West, Ian M. 2017. Mendip Hills, excluding Cheddar Gorge - Geological field guide. Supplement to Geology of the Wessex Coast. Internet site: www.southampton.ac.uk/~imw/Mendip-Hills.htm. By Ian West, Romsey, and School of Ocean and Earth Science, Southampton University. Version: 26th April 2017. Revised version

Geology of the Mendip Hills, excluding Cheddar Gorge

Ian West,

Romsey, Hampshire

Faculty of Natural and Environmental Sciences,
Southampton University,

Webpage hosted by courtesy of iSolutions, Southampton University

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Cheddar Gorge - Geology

Mendip Hills - Geology (other than Cheddar Gorge)

Carboniferous Geology Bibliography (this mainly related to the Mendip Hills, including Cheddar Gorge, but also includes some references regarding the Tweed Borehole, north of England).

[Note on progress, at 13th September 2015. There are three webpages by Ian West, in preparation on the Carboniferous Limestone of the Mendip area. The first - Cheddar Gorge, is fairly advanced and already providing field guidance, but it will be expanded in due course (there is a large reserve stock of unused Cheddar photographs). The second, webpage on other localities of the Mendip Hills is in very preliminary state and will be enlarged and improved later, when time permits (there is no large reserve stock yet). The third webpage is a Bibliography on the Carboniferous Limestone of the Mendips and other areas. This is already fairly large and will be expanded progressively.]

(Preliminary draft webpage in preparation.)

Click on images for large, high resolution versions!
(do not use browser zoom on the low resolution versions)

Cheddar Gorge, Mendip Hills, Somerset, introductory view down the gorge, just up from The Horseshoe, 29th July 2015



Cheddar Gorge - Geology

Mendip Hills - Geology (other than Cheddar Gorge) This webpage!

Carboniferous Geology Bibliography (this mainly related to the Mendip Hills, including Cheddar Gorge, but also includes some references regarding the Tweed Borehole, north of England).

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CONTENTS: (in preparation)


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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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Geological Maps Geological Maps

Purchase of Available BGS Geological Maps of the Mendips and Cheddar Gorge is, of course, recommended. They are usually available from the BGS (British Geological Survey) Bookshop online, and at low cost. They are well-worth buying if you are interested in the geology of the area.

A special Geological Sheet ST 45 Cheddar, scale 1:25,000 covers the Cheddar area in detail. This map really should be obtained by anyone interested in the geology of this area. It is very clear and easy to read and it explains the Cheddar area in its structural perspective.

For the eastern Mendips in broader terms, 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.

Example part of the BGS Geological Map, 1:25,000, Sheet 45, and which includes Cheddar Gorge, Mendip Hills, Somerset

See also the following:
Institute of Geological Sciences [now the British Geological Survey], Bristol District, Solid and Drift, One Inch Series, parts of sheets 250, 251, 264, 265, 280 and 281. The old 1946 edition, with additions in 1956, is largely the basis for the redrawn map below (which not a direct copy and not a photocopy), but other maps have also been used. See the latest version of the Bristol District geological map from BGS.


A simplified, redrawn, general geological map of part of the Mendip Hills area, to show the structure and its relationship to Cheddar Gorge, Somerset, based mainly on old geological maps, and with topography not shown

Above is the very simplified geological map of the Mendips and the area around Cheddar Gorge. It has been completely redrawn largely based on old maps, but with some updating of nomenclature. No topography is shown.


A completely redrawn, geological map of Cheddar Gorge, Mendip Hills, Somerset, with various modifications but based on Green and Welch, 1965, with minor additions from the BGS 1966 map and other sources


The following are some brief comments on the simplified geological map, provided above. This map has been completely manually redrawn, pixel by pixel, with basic geological information and no topographic data. Colours are different. It is based, without geological revision (except some nomenclatural changes) on the two British Geological Survey maps of the area of interest around Cheddar Gorge, and these are referred to below. It is merely introductory and for general educational purposes and is not at all intended as any substitute for obtaining and studying the appropriate British Geological Survey Maps. Indeed, this effectively advertises them. The British Geological Survey Maps also show topographic data and place names (which are not on the simplified map provided here.

Various editions exist of the following relevant maps and purchase of appropriate available versions is strongly recommended from the BGS Bookshop Online. They are both very good and very interesting maps and have been available in various editions at different years [the specialist will notice terminological differences]:

BGS. Geological Sheet ST 45 (Solid and Drift Edition) Cheddar, Scale 1: 25,000, [About 2 and a half inches to one mile}. Institute of Geological Science or British Geological Survey. Classical Areas of British Geology Series. 1959; 1969. There is a new edition of 1983. Six Inch Sheets are also available from BGS.

BGS. Geological Sheet - Bristol District, Special Sheet, (Solid and Drift Edition). Parts of BGS Geological Maps - sheets 250, 251, 264, 265, 280 and 281. (These specific sheets may also be relevant, if available).

BGS. Geological Sheet - Bristol 264. [not to be confused with the Bristol District Sheet]. 1:50,000 edition. This does not include Cheddar Gorge or the nearby Mendip Hills. It deals with an area to the northwest, including Bristol, Avonmouth, Filton, Clevedon, Yatton, Flax Bourton and Keynsham.


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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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[Note, this aerial photo is expandible by clicking-up]

Moon's Hill Adesite Quarry - Aerial Photograph



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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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Please go to: Carboniferous Geology Bibliography (this mainly related to the Mendip Hills, including Cheddar Gorge, but also includes some references regarding the Tweed Borehole, north of England).


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Copyright © 2015 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.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea, caves, unstable quarry faces or other causes. This web page is not a caving guide and it does not recommend entry into any caves, other than commercial show caves. Individuals or group leader should take suitable observations and precautions regarding risk from road traffic, particularly in Cheddar Gorge, and industrial traffic in quarries. Not all places discussed in this website need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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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.