pp. [This is an excellent book with numerous clear colour photographs, and is superb for recognition of sedimentary rock types and sedimentary structures.]
Van De Kemp, P.C. The Silurian rocks of the Mendip Hills, Somerset; and the Tortworth area, Gloucestershire, England. Geological Magazine, Vol. 106, No. 6, 1969, pp. 542-553, plates 28-29.
Available online:
Van de Kemp, 1969. Silurian rocks of the Mendip Hills.
Field, petrograph and chemical studies on the Silurian volcanic rocks of the Mendip Hills show that there are probably 15 or more rock units in the series including andesite and rhyodacite lavas, rhyodacite tuffs, agglomerates, and a dolerite dyke. The predominant rock type is rhyodacite which may be as much as 80 percent of the volcanics. Volcanics of Silurian age from the Tortworth area, Gloucestershire, are of latite-andesite composition.
The Mendip rocks have been deuterically altered. Calcite-quartz-laumonite veins are common in fractures in these rocks. The agglomerates are particularly susceptible to weathering and some bombs are extensively altered to clays. Twelve rocks were chemically analysed for 36 elements each. no anomalous base metal concentrations were found in the volcanics although Pb, Zn, and Cu mineralisation is known in the are. K/Rb varies from 202 to 909 in these calc-alkaline rocks.
Wainwright (a company supplying aggregrate). 2008. Geology: Moons Hill and Stoke Quarries. Go to website: Wainwright: Geology: Moons Hill and Stoke Quarries.
Example extract follows, but see the full website with photographs.
The rocks extracted from Moons Hill and Stoke Quarries are volcanic in origin and were formed 425 million years ago during the Silurian period of geological time. They form part of a renowned feature of British geological history and have been the subject of much debate and research over the years. The volcanoes which produced the lavas, ashes and other rock types extended over a wide area of what is now south west England. Rocks of the same type and age can be found in Devon and Gloucestershire.
Volcanic lavas, described as Andesites, were extruded over a wide area forming beds up to 100 to 150 metres deep. During this period of volcanic activity, accumulations of volcanic ash also built up, sometimes overlying the lavas, sometimes interbedded with them. The fine-grained ashes are known as Tuffs and often contain blocks or cobbles of Andesite lavas; these formations are known as Agglomerates. Because of the presence of shale and mudstone below the volcanic rocks and sometimes interbedded with them, we know that this volcanic activity was taking place in a marine environment. The whole sequence of lavas, ashes and agglomerates can be readily identified and measured in the Quarry because the beds are now standing vertically on end like books on a bookshelf.
After being deposited in a semi-molten state in horizontal layers, the volcanic rocks cooled and solidified and were eventually buried by thousands of metres of sedimentary rocks including terrestrial sandstones and limestones formed in oceans and lagoons. Many millions of years later these rock beds were subjected to an intense period of structural uplift resulting in the formation of the Mendip anticlinal folds, in the core of which the most ancient formations can be found. The idealised cross section through Moon's Hill quarry shows how the volcanic rocks now lie in relation to the other formations.
At Moons Hill and Stoke Quarries we concentrate on producing aggregate from the Andesite beds, these being generally stronger and more durable than the Tuffs. The latter are however quite suitable for most specifications. The Andesite aggregates are regarded as roadstone of the highest quality.
Wall , G.R.T. and Jenkyns, H.C. 2004. The age, origin and tectonic significance of Mesozoic sediment-filled fissures in the Mendip Hills (SW England): implications for extension models and Jurassic sea-level curve. Geological Magazine, July 2004, vol. 141, no. 4, pp. 471-504.
In the eastern Mendip Hills, on the northern margin of the Wessex Basin, SW England, the Carboniferous Limestone is cut by numerous fissures that are filled with Mesozoic sediments (sedimentary dykes, neptunian dykes). The fissures contain a record of Triassic-Lower Jurassic sediments that are only sparingly preserved in their normal stratigraphical position between the Carboniferous Limestone and the unconformably overlying Upper Inferior Oolite of Bajocian age. Detailed analysis of cross-cutting relationships, facies analysis, biostratigraphy, lithostratigraphy and strontium-isotope ages of relevant Mesozoic sediments has allowed the construction of an Upper Triassic-Lower Jurassic fissure-fill stratigraphy for the eastern Mendip area. Most fissures were clearly formed by rapid influx of unlithified sediment from the land surface or sea floor. Some smaller cavities, or larger cavities with restricted access to the unconformity, were apparently filled by sediment that trickled down into the fissure system. The vast majority of the Mendip fissures are interpreted as having formed as a response of the Carboniferous Limestone, north of major basin-bounding faults, to pulses of tectonic extension during Ladinian-Norian/Rhaetian, late Hettangian-early Sinemurian, late Sinemurian-early Pliensbachian, mid-Pliensbachian, late Pliensbachian and Bajocian times. Triassic-earliest Jurassic fissures have a broad spread of strike from E-W to NW-SE to N-S, accommodating extension in a roughly NE-SW direction. Younger Jurassic fissures show well-defined E-W and N-S trends with the former becoming dominant through time. Total extension of about 4.7 percent N-S and about 0.6 percent E-W was produced by the formation of Triassic-Jurassic fissures within the Carboniferous Limestone. Such patterns of extension are thought likely to be characteristic of the subsurface geology in much of southern England and Wales. Major implications of this study are that: (1) the presence of seismically unresolvable sediment-filled fissures in supposedly rigid fault blocks can lead to a significant underestimate of regional extension based on the restoration of motion on normal faults on seismic-reflection profiles, and (2) the isolation of pulses of tectonic activity with a temporal resolution of 105-106 years may provide a means of identifying a tectonic signal in relative sea-level curves derived from the Jurassic sedimentary record.
Welch , F.B.A. 1929. The geological structure of the central Mendips. Quarterly Journal of the Geological Society, London, vol. 85, issue 1-4, pp. 45-76. Extract: "The area here termed the Central Mendips comprises that part of the Mendip Hills which lies between Cheddar and Shepton Mallet. This paper deals only with the Palaeozoic rocks, which occupy the greater part of the area, and appear in inliers among the Mesozoic rocks on the south."
Available online as a pdf from the Lyell Collection of the Geological Society, London.

