MENDIP CARBONIFEROUS WEBPAGES:
Cheddar Gorge - Geology - (this webpage)
Burrington Combe, Mendip Hills (website in preparation)

Mendip Hills - Geology (other than Cheddar Gorge)
Carboniferous Geology Bibliography

1 - INTRODUCTION TO CHEDDAR GORGE
1-INT-1. 1-INTRODUCTION - General
1-INT-2. 1-INTRODUCTION - Safety
1-INT-3. 1-INTRODUCTION - Topographic
1-INT-4. 1-INTRODUCTION - Geological Maps
1-INT-5. 1-INTRODUCTION - Geology - Introduction
1-INT-6. 1-INTRODUCTION - Geology - Introduction continued

1 - INTRODUCTION - 6

Carboniferous Limestone - General Introduction

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2 - STRATIGRAPHY OF CHEDDAR GORGE
2-STRAT-1. 2-STRATIGRAPHY - Stratal Succession ]
2-STRAT-1a. 2-STRATIGRAPHY - Stratal Succession - Hypersaline Lagoonal Phases (Modiola Phases)
2-STRAT-2. 2-STRATIGRAPHY - Cheddar Oolite Formation
2-STRAT-3a. 2-STRATIGRAPHY - Clifton Down Limestone Formation (1) Introduction
2-STRAT-3b. 2-STRATIGRAPHY - Clifton Down Limestone Formation (2) Sequence
2-STRAT-4. 2-STRATIGRAPHY - Triassic, Dolomitic Conglomerate Formation
2-STRAT-5. 2-STRATIGRAPHY - Pleistocene Head
2-STRAT-6. 2-STRATIGRAPHY - Cave Deposits
2-STRAT-7. 2-STRATIGRAPHY - Alluvium

2a - STRUCTURE AND TECTONICS OF CHEDDAR GORGE
2a-STRUCTURE-1 Structure and Tectonics - Introduction
2a-STRUCTURE-2 Structure and Tectonics - Strike-Slip-Faulting-
2a-STRUCTURE-3 Structure and Tectonics - Extensional Fractures -
2a-STRUCTURE-4 Structure and Tectonics - Other Tectonism-

3 - GEOMORPHOLOGY AND HYDROGEOLOGY OF CHEDDAR GORGE
3-GEOMORPH-1. Geomorphology - Introduction
3-GEOMORPH-2. Geomorphology - The Origin of Cheddar Gorge (section to be added)
3-GEOMORPH-3. Geomorphology - Quaternary Erosion.
3-GEOMORPH-4. Hydrogeology - Origin of Cave Systems.

4 - LOCATIONS IN CHEDDAR GORGE
(Up-gorge to Down-Gorge and then high south cliffs)
4-LOCAT-1. Location - Black Rock Gate
4-LOCAT-2. Location - Black Rock Gate to the old Reservoir
4-LOCAT-3. Location - Old Reservoir
4-LOCAT-4. Location - Above Horseshoe Bend
4-LOCAT-5. Location - Horseshoe Bend
4-LOCAT-5a. Location - Horseshoe Bend - Brachiopod Beds
4-LOCAT-5b. Location - Horseshoe Bend - Stromatolites
4-LOCAT-5c. Location - Horseshoe Bend - [other]
4-LOCAT-6. Location - Between Horseshoe Bend and Landslip Quarry
4-LOCAT-7. Location - Landslip Quarry
4-LOCAT-7a. Location - Amphitheatre and High Climbing Wall(almost opposite Landslip Quarry)
4-LOCAT-8. Location - Cooper's Hole Area
4-LOCAT-9. Location - Strike-Slip Fault Area
4-LOCAT-10. Location - Gough's Cave Entrance and Cliffs
4-LOCAT-11. Location - Resurgence
4-LOCAT-12. Location - Cox's Cave area
4-LOCAT-13. Location - Lion Rock
4-LOCAT-14. Location - Pond Area
4-LOCAT-15. Location - Cheddar Village
4-LOCAT-16. Location - Jacob's Ladder and Lookout Tower
4-LOCAT-17. Location - High Rock Viewpoint (top cliffs)
4-LOCAT-18. Location - Pinnacles Area (top cliffs)
4-LOCAT-19. Location - Top Cliffs (up-gorge)
4-LOCAT-20. Location - Batts Coombe Quarry (NW of Cheddar Gorge)

5 - FOSSILS IN CHEDDAR GORGE
5-FOSSILS-1. Fossils - Corals of the Lower Clifton Down Limestone.
5-FOSSILS-2. Fossils - Brachiopods of the Lower Clifton Down Limestone

5-FOSSILS-3. Other Fossils

6 - CAVES IN CHEDDAR GORGE
6-SHOWCAVES-1. Caves - Introduction - Show-Caves Location
6-SHOWCAVES-2. Caves - Introduction - Cave Systems

6a - SHOW CAVES IN CHEDDAR GORGE - GOUGH'S CAVE
6a-GOUGHS-1. Gough's Cave - Introduction
6a-GOUGHS-2. Gough's Cave -
6a-GOUGHS-3. Gough's Cave - Stanton's Survey
6a-GOUGHS-4. Gough's Cave -
6a-GOUGHS-5. Gough's Cave -

6b - SHOW CAVES IN CHEDDAR GORGE - COX'S CAVE
6b-COX-2. Cox's Cave - Introduction
6b-COX-2. Cox's Cave -

7 - OTHER CAVES IN CHEDDAR GORGE
7-OTHERCAVES-1. Other Caves -
7-OTHERCAVES-2. Water Input above Horseshoe Bend
7-OTHERCAVES-3. Other Caves -
7-OTHERCAVES-4. Great Oones Hole -
7-OTHERCAVES-5. Pride Evans Hole -
7-OTHERCAVES-6. Caves -
7-OTHERCAVES-7. Caves -

8 - MISCELLANEOUS IN CHEDDAR GORGE
8-MISC-1. Red Staining, probably Triassic
8-MISC-2. Miscellaneous -
8-MISC-3. Miscellaneous -
8-MISC-4. Miscellaneous -

9 - ACKNOWLEDGEMENTS
9-ACKNOW-1. Acknowledgements -

10 - REFERENCES
10-REFERENCES-1. References -
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1 - INTRODUCTION - 1

General

Cheddar Gorge is a famous locality in southern England. It is notable as a canyon-like feature, unusual in this part of the world, and with extensive cave systems. These are in Carboniferous Limestone of about 350 million years old. The gorge is visited by vast numbers of people. There are many car parks, often completely or nearly filled. It is a busy place, particularly in the lower, western part, where people go to visit Gough' Cave and Cox's Cave, both of them being well-known show-caves with entry fees. In geological terms the gorge is interesting in terms of geological structure, and it has a consistent dip of about 20 degrees in a direction towards the south-south west. The Carboniferous Limestone here is unusually thick, about a kilometre in total, although only a part, high in the succession is exposed in the gorge. The place is rather disappointing in terms of visible fossils and not many are at all conspicuous. However, the limestones are at certain levels full of brachiopods and corals. The hard lithification of the limestone and the peculiar dark, almost black, weathering, makes them very difficult to see. The origin of the gorge is particularly interesting and usually attributed to erosion by the outflow of melt-water in the late Pleistocene ice age. This topic will be discussed further, in a part of the webpage yet to be added.

2 - INTRODUCTION - SAFETY

Respect private property; both sides and the base of Cheddar Gorge is on private land, and abide by safety rules, regulations and official notices. Do not enter caves, other than show-caves, without permission and the necessary expertise and equipment. Do not climb the cliffs to get to promontories or caves, without proper equipment and necessary expertise. To the casual visitor the road traffic is the greatest hazard.

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.

In the old quarry, and particularly in the road or road-side footpaths, take care with regard to risk of falling rock. There is risk at various places and there are small signs on the cliff-base rocks, indicating the danger. In some places, as at Horsehoe Bend, some blocks of fallen rock can be seen at the foot of cliffs.

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 particularly careful with the steep walls of Cheddar Gorge. Do not climb these, and do not ascend other than by using proper footpaths. Take car on irregular and/or slippery rocks in and around Cheddar Gorge and other localities. Beware of the risk of occasional falling rocks in the gorge. Do not enter caves, other than commercially operated ones, and do not undertake caving without proper equipment and without the assistance of experienced people and knowledge of the cave systems.

When making geological studies in Cheddar Gorge, Burrington Coombe or other places, take care not to ignore road traffic. Fast-moving cars and other vehicles can be a significant risk, to individual geologists or students and particularly to parties walking on the roads. Note that there is no roadside pavement in Cheddar Gorge, there are many bends and vehicles, particularly cars can pass close by and at some speed. It is easy to be involved in geological study or geological photography and to accidently walk backwards onto the busy and rather narrow road.

Take care on irregular or loose blocks, as in Landslip Quarry, Cheddar Gorge and other places. It is easy to slip and to put a foot down a fissure.

In the coastal extension of the Mendip Hills in the Weston-super-Mare area, there are risks from rising tide and from walking into soft mud at low tide. This webpage is purely descriptive and it does not recommend that any particular activity takes place. It is not an itinerary and not a climbing guide, collecting guide, caving guide or even a walking route guide.

You do not have to go anywhere and if you do go to a particular place, you go at your own risk. You are not advised or encouraged here to rock-climb, enter caves or take risks with traffic; you are not advised to hammer anything, remove anything or make yourself a nuisance to tourists. It is your own responsibility to avoid falling down rocks, and if a rock falls on you, it is the result of risk which you take.

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1 - INTRODUCTION - 3

Introductory Topographic Maps

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1 - INTRODUCTION - 4

Geological Maps and Geological Cross-Sections

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.

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.

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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.

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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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1 - INTRODUCTION - 5

Geology General Introduction

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Thickness of Carboniferous Limestone

The thickness of the Carboniferous Limestone in the Mendip area and in relation to South Wales has been given by George (1972), fig. 1. The thickness is very great in the Mendips, being between 900 metres in the northwestern part and increasing to 1100 metres in the southeastern part of the range of hills. At Cheddar Gorge the thickness is roughly about 1 kilometre. Compared to other areas, this great thickness is similar to that in the Gower Peninsula, South Wales. The thickness is less to the north, and is soon zero, as St. George's Land is approached (i.e. now central Wales). The increase in thickness towards the south is greatest at Bosherton in the Pembrokeshire area, where the Lower Carboniferous or Dinantian (Avonian in old terminology) reaches almost 1500 metres. Thus the Mendip sequence is a very thick carbonate facies. It is also near the southern limit of the limestone facies. Further to the south, in Devon, the non-carbonate Culm facies occurs. Note that in spite of the great thickness of Carboniferous Limestone, there, only the uppermost part is seen in the lower, much publicly visited, part of Cheddar Gorge

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The Carboniferous Limestone of the Mendips is of shelf facies. It has several oolite horizons which are clear indicators of high-energy, clear shallow water. Fossil coral remains are common, again indicating shallow, marine water. So, it is a major and very thick, carbonate shelf facies, showing only minor oscillations in depth and of specific type of carbonate rock. It is quite surprising that carbonate shelf facies could be maintained in fairly uniform conditions for such a long interval of time.

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1 - INTRODUCTION - 6

Carboniferous Limestone - Introduction continued.

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Mineral Composition

The Carboniferous Limestone is a thick, hard, old limestone. dominantly composed of the calcium carbonate (CaCO3) in the form of the hexagonal (rhombohedral) mineral calcite [low-Mg calcite]. This is mostly as anhedral crystals, forming both the bioclasts and the matrix (either finer micrite or a coarser calcite cement).

The less stable, calcium carbonate mineral, aragonite (also CaCO3 but orthorhombic and often with some fairly high, strontium carbonate content) was almost certainly originally present here within shells. It was probably common in Carboniferous ammonoids, nautiloids and bivalve etc., as in Jurassic strata. The situations were not drastically different. However, aragonite has mainly been lost by diagenesis in this older strata, and only preserved in exceptional circumstances (particularly bituminous shales-. See the papers of Stehli and of Yochelson for more information on this topic.