Welch, F.B.A. 1933 (for 1932). Geological structure of the eastern Mendips. Quarterly Journal of the Geological Society, London, vol. 89 for 1932, pp. 14 - 52. Accessible as a pdf file from the Geological Society Lyell Collection. By Francis Brian Awburn Welch.
No abstract. Example extract from Introduction:
Geologically, the area consists of the Beacon Hill pericline, which is the most southerly situated of the four echeloned periclines constituting the Mendip Hills. This pericline forms an approximately east-and-west ridge which attains at its western end a maximum altitude of 974 feet O.D. Its ridgelike character, especially prominent in its western part, is due to the resistant nature of the rocks of which it is composed: namely, Carboniferous Limestone, 'Millstone Grit,' and a central core of Old Red Sandstone and Silurian rocks. Succeeding these hard beds on the north are the soft Coal Measure shales (Lower Coal Measures), whilst the low ground on the south is occupied by Mesozoic strata banked against the Palaeozoic massif.

West , I.M., Brandon, A. and Smith, M. 1968. A tidal flat evaporitic facies in the Visean of Ireland. Journal of Sedimentary Petrology, vol. 38, No. 4, pp. 1079-1093, Figs 1- 14, December 1968. By Ian M. West, Alan Brandon and Melvyn Smith, Department of Geology, The University of Southampton, England.

A slightly revised version of this paper is available as a webpage of this website. Please go to:

West, Brandon and Smith. A tidal flat evaporitic facies in the Visean of Ireland. (revised version online, 2010).

(To see a rather poor copy of the original, unrevised paper go to:
Appendix - Carboniferous Evaporites - Ireland - West, Brandon and Smith.)

A sequence of thinly bedded limestones, dolomites, shales and sandstones contains evaporitic beds in County Leitrim and Cavan, Republic of Ireland. This sequence constitutes the Aghagrania Formation (new name) of B2 to PIc age, (Upper Visean), with a type section east of Drumshanbo, County Leitrim. The evaporitic beds which have not previously been recorded from this horizon or locality, are mostly unfossiliferous laminated limestones and dolomites with macrocells [nodules of the usual calcium sulphate type, i.e. chickenwire] and with pseudomorphs after gypsum, anhydrite, and halite. This facies of the Aghagrania Formation also includes evaporitic breccias, and celestite and carbonate replacements of calcium sulphate. Blocks of gypsum in boulder clay, on the shore of Lough Allen, are probably derived from these beds. The evaporitic strata alternate with shales and limestones containing marine faunas, and with unfossiliferous sandstones. All the facies show evidence of shallow water deposition and were probably formed in an area of low relief subjected to transgressions and regressions of a shallow sea. The evaporitic beds may be compared to the dolomite and gypsum deposits of present day tidal flats and associated shallow lagoons. They also resemble certain other occurrences of ancient laminated dolomite and limestone beds which been recently described and attributed to a tidal flat origin.

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Copyright © 2013 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

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Dr Ian West, author of these webpages

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.


at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.