Aragonite is not stable in meteoric water (i.e. fresh land water, not seawater), except when sometimes protected by clays and/or organic matter. It is not usually found in the old, hard Carboniferous limestone of southern England, but it can survive quite easily in younger geological carbonate sediments if they are within clay or shale. The general lack of aragonite would be expected in such old carbonate rocks that have been subject to tectonic action, deep burial and subsequent uplift and a long time phase. Aragonite is known from oil shales in Scotland, where the Carboniferous is not thermally mature and there is preserving organic matter. It is also in natural asphalt deposits. Elsewhere it is usually rare or absent, or replaced by calcite. See the papers mentioned above and also Hallam and O'Hara for more information. Aragonite would not generally be expected in the old, hard and rather massive limestones of Cheddar Gorge, only abundant low-Mg calcite, but exceptional occurrences are not impossible.

Porosity and Permeability

The Clifton Down Limestone Formation of Cheddar Gorge is hard, dark micritic rock that seems to be of very low porosity. In contrast the Cheddar oolite etc, further down in the sequence is likely to have much higher porosity and permeability in places. A very low porosity is common for certain parts of the Carboniferous sequence at certain localities . Some Irish Carboniferous Limestone has less than 1 percent porosity. A Buxton Carboniferous limestone was found to have only 2.9 percent porosity and a dry density of 2.71. This accords with the theoretical figure for 100 percent calcite which is 2.71 (i.e. the specific gravity of calcite). The permeability of the Buxton rocks was as low as 0.3. Some poroperm data on a small number of Carboniferous Limestone samples, but not specifically for Cheddar Gorge is given in : Bell (2013), p. 570..

2 - STRATIGRAPHY - 1

Stratal Succession - Introduction

2 - STRATIGRAPHY - 1a

Stratal Succession

Regressive Hypersaline Lagoonal Phases
("Modiola Phases" with stromatolites)

[Go also to related subject matter -
Stromatolites at Horseshoe Bend, Cheddar Gorge.]

While most of the Carboniferous Limestone is of shallow marine facies with crinoidal limestones, coral-rich limestones, etc, some parts of it are of carbonate lagoon type, sometimes with raised salinities and environments just approaching those suitable for evaporite deposition. This has been recognised since the beginning of the 20th century. More recently the transgressions and regressions were discussed more specicifically by Ramsbottom (1973). In summary, the transgressive phases on the shelf areas are represented by bioclastic limestones. However, the regressive phases are characterised by oolitic limestones and calcite mudstones, the latter often containing stromatolites. These are interpreted as the early stages of evaporite deposition. There may be dolomitisation beneath these regressive phases, according to Ramsbottom (1973). Above the base of D1 the minor cyclicity affects the whole area of England and Wales and was the result of eustatic changes in sea level.

The regressive phases are of special interest and easily recognised by the presence of stromatolites. It is also likely some minor evaporites in the Mendip area might be associated with these. Within the Carboniferous Limestone of South Wales, the Avon Gorge and the Mendip Hills there are three sequences of limestone with evidence of raised palaeosalinity. These broad regressive phases were known in the past as the "Modiola Phases". They have always been regarded as calcareous lagoon phases and the higher salinity has long been recognised. They were described back long ago by Dixon (1907) and Dixon and Vaughan (1911). The old discussion was mainly in relation to the coastal exposures of the Gower Peninsula, South Wales, where the exposure is particularly good. They are also of broader application to Bristol, Bath and Mendip area. The authors, mentioned above, discussed the following (based with modifications on their Table 3, p. 514). The occurrence of stromatolites at Cheddar Gorge has been reported by Green and Welch (1965). They found them in the upper 55m. of the Clifton Down Limestone, 47m from the top, and about 13m. above the Lithostrotion Beds. The examples seen at Cooper's Hole, mentioned below might be closer to the Lithostrotion beds than that but distance has not been measured. It is quite likely that there are move stromatolite horizons than are clearly exposed. Green and Welch (1965), p. 31, referred to "algal limestone" (i.e. stromatolites), including Ortonella sp. The present writer has not seen the Ortonella and they may well be referring to a different horizon but still within the high Clifton Down Limestone. The matter of finding particular features is not at all easy because of very high cliffs and most exposures being of dark limestone, hard and oscure and not showing details. Cheddar Gorge is not easy for geological study and it is almost certain that much is missed by the casual geological visitor and, perhaps, even by the experienced surveyor working in detail in difficult places.

This present website on Cheddar Gorge, reports then only two occurrences, which seem to be from the upper two of these lagoonal phases (and both are easily accessible).

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1. Lowest Modiola Phase.

(not seen at Cheddar Gorge - beneath the lowest strata)

The hypersaline sequence at the base of K [i.e. basal Tournasian]. "Some beds yield little, except: - lamellibranchs [bivalves] such as Modiola and Sanguinolites [bivalve - Cardiidea], Spirobis (?);abundant ostracods such as Cytherella.".. "Some beds show brecciation in which the fragments are but slightly separated. Beds with fragments [clasts] of contemporaneous sediments. Presence of pure oolites of fine grain [c.f. Basal Purbeck, hypersaline, lagoonal, fine-grained oolites (see West, 1975)]... "Packing of ostracods within one another by long-continued drifting". Fotenote says "First noticed in [lagoonal] Purbeck limestone by Mr. F. Chapman, 1906 ". Purple and green shales and fine conglomerate are sometimes present. Some plant remains from nearby land have been found in this regressive phase, but not in Cheddar Gorge.

2. Middle Modiola Phase.

(the Horseshoe Bend Stromatolite Bed at Cheddar Gorge)

This at the base of C2 + S1 [i.e. below the Seminula Pisolite and the Fluorite Bed]. The presence of some stromatolites is described: "calcite mudstones [micrite] and dolomite-mudstones to finely lamined; a little of the latter crumpled as in landscape marble". Almost all the beds are unfossiliferous or yield only particular bivalves, such as the high-salinity tolerant Sanguinolites. Generally there is an absence of normal marine beds to within this particular hypersaline Modiola phase (Dixon and Vaughan, 1911). [but with a minor exception of a thin crinoid parting at Caswell Bay, Gower]. Some plant remains from nearby land have been found in this regressive phase, but not in Cheddar Gorge.

[For more information on this bivalve, see the following website descriptive summary; note the reference to tolerance of hypersaline water.
"Sanguinolites variabilis
Description: Specimen of a fossil bivalve, Sanguinolites variabilis, collected from the Scremerston Coal Group, Redesdale Ironstone of Early Carboniferous, Dinantian, Visean, Asbian age from Comb (?), Northumberland (NE England). Collected by John Dunn. This bivalve burrowed into the surface of shallow tropical sea floor. It fed on small food particles that it filtered out of the sea water. The bivalve probably died in its burrow because its two shells have remained joined together. Sanguinolites is usually found in a specialised high density - low diversity fauna that could tolerate extremely salty (hypersaline) sea water such as that found in coastal lagoons. It is often found with another bivalve Modiolus after which this specialised community is named. The specimen was found in Northumberland. It is from the Carboniferous period (362 - 290 million years ago)"]
[From the website: Sanguinolites variabilis.]

3. Highest Modiola Phase

(the Cooper's Hole Stromatolite Beds, and possibly others, not seen)

The Modiola Phase at the top of S2, the uppermost Clifton Down Limestone Formation, and present, amongst other places at Cheddar Gorge. It includes pisolites (in Gower) and stromatolites or thrombolites (sometimes referred to in old literature as "Landscape Marble") at the Avon Gorge, Chipping Sodbury and Cheddar Gorge. According to Dixon and Vaughan Spirorbis - like annelids occur in places [These ancient, small coiled forms with a shell are now not considered to be Spirorbis but actually microconchid "worm" tubes.]. A "Mitcheldeania - type alga" [Ortonella]has been reported from southwestern Gower. This Modiola phase is not a simple and continuous unit derived from hypersaline conditions. There has been frequent oscillation of salinity with brachiopod and crinoidal beds. At Cooper's Hole, Cheddar Gorge some scattered brachiopods are present between stromatolite beds. Plant remains are not know from this highest lagoonal phase.

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Modiola Phases - Palaeoenvironments

The three Modiola Phases in the Lower Carboniferous of the southern England and south Wales area are to some extent also discussed a little later by (Reynolds, 1921).

With regard to the Modiola Phases, the interesting problem is that of the salinity tolerance of the Carboniferous mussel-like bivalve, Modiola. In the Carboniferous of the region, it characterises lagoonal facies, ofen with stromatolites and therefore probably sometimes hypersaline. Minor evaporites are present in the Carboniferous Limestone of this region. This apparently euryhaline bivalve also occurs in parts of the Tournasian evaporitic sequence from the Tweed Borehole, from near Berwick-on-Tweed. There it is under investigation by the British Geological Survey and several researchers. There is a possible modern analogue. The modern mussel-like bivalve Brachidontes pharaonis has a salinity tolerance from 35 to 53 psu [practical salinity units]. Brachidontes lives in the waters of Qatar and theoretically could approach the gypsum-precipitating sabkhas. Some of the Arabian Gulf facies have obvious similarities to those of the British Lower Carboniferous.

A significant comment made in the old but thoughtful paper of Dixon and Vaughan (1911) is that the occurrence of the Modiola Phases indicates a very limited tidal range. This may be true but there are also factors of the extent of restriction of water-flow and of scale of the lagoons which have also to be considered. This is a general problem, not restricted to the Carboniferous of southern Britain.

Another factor to be considered, but not discussed by those authors, is that there may be evidence (at least at Cheddar Gorge)of fairly rapid change in salinity. Thus the stromatolite beds at Cooper's Hole, Cheddar Gorge have some brachiopods between them. If comparison is made with the basal Lulworth Formation, (late Jurassic) Purbeck Group of southern England then it will seen that in that case the truely marine water almost never impinge on the lagoonal evaporite facies. The palaeosalinities certainly changed, but open marine influence is not seen there. Rapid marine transgression is seen in the Carboniferous Limestone Modiola Phases. We now leave this topic and consider other aspects of the Lower Carboniferous of Cheddar Gorge.

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2 - STRATIGRAPHY - 2

Stratal Succession.

The Cheddar Oolite Formation

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The Cheddar Oolite Formation is not well exposed in the main part of Cheddar Gorge. On the geological map a small inlier is shown at the Horseshoe Bend. This is not obvious at the surface but road material, a small car park and grass may have covered it.

2 - STRATIGRAPHY - 3a

Stratal Succession

The Clifton Down Limestone Formation (1) - Introduction

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The part of the Carboniferous Limestone Group into which Cheddar Gorge is cut is the Clifton Down Limestone Formation. See the BGS Lexicon of Named Rock Units (just Google - "Clifton Down Limestone Formation"). This is from a higher part of the Carboniferous Limestone Group. It is Arundian to Holkerian in age [about middle Visean, about 340 million years old, in simple terms].

Its main characteristics can be summarised as follows:
This formation consists of splintery dark grey calcite and dolomite mudstones, pale grey oolitic, dark grey bioclastic and oncolitic limestones and some mudstones (after BGS). Scattered cherts and silicified fossils occur within part of it. In Cheddar Gorge it is not easy to see the details because of dark joint surface, which rarely shows much detail. A favourable exposure with good weathering is needed.

The Clifton Down Limestone Formation is 266m. in the Avon Gorge but thins to 150 to 200m in the Mendips (BGS Lexicon).

This is carbonate sediment from shallow water in a hot, nearly equatorial environment. Cheddar Gorge area was just south of the equator at the time. The sea was shallow and clear and suitable for the growth of corals. The limestone was deposited in a variety of settings, from shallow carbonate shelf to barrier or back barrier (or even lagoon).

The fauna of the Clifton Down Limestone is marked by an abundance of individuals in places. In Cheddar Gorge a rich fauna will not easily be found because of the type of exposure, as mentioned above. In reality there is an abundance of individuals but a paucity of species (Green and Welch, 1965). It includes the coral Lithostrotion martini (this has groups of corals which are circular in cross-section). Basaltiform (i.e hexagonal) Lithostrotion corals also occur. In practice, in the field the species cannot be identified but lithostrotion is quite easily found, particularly at Landslip Quarry. Composita ficoidea ["Seminula ficoidea" in old literature] is a smooth-shelled brachiopod that is apparently common, but not easy to see. A large productid brachiopod Productus corrugatohemisphericus has been found in this limestone, although not necessarily in Cheddar Gorge. In summary, you are most likely to see colonies of Lithostrotion, usually of a non-basaltiform type. Some photographs of rather poor specimens are shown in this webpage.

Some obvious palaeoenvironmental conclusions can be drawn from the Lithostrotion corals. Water salinity could not have exceeded about 50 ppt (parts per thousand). Condition in much of the Arabian Gulf are broadly similar today, but close inshore to UAE the salinity rises above 50 and it is too hypersaline to compare to this Carboniferous Limestone of Cheddar Gorge. This is compatible with the rarity of evaporites in the limestone sequence here. Carboniferous Limestone evaporites do occur in the Mendips but are not abundant here (elsewhere Lower Carboniferous evaporites are well-developed; the climate was appropriate for their precipitation, but the environments were not always appropriate.).

2 - STRATIGRAPHY - 3b

Stratal Succession

The Clifton Down Limestone Formation (2) - Sequence

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For details of the Clifton Down Limestone succession at Cheddar Gorge see particularly:
Green and Welch (1965), p. 19 and p. 31. This formation is very thick but there is easy access to it in the area of the Horseshoe Bend (lower part of the Lower Clifton Down Limestone Formation), the cliffs just west of Horseshoe Bend (chert beds, with a single continuous chert bed at the base), and at the lower part of Landslip Quarry (Lithostrotion Beds). The upper part of the old quarry, with the stepped, quarried exposure in the calcite mudstone above the Lithostrotion Beds.

2 - STRATIGRAPHY-4

Stratal Succession.

The Triassic Dolomitic Conglomerate Formation

The Dolomitic Conglomerate is a breccia or conglomerate consisting of blocks or fragments of Carboniferous Limestone in a matrix of sandy marl or fine-grained limestone debris (Green and Welch, 1965). The clasts range in size from about a centimetre or smaller up to more than a metre. The cemented rock is hard and can form crags like those of the Carboniferous Limestone. Although widely developed at the margins of the Mendip Hills and in former wadis or valleys (such as at Burrington Coombe) it is not a significant component within the main cliffs of Cheddar Gorge. They are almost entirely Carboniferous Limestone. As the name suggests, it is extensively dolomitised, although not necessarily fully. Dolomitisation can be accompanied by hydration or partial hydration of the original hematite to limonite (or goethite) so that the colour can change from red to yellow to yellowish brown (Green and Welch, 1965).

2 - STRATIGRAPHY - 5

Stratal Succession.

Pleistocene Head

Head is poorly sorted, stony, gravelly loam and clay on top of the older deposits. It is of Pleistocene age and is considered to have been formed by solifluction in periglacial conditions.

2 - STRATIGRAPHY - 6

Stratal Succession.

Cave Deposits

Cave deposits range from water-laid sand, silt and gravel, to frost breccias and brown and reddish brown cave earths (Green and Welch, 1965). They are important deposits which may in some places contain human remains and artefacts. [This section will be expanded later.]

2 - STRATIGRAPHY-7

Stratal Succession.

Alluvium

Alluvial gravel occurs on the lower ground, normally in stream valleys. [Further details will be added.]

2a - STRUCTURE AND TECTONICS OF CHEDDAR GORGE

2a-STRUCTURE-1

Structure and Tectonics - Introduction

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2a - STRUCTURE 2

Strike-Slip-Faulting

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2a - STRUCTURE 3

Extensional Fractures

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2a - STRUCTURE 4

[ready for use]

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3 - GEOMORPHOLOGY OF CHEDDAR GORGE

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3 - GEOMORPHOLOGY - 1.

Introduction

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3 - GEOMORPHOLOGY - 2.

The Origin of Cheddar Gorge

(a new section to be added)

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3 - GEOMORPHOLOGY AND HYDROGEOLOGY - 3.

Quaternary Erosion

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Cheddar Gorge is very youthful, probably almost entirely of Pleistocene origin and it lacks river terraces. Most stream and river valleys in the south of England have had a progressively downcutting development, leaving terraces as indicators of their history. There are usually recent river or stream valleys within older valleys. In the Mendip Hills some of the valley systems are very old. Burrington Coombe, for example, has a precursor of Triassic age, now partly filled with Triassic debris, the Dolomitic Conglomerate. There was a desert wadi there about 250 million years ago. This is not the case in the lower and main part of Cheddar Gorge. As the photograph above shows, there is no trace of a pre-existing valley here.

4 - GEOMORPHOLOGY - HYDROGEOLOGY - ORIGIN OF CAVE SYSTEMS

Origin of Caves - Hydrogeology

The Carboniferous Limestone of the Mendip Hills is an important aquifer. It is the most important in the region (Green and Welch, 1965). It is, of course, well known that the water table is deep beneath the surface under the Mendips. The situation is not simple, though. The Clifton Down Limestone of the area, in which the major cave systems occur has in terms of individual blocks of rock low porosities. However, it is characterised by many joints, fissure and caves.

Of course the caves and the hydrogeology of the Cheddar Gorge region has long been studied, and in various ways. It is not necessarily easy to understand, partly because the gorge has a long history in human terms (very short in geological terms) during which the water table was in various different positions and the climate was varied from periglacial to temperate and relative warm at present. In addition there has long been a special emphasise on particular caves, in addition to overview studies.

A particularly useful and recommended paper that explains to some extent the Cheddar Gorge aquifer and the development of the caves is that of Worthington and Ford, 2009, Self-Organised Permeability in Carbonate Aquifers. It is relevant to Cheddar Gorge (see p. 330). It is quantitative and moderately technical, but the general geological reader can understand the formulae in general with a little guesswork, if they are not experienced in hydrogeology. The general themes can be easily understood by the general (geological) reader anyway, and the paper is conveniently available online. It is recommended as a background to discussion of Cheddar Gorge cave and hydrogeological problems. Some points relevant to Cheddar Gorge and its caves are given in some notes below that are based on the paper. You are strongly advised to obtain the paper and study it in full, though.

Worthington and Ford, 2009 - Carbonate Aquifers - Some summary notes.

Some major points, mainly from Worthington and Ford, 2009 () relevant to the limestones of Cheddar Gorge, are summarised here in simplified form. Carbonate rocks, i.e. limestones etc, are unusual in being moderately soluble in infiltrating water, with dissolved solids (i.e. calcium bicarbonate). Typical dissolved solid concentrations are 100 to 300 milligrams per litre [milligram - thousanth of a gram). At Cheddar the emerging water will be "hard water" with high calcium carbonate (dissolved as bicarbonate). The high carbonate content is not a health problem and may be beneficial.
Carbonate aquifers can have groundwater velocities on average, faster than in other rock types. Sampling in wells shows that are commonly contaminated by microbes Worthington and Ford, 2009 ()(referring to Embrey and Runkle, 2006). In other words, water from rain, acquiring bacteria etc. at the surface, travels relatively quickly through fissures and joints and caves in some hard limestone rocks. In contrast it may filter very slowly through beds of siltstone (or the special case of fine-grained Chalk), spending a long time underground and away from surface organisms. Perhaps, as expected, the stream at Cheddar is able to rise quite fast after a spell of heavy rain. If this is the case then transport of microbes may be faster than elsewhere, although this does not imply that there is any real health hazard in this clean natural environment. [However, in the unlikely case of some sudden pollution affecting the Mendip Hills, nuclear for example, the relatively quick-response water system of Cheddar Gorge would soon carry polluted waters to the outflowing resurgence. In contrast, deep boreholes in Chalk, further east would long continue to yield uncontaminated water.]

Practical and theoretical models have used to understand dissolution in carbonate aquifers; that is to say the natural underground dissolving of limestone rock so as to form natural underground reservoirs of water. The pattern of the natural processes that takes place, including the enlargement of joints, fissures and caves has been much researched and largely determined. Therefore there is a theoretical model for the formation of solution features in limestone.

Worthington and Ford, 2009 explain these natural processes in theory. They only mention Cheddar Gorge, almost in passing, on p. 330. However, the principles that they recognise are important.

In theory rainwater should start to dissolve the first limestone which it first encounters at the surface. If the water can become saturated for calcium carbonate, it will not,of course, dissolve any more carbonate. However, it reality, the dissolution of calcium carbonate does not take place quickly and completely near the surface. The descending water is not fully saturated. Thus it still has the capacity to dissolve more limestone underground. Fast-moving water, that is not saturated, flows through natural fractures, dissolving limestone and enlarging them. Of course that would be expected. It should be noted, though, that the faster the water flows, the more the limestone is dissolved.

When the cavity is sufficiently wide the water becomes turbulent and then enlarges rapidly. The critical turbulent phase is known as "breakthrough". As this rapid enlargement along the channels takes place there is then a drop in the head of water. This causes a reorientation of the flow field so that flow is towards the main channels. Tributary channels then form. There are also many very small channels formed, and these may be only one to ten millimetres in width. Some but not all carbonate aquifers develop large cavities, i.e. caves, large enough for people to walk in. The relatively fast-flowing water can develop a system with channels and tributaries heading towards a spring or outflow. In other words, with turbulent flow, the underground water can organise itself in a manner that resembles a surface river system [see Fig. 1 of Washington and Ford, 2009].

In Britain the Chalk, accounting for 55 percent of total groundwater production in England and Wales is very different from the Carboniferous Limestone. The Chalk has traditionally been thought of as a porous medium and it actually has a high porosity. However, the most of the actual water-flow is not straight through the bulk of the Chalk, but in channels with apertures (i.e. joints, fissures etc) of a few millimetres up to a few centimetres. These small apertures are relatively numerous but large apertures (i.e. caves) are rare.

Carboniferous Limestone Channel Aquifers (Caves)

The Carboniferous Limestone is considered to be a well-developed, karst aquifer Worthington and Ford, 2009. In the Carboniferous Limestone of the Yorkshire Dales stream passages form a dendritic (tree-like) pattern, like a surface river system. There can be one main spring or outflow, but others slightly higher and liable to operate in flood conditions. The larger channels are caves. Worthington and Ford, 2009. The caves, however, only represent a very small fraction of the volume of the aquifer. The Carboniferous Limestone has fewer larger channels and these are the caves. However, the actual percentage volume of caves (channels) in the saturated zone, that is below the water table, at Cheddar Gorge is only 0.02 percent Atkinson (1977). It seems to be proven that the cave channels represent only a very small proportion of the overall pore space for water within the Carboniferous Limestone at Cheddar.

Triple Porosity Types in Carbonate Aquifers (i.e. Cheddar limestones)

Much of the underground water is in joints, small fissures and matrix pores. In carbonate rocks in general the matrix [small pores] provides almost all storage but has very long residence times (Worthington, 2007). It will not change fast or become polluted quickly. Channels, i.e. caves etc, provide little storage, account for most of the flow and have very short residence times. The Worthington paper confirms the view that the water of the Cheddar Caves comes in and out fast. Thus it is, in theory, liable to pollution from above, should the land surface be polluted. The fractures, numerous in the Clifton Down Limestone, have an intermediate role between matrix and channels and have low storage and moderate residence times. The author makes the point that the carbonate aquifers are not so much marked by ranging from conduit [cave] flow to diffuse flow [through the small pores of the matrix] but rather have triple porosity with contrasting properties of matrix, fractures and channels.

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4 - LOCATIONS IN CHEDDAR GORGE

(From the high ground in the south and in a dowwn-gorge direction. Then with some additional comment on the high cliff tops of the south side. )

4 - LOCATIONS - 1

Black Rock Gate

Black Rock Gate is near the highest end of the gorge, really just a valley at this point. The bedrock is the Cheddar Limestone Formation of probable Chadian age. This is the basal unit of the Visean.

There is an old limekiln nearby. This is shown in the National Trust website:
Cheddar Gorge - Black Rock Limekiln Walk. The bedrock is the Cheddar Limestone Formation

4 - LOCATIONS - 2

Black Rock Gate to Old Reservoir

In the upper part of Cheddar Gorge, before the main cliffs are reached there are small cliffs approaching the road side and some minor slopes of scree. Some of this scree is fossiliferous, with remains of brachiopods, mostly broken

4 - LOCATIONS - 3

The Old Reservoir

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The old reservoir in the higher part of Cheddar Gorge is a useful indicator of position. It is further up than the bus-turning layby. The top of the Cheddar Oolite Formation is in contact with the higher Clifton Down Limestone Formation, beneath the reservoir. The cliffs at the back only show the Clifton Down Limestone, and Cheddar Oolite exposure have to be sought further up the gorge above the position of the reservoir. Incidently, notice the gorge is fairly wide at this point, thus giving space for the reservoir. There is a major, circular, open reservoir now on lower ground near Cheddar village.

4 - LOCATIONS - 4

Above Horseshoe Bend
(i.e. up-gorge from Horseshoe Bend - Below The Pinnacles)

(Cheddar Oolite to Lower Clifton Down Limestone Formation)

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Extensional fissuring in the Carboniferous strata of the Mendip Hills has been described and discussed by Wall and Jenkyns (2004) [ Wall, and Jenkyns, 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 curves.Geol. Mag.]. They reported that 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 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.

In Cheddar Gorge, particularly in the area of the Pinnacles and the Horseshoe Bend, there are numerous, well-developed, open joints or fissures. These trend NE to SW. They do not usually show slickensides nor obvious sediment fills, although it does not mean that they are not necessarily present somewhere in the area.

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4 - LOCATIONS - 5

5. Horseshoe Bend - General
(introductory)

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The above diagrams are provided as introductory maps or aerial photographs or sections of the Horseshoe Bend area of Cheddar Gorge with a scale. They are given as a simple basis for future more detailed work, including, so it is intended, vertical cliff and stratal measurements.

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4 - LOCATIONS - 5a

5. Horseshoe Bend - Brachiopod Beds

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At Horseshoe Bend, the stromatolite bed and the articulated brachiopod bed, a biocoenosis, can be seen at certain points but they are not immediately obvious. There also seem to be finely laminated beds that might be of microbial mat origin. Note that at the bottom of the cliff there is the apparent junction, based on the BGS map, between the Cheddar Oolite Formation (below) and the Lower Clifton Down Limestone Formation (above and forming most of the cliff). The images above should facilitate finding the units, but it may be difficult to trace them laterally. Study is not helped by the busy road traffic at this rather hazardous bend, and great care must be taken when visiting as a geologist. Note that it would not be easy to lead a large field party here because of the traffic and this is not recommended as a very safe place. One or two people can examine the cliff here but it is necessary to listen for and to watch for approaching road traffic. It is a little safer on the roadside footpath, on the inside of the bend.

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Brachiopods in the East Cliff of Horseshoe Bend

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Brachiopod beds can also be seen in the eastern, inner cliff of Horseshoe Bend. They are also present in the lower part of the Lower Clifton Down Limestone. The example shown above is of a brachiopod shell debris bed, not a biocoenosis

4 - LOCATIONS - 5b

5b. Horseshoe Bend -
Stromatolite Mounds and Mats in the Lower Clifton Down Limestone.

(The Lower S2 stromatolites already known from Avon Gorge, Burrington Coombe and Chipping Sodbury etc. continue through to Cheddar Gorge.)

[Addtional Notes: With regard to Carboniferous stromatolites in general see also the relevant part of the stratigraphical introduction. Go to: Cheddar Gorge - Stratigraphical Introduction - Modiola Phases]

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Description - Laterally-Linked Hemispherical Mounds (LLH)

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Compare these Carboniferous stromatolites with the almost identical modern (LLH - laterally linked hemisphaeroid), algal stromatolites of Marion Lake, South Australia, particularly as shown by Von der Borch (1977) in Fig. 4E (p.699). The modern examples from Marion Lake are aragonite, with blue-green algal fibres, and with much associated gypsum as lenticular crystals.

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[the following is from a higher stromatolite horizon at Cooper's Hole, and shown here again for comparison]

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Obvious hemispherical stromatolite structures, of microbial mat or "algal mat" origin, are visible in the Lower Clifton Down Limestone Formation at the outer curve of Horseshoe Bend. They are present very low in the Clifton Down Limestone, beneath the Chert Beds and just above the brachiopod beds that are a notable feature of this locality.

The bed can be traced, to some extent, round the Horseshoe Bend cliffs, although it is mostly high and out of reach. Just round the upper corner, where there is a major joint and a partially collapsed block, the bed can be seen again. Here the lamination is obvious, but laterally-linked hemispheroids can not be proven.

The exact stratigraphical position this Lower stromatolite Bed has not been measured, but they may be about 10 metres or less from the base of the Clifton Down Limestone Formation (lower part). The location is shown in photographs, but no accurate surveying has been undertaken.

The boundaries of the stromatolite at Horseshoe Bend, Cheddar Gorge, are not clearly seen, and the overall size and shape is not known. The bedding is broadly parallel though, it is unlikely the stromatolite structure is very thick. Since stromatolites are obvious adjacent to road, where people walk past, there are probably many more in the less accessible, high, cliffs of Cheddar Gorge.

Particularly note that this small exposure of stromatolites suggests that there is a bed of stromatolites at this horizon, resembling the superb surface shown by the Avon RIGS group in Barnhill Quarry, Chipping Sodbury. Go to:
Outcrop, the blog of the Avon RIGS group, 19th March 2012. See the photograph here, part-way down the page. The photograph credit is to: WYG Environment, Barnhill Quarry Geoconservation Assessment

Stromatolites have already been recorded from the Clifton Down Limestone Formation in the general Bristol-Mendips region by Kirkham (2005), although his paper is mainly concerned with Ortonella and with thrombolites, rather than typical, laminated stromatolites. There appears to be some thrombolitic fabric within the Horseshoe Bend stromatolite, but it has not yet been studied in detail. The thrombolites discussed by Kirkham are mainly concentrated in the "Concretionary Beds" of the Bristol area. The strata he discussed are at the top of the Clifton Down Limestone (and top of the Holkerian) and thus significantly higher then the stromatolite shown here.

Rather further north, in the Clifton Down Limestone Formation at Chipping Sodbury and Wick Murray and Wright (1971) recorded "algal mats" or microbial mats. These intertidal features were found at the bottom and top of a cycle of sedimentation. The sequence, which they studied at Chipping Sodbury is much thinner than the thick sequence at Cheddar Gorge. At Cheddar the intertidal stromatolite occurs near the bottom of the Clifton Down Limestone but, as noted, is not exactly at the base.

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In the Avon Gorge the basal part of the Clifton Down Limestone (S2) contains stromatolites. They are smaller than those at Cheddar Gorge, but may be from near the same horizon. They have been reported as "Algal Limestone, Concretionary Beds, S2(d), Observatory Hill, Clifton." (Reynolds, S.H. (1921) re Avon section, Bristol. Reynolds described and photographed what are in effect stromatolites. (As at Cheddar Gorge they occur some way below the beds with abundant Lithostrotion corals, i.e. at the Landslip Quarry).
On p. 232 "Massive limestone, occasionally oolitic, alternating with 'Concretionary' (algal) and Seminula Bands. Upper Beds with shaley partings. 125ft. [38m] - The 'Concretionary Beds' are [were] ill-exposed in the southern part of the wooded slope south of the Great Quarry." They are referred to as part of the Third 'Modiola' Phase.

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Another occurrence of stromatolites is at Burrington Coombe, not far to the north of Cheddar Gorge. Stromatolites have been recorded in S2, i.e. the Holkerian Stage and broadly equivalent to the Clifton Down Limestone. However, the particular records seem to apply to higher parts of the S2 sequence than is seen at Horseshoe Bend, where the stromatolites occur near the base of the Clifton Down Limestone Formation and below the limestones rich in Lithostrotion. It is quite reasonable that more than one horizon of stromatolite may occur, as at Chipping Sodbury (mentioned above). It is just a very shallow, hypersaline facies.

The reference to the Burrington Coombe stromatolites is as follows:
See:
Reynolds and Vaughan (1911), p. 345:
The "concretionary beds" which are best seen at about the middle of the quarry, are essentially the same as those of the Avon section, and the account given in the paper already referred to of the peculiar 'Landscape' or 'Cotham Marble' [stromatolite in modern terminology] is applicable to the Burrington rocks. Section (see Plate XXVIII, fig. 1) show that this type of rock is to a large extent made up of a mass of small interlacing tubules, which Dr. Bather suggests may be calcareous algae. Dr. G.H. Hinde has .. [to be continued]

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Elsewhere, further afield, Carboniferous stromatolites are associated, as would be expected, with evaporites: e.g. West, Brandon and Smith (1968). A stromatolite bed from Bellavally Gap in Ireland is shown above, but it does not show the mound-like or laterally-linked hemispheroid form. The example in Cheddar Gorge clearly show the mound-like development of the stromatolite structure. It is similar to the Barnhill Quarry stromatolites. It is much like the classic Shark Bay stromatolites, which are in hypersaline water and in proximity to gypsum deposits.

It is also of interest that Kirkam found, in the Bristol area, calcitised evaporite horizons including enterolithic anhydrite of sabkha type Kirkham (2005). Incidently this is very abundant in non-calcitised condition, still as anhydrite, in the Lower Carboniferous Tweed Borehole of BGS, near Berwick-on-Tweed [at present, 2015, this is being studied in detail, and the writer has seen some thin-sections].

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(1)Description - Microbial Mat Limestone(?)

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A very finely laminated limestone sequence, slightly reddish (on the exposed surface, but without domal stromatolites occurs a little lower in the same sequence at the same locality of Horseshoe Bend, Cheddar Gorge. This has not been investigated in thin-section. It is likely to be, but not proven to be, also of microbial mat origin. The evidence is the very fine lamination, the absence of ripple-bedding or cross-bedding and the apparent absence of brachiopods, corals or other fossils.

The bed can be clearly seen in the cliff of the spur, just across the road from the domal stromatolites. It is stratigraphically lower by a few metres but the vertical separation has not been measured.

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(2) Analogues - Other Carboniferous Microbial Mats and Stromatolites

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Stromatolites or microbial mat limestones [i.e. algal limestones] are well-known features, both of the Planet Earth and the related planet Mars (although hummocky and dome-shaped thrombolites are more conspicuous there than are the laminated stromatolites; but of course the thrombolites are more easily seen and photographed by a Mars rover machine). It is not clear which planet they first originated on, before being transported by meteorites, but this is a sideline, not relevant to the present study.

Stromatolites common in the Lower Carboniferous limestones at many places. They are well-developed, for example, in the Visean of Leitrim in Ireland ( West et al., 1968) where they are associated with evaporites. They are undoubtedly of hypersaline origin, at least on this planet.

Similar occurrences microbial mats associated with sabkha, nodular gypsum or anhydrite occur repetitively in the Tweed Borehole of early Carboniferous Limestone, near Berwick-upon-Tweed. Of course this stromatolite -evaporite associated is very familiar in other Formations such as the Purbeck Group in Dorset (West (1964) and West (1975) , etc.)

At no great distance from Cheddar Gorge, at Chipping Sodbury outlier to the north, [and at the Avon Gorge and Burrington Coombe], stromatolites are well-developed in thin stratal units within the Clifton Down Limestone Formation. the same formation at which they occur at Cheddar Gorge (and elsewhere). The Clifton Down Limestone, is the Visean unit in which they mainly occur. They are also known from the older Tournasian strata elsewhere in the region. The stromatolites occur in fairly thin units within a thicker limestone sequence and represent temporary increases in salinity to hypersaline. For details of the Chipping Sodbury occurrence see: Murray and Wright (1971). A very good surface of stromatolites has been preserved in the area in more recent times.

The stromatolite bed in Cheddar Gorge roadside section is not unusual for the Carboniferous of Britain and Ireland. Dolomitisation can occur in associated beds, but this is not always the case and there seems not to be a close relationship at Cheddar Gore. Evaporites, can occur with stromatolies but and this is the normal situation for a sabkha cycle. Precipitation of calcium sulphate often occurs in the sequence directly above microbial mats or stromatolites, and the loss of calcium from the waters increases the local Mg/Ca ratio. A possible implication of the Horseshoe Bend stromatolite bed is that evaporites might have been present above. The strata immediately above need examining. Consideration might also be given to the major bedding plane a short distance above, which causes one of the "steps" in the Cheddar Gorge cliffs. Find traces of evaporites in the thick and hard Clifton Down Limestone Formation is not easy.

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The Horseshoe Bend occurrence is not a first discovery of stromatolites in the Mendips; it is just a conveniently accessible occurrence at a well-known locality. Indeed, the British Geological Survey in a webpage on Mendip rocks has already mentioned the occurrence of stromatolites in the highest part of the Clifton Down Limestone Formation. They are also present, as shown in photographs below in the lower part of the Clifton Down Limestone at Horseshoe Bend, Cheddar Gorge. Here, for comparison, is a brief extract from the BGS descriptions of the strata, with reference to the Clifton Down Limestone. Go to the following webpage, a small relevant extract from which is given below: