A show cave —also called tourist cave , public cave , and, in the United States, commercial cave —is a cave which has been made accessible to the public for guided visits.
121-784: The Mendip Hills (commonly called the Mendips ) is a range of limestone hills to the south of Bristol and Bath in Somerset , England. Running from Weston-super-Mare and the Bristol Channel in the west to the Frome valley in the east, the hills overlook the Somerset Levels to the south and the Chew Valley and other tributaries of the Avon to the north. The highest point, at 325 metres above sea level,
242-455: A Mohs hardness of 2 to 4, dense limestone can have a crushing strength of up to 180 MPa . For comparison, concrete typically has a crushing strength of about 40 MPa. Although limestones show little variability in mineral composition, they show great diversity in texture. However, most limestone consists of sand-sized grains in a carbonate mud matrix. Because limestones are often of biological origin and are usually composed of sediment that
363-457: A bloom of cyanobacteria or microalgae . However, stable isotope ratios in modern carbonate mud appear to be inconsistent with either of these mechanisms, and abrasion of carbonate grains in high-energy environments has been put forward as a third possibility. Formation of limestone has likely been dominated by biological processes throughout the Phanerozoic , the last 540 million years of
484-415: A campaign has been started to halt the creation of any new quarries and to restrict the activities and expansion of the existing ones. The Mendips are home to a wide range of outdoor sports and leisure activities, including hunting , caving, climbing, and abseiling . The rich variety of fauna and flora also makes it attractive for hillwalking and those interested in natural history. Mendip Activity Centre
605-434: A carbonate rock outcrop can be estimated in the field by etching the surface with dilute hydrochloric acid. This etches away the calcite and aragonite, leaving behind any silica or dolomite grains. The latter can be identified by their rhombohedral shape. Crystals of calcite, quartz , dolomite or barite may line small cavities ( vugs ) in the rock. Vugs are a form of secondary porosity, formed in existing limestone by
726-612: A central quartz grain or carbonate mineral fragment. These likely form by direct precipitation of calcium carbonate onto the ooid. Pisoliths are similar to ooids, but they are larger than 2 mm in diameter and tend to be more irregular in shape. Limestone composed mostly of ooids is called an oolite or sometimes an oolitic limestone . Ooids form in high-energy environments, such as the Bahama platform, and oolites typically show crossbedding and other features associated with deposition in strong currents. Oncoliths resemble ooids but show
847-449: A change in environment that increases the solubility of calcite. Dense, massive limestone is sometimes described as "marble". For example, the famous Portoro "marble" of Italy is actually a dense black limestone. True marble is produced by recrystallization of limestone during regional metamorphism that accompanies the mountain building process ( orogeny ). It is distinguished from dense limestone by its coarse crystalline texture and
968-538: A complex suite of mountain and hill ranges across what is now southern Ireland, south-western England, Brittany , and elsewhere in western Europe. As a result of the Variscan mountain-building, the Mendip area now comprises at least four anticlinal fold structures, with an east–west trend, each with a core of older Devonian sandstone and Silurian volcanic rocks . The latter are quarried for use in road construction and as
1089-949: A composition reflecting the organisms that produced them and the environment in which they were produced. Low-magnesium calcite skeletal grains are typical of articulate brachiopods , planktonic (free-floating) foraminifera, and coccoliths . High-magnesium calcite skeletal grains are typical of benthic (bottom-dwelling) foraminifera, echinoderms , and coralline algae . Aragonite skeletal grains are typical of molluscs , calcareous green algae , stromatoporoids , corals , and tube worms . The skeletal grains also reflect specific geological periods and environments. For example, coral grains are more common in high-energy environments (characterized by strong currents and turbulence) while bryozoan grains are more common in low-energy environments (characterized by quiet water). Ooids (sometimes called ooliths) are sand-sized grains (less than 2mm in diameter) consisting of one or more layers of calcite or aragonite around
1210-413: A concrete aggregate. The Mendips were considerably higher and steeper 200 to 300 million years ago, and subsequent erosion has resulted in varying geological features including gorges, dry valleys, screes , swallets and others typical of karst landscapes. Beneath the southern escarpment and plateau are caves . There are also areas of limestone pavement and other karst features. Dissolution of
1331-412: A considerable fraction of the limestone bed. At depths greater than 1 km (0.62 miles), burial cementation completes the lithification process. Burial cementation does not produce stylolites. When overlying beds are eroded, bringing limestone closer to the surface, the final stage of diagenesis takes place. This produces secondary porosity as some of the cement is dissolved by rainwater infiltrating
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#17328442977551452-483: A drop of dilute hydrochloric acid is dropped on it. Dolomite is also soft but reacts only feebly with dilute hydrochloric acid, and it usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers. Impurities (such as clay , sand, organic remains, iron oxide , and other materials) will cause limestones to exhibit different colors, especially with weathered surfaces. The makeup of
1573-621: A few million years, as this is the most stable form of calcium carbonate. Ancient carbonate formations of the Precambrian and Paleozoic contain abundant dolomite, but limestone dominates the carbonate beds of the Mesozoic and Cenozoic . Modern dolomite is quite rare. There is evidence that, while the modern ocean favors precipitation of aragonite, the oceans of the Paleozoic and middle to late Cenozoic favored precipitation of calcite. This may indicate
1694-455: A few thousand years. As rainwater mixes with groundwater, aragonite and high-magnesium calcite are converted to low-calcium calcite. Cementing of thick carbonate deposits by rainwater may commence even before the retreat of the sea, as rainwater can infiltrate over 100 km (60 miles) into sediments beneath the continental shelf. As carbonate sediments are increasingly deeply buried under younger sediments, chemical and mechanical compaction of
1815-490: A high percentage of the mineral dolomite , CaMg(CO 3 ) 2 . Magnesian limestone is an obsolete and poorly-defined term used variously for dolomite, for limestone containing significant dolomite ( dolomitic limestone ), or for any other limestone containing a significant percentage of magnesium . Most limestone was formed in shallow marine environments, such as continental shelves or platforms , though smaller amounts were formed in many other environments. Much dolomite
1936-588: A large proportion of the annual precipitation falls from showers and thunderstorms at this time of year. Average rainfall is around 800–900 mm (31–35 in). About 8–15 days of snowfall is typical. November to March have the highest mean wind speeds, with June to August having the lightest; the prevailing wind direction is from the south-west. A combination of the rainfall and geology leads to an estimated average daily runoff from springs and boreholes of some 330,000 m (72 million imperial gallons). Bristol Waterworks Company (now Bristol Water ) recognised
2057-435: A limestone sample except in thin section and are less common in ancient limestones, possibly because compaction of carbonate sediments disrupts them. Limeclasts are fragments of existing limestone or partially lithified carbonate sediments. Intraclasts are limeclasts that originate close to where they are deposited in limestone, while extraclasts come from outside the depositional area. Intraclasts include grapestone , which
2178-466: A lower Mg/Ca ratio in the ocean water of those times. This magnesium depletion may be a consequence of more rapid sea floor spreading , which removes magnesium from ocean water. The modern ocean and the ocean of the Mesozoic have been described as "aragonite seas". Most limestone was formed in shallow marine environments, such as continental shelves or platforms . Such environments form only about 5% of
2299-525: A lower diversity of organisms and a greater fraction of silica and clay minerals characteristic of marls . The Green River Formation is an example of a prominent freshwater sedimentary formation containing numerous limestone beds. Freshwater limestone is typically micritic. Fossils of charophyte (stonewort), a form of freshwater green algae, are characteristic of these environments, where the charophytes produce and trap carbonates. Limestones may also form in evaporite depositional environments . Calcite
2420-523: A mechanism for dolomitization, with one 2004 review paper describing it bluntly as "a myth". Ordinary seawater is capable of converting calcite to dolomite, if the seawater is regularly flushed through the rock, as by the ebb and flow of tides (tidal pumping). Once dolomitization begins, it proceeds rapidly, so that there is very little carbonate rock containing mixed calcite and dolomite. Carbonate rock tends to be either almost all calcite/aragonite or almost all dolomite. About 20% to 25% of sedimentary rock
2541-586: A mix of barbed wire and sheep fencing. These dry-stone walls are of botanical importance as they support important populations of the nationally scarce wall whitlowgrass ( Draba muralis ). Amongst the plants which occur in the area are the Cheddar pink ( Dianthus gratianopolitanus ), purple gromwell ( Lithospermum purpurocaeruleum ), white rock-rose ( Helianthemum apenninum ), Somerset hair-grass ( Koeleria vallesiana ), and starved wood-sedge ( Carex depauperata ). Twenty Palaeolithic sites have been identified in
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#17328442977552662-448: A plausible source of mud. Another possibility is direct precipitation from the water. A phenomenon known as whitings occurs in shallow waters, in which white streaks containing dispersed micrite appear on the surface of the water. It is uncertain whether this is freshly precipitated aragonite or simply material stirred up from the bottom, but there is some evidence that whitings are caused by biological precipitation of aragonite as part of
2783-449: A radial rather than layered internal structure, indicating that they were formed by algae in a normal marine environment. Peloids are structureless grains of microcrystalline carbonate likely produced by a variety of processes. Many are thought to be fecal pellets produced by marine organisms. Others may be produced by endolithic (boring) algae or other microorganisms or through breakdown of mollusc shells. They are difficult to see in
2904-489: A wide distribution across the Mendips and are often found in flooded disused quarries. Several rare butterflies are indigenous to the area, including the nationally scarce pearl-bordered fritillary ( Boloria euphrosyne ), Duke of Burgundy ( Hamearis lucina ), and white-letter hairstreak ( Satyrium w-album ). The large blue butterfly ( Maculinea arion ) became extinct in the hills in the late 1970s. The white-clawed crayfish
3025-486: Is Beacon Batch which is the summit area atop Black Down . The hills gave their name to the former local government district of Mendip , which administered most of the local area until April 2023. The higher, western part of the hills, covering 198 km (76 sq mi) has been designated an Area of Outstanding Natural Beauty (AONB), which gives it a level of protection comparable to a national park . The hills are largely formed from Carboniferous Limestone , which
3146-795: Is a cave that has been made accessible to the public for guided visits, where a cave is defined as a natural occurring void beneath the surface of the earth, per the International Show Caves Association. A show cave may be managed by a government or commercial organization and made accessible to the general public, usually for an entrance fee. Unlike wild caves, they may possess regular opening hours, guided group tours, constructed trails and stairs, color artificial illumination and other lighting, musical/video/laser shows and concerts, elevators, small trains, and boats if they contain underground water features. Some caves (mainly in Asia) open to
3267-752: Is a 58-kilometre (36 mi) long-distance footpath from the Mendips to the Cotswolds, and the Mendip Way covers 80 km (50 mi) from Weston-super-Mare to Frome. The western section runs from the Bristol Channel at Uphill Cliff , affording views over the Somerset Levels, crosses the central Mendip plateau leading down to Cheddar Gorge, and then continues to Wells and Frome. The much longer Monarch's Way runs for 990 km (620 mi), from Worcester to Shoreham-by-Sea in West Sussex. It closely follows
3388-475: Is about 10 °C (50 °F) with seasonal and diurnal variations, but the modifying effect of the sea restricts the range to less than that in most other parts of the United Kingdom. January is the coldest month, with mean minimum temperatures between 1 °C (34 °F) and 2 °C (36 °F). July and August are the warmest, with mean daily maxima around 21 °C (70 °F). In general, December
3509-568: Is accomplished by the effect of gravity on the runoff. Water from the Mendips is also collected in Cheddar Reservoir , which was constructed in the 1930s and takes water from the springs in Cheddar Gorge. The area hosts three semi-natural habitats of national importance: ash – maple woodland ( Fraxinus spp. and Acer spp.) often with abundant small-leaved lime ( Tilia cordata ), calcareous grassland and mesotrophic grassland . Much of
3630-560: Is also favored on the seaward margin of shelves and platforms, where there is upwelling deep ocean water rich in nutrients that increase organic productivity. Reefs are common here, but when lacking, ooid shoals are found instead. Finer sediments are deposited close to shore. The lack of deep sea limestones is due in part to rapid subduction of oceanic crust, but is more a result of dissolution of calcium carbonate at depth. The solubility of calcium carbonate increases with pressure and even more with higher concentrations of carbon dioxide, which
3751-535: Is also nationally rare and is a declining species with small populations in a tributary of the Mells River and the River Chew . The dry stone walls that divide the pasture into fields are a well-known feature of the Mendips. Constructed from local limestone in an "A frame" design, the walls are strong yet contain no mortar; many have been neglected and allowed to disintegrate, or have been replaced or contained by
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3872-482: Is an uncommon mineral in limestone, and siderite or other carbonate minerals are rare. However, the calcite in limestone often contains a few percent of magnesium . Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. High-magnesium calcite retains the calcite mineral structure, which is distinct from dolomite. Aragonite does not usually contain significant magnesium. Most limestone
3993-639: Is based at the Charterhouse Centre in the heart of the AONB. The AONB Unit consists of four staff: a manager, project officer, support officer and part-time planning officer and fixed term project officers. They are supported by volunteer rangers. In 2005 a proposal was submitted to the Countryside Agency to extend the Area of Outstanding Natural Beauty to Steep Holm and Brean Down in the west and towards Frome in
4114-402: Is carbonate rock, and most of this is limestone. Limestone is found in sedimentary sequences as old as 2.7 billion years. However, the compositions of carbonate rocks show an uneven distribution in time in the geologic record. About 95% of modern carbonates are composed of high-magnesium calcite and aragonite. The aragonite needles in carbonate mud are converted to low-magnesium calcite within
4235-474: Is clusters of peloids cemented together by organic material or mineral cement. Extraclasts are uncommon, are usually accompanied by other clastic sediments, and indicate deposition in a tectonically active area or as part of a turbidity current . The grains of most limestones are embedded in a matrix of carbonate mud. This is typically the largest fraction of an ancient carbonate rock. Mud consisting of individual crystals less than 5 μm (0.20 mils) in length
4356-416: Is commonly white to gray in color. Limestone that is unusually rich in organic matter can be almost black in color, while traces of iron or manganese can give limestone an off-white to yellow to red color. The density of limestone depends on its porosity, which varies from 0.1% for the densest limestone to 40% for chalk. The density correspondingly ranges from 1.5 to 2.7 g/cm . Although relatively soft, with
4477-420: Is controlled largely by the amount of dissolved carbon dioxide ( CO 2 ) in the water. This is summarized in the reaction: Increases in temperature or decreases in pressure tend to reduce the amount of dissolved CO 2 and precipitate CaCO 3 . Reduction in salinity also reduces the solubility of CaCO 3 , by several orders of magnitude for fresh water versus seawater. Near-surface water of
4598-545: Is converted to low-magnesium calcite. Diagenesis is the likely origin of pisoliths , concentrically layered particles ranging from 1 to 10 mm (0.039 to 0.394 inches) in diameter found in some limestones. Pisoliths superficially resemble ooids but have no nucleus of foreign matter, fit together tightly, and show other signs that they formed after the original deposition of the sediments. Silicification occurs early in diagenesis, at low pH and temperature, and contributes to fossil preservation. Silicification takes place through
4719-503: Is deposited close to where it formed, classification of limestone is usually based on its grain type and mud content. Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera . These organisms secrete structures made of aragonite or calcite, and leave these structures behind when they die. Other carbonate grains composing limestones are ooids , peloids , and limeclasts ( intraclasts and extraclasts [ ca ] ). Skeletal grains have
4840-460: Is described as coquinite . Chalk is a soft, earthy, fine-textured limestone composed of the tests of planktonic microorganisms such as foraminifera, while marl is an earthy mixture of carbonates and silicate sediments. Limestone forms when calcite or aragonite precipitate out of water containing dissolved calcium, which can take place through both biological and nonbiological processes. The solubility of calcium carbonate ( CaCO 3 )
4961-617: Is described as micrite . In fresh carbonate mud, micrite is mostly small aragonite needles, which may precipitate directly from seawater, be secreted by algae, or be produced by abrasion of carbonate grains in a high-energy environment. This is converted to calcite within a few million years of deposition. Further recrystallization of micrite produces microspar , with grains from 5 to 15 μm (0.20 to 0.59 mils) in diameter. Limestone often contains larger crystals of calcite, ranging in size from 0.02 to 0.1 mm (0.79 to 3.94 mils), that are described as sparry calcite or sparite . Sparite
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5082-462: Is distinguished from micrite by a grain size of over 20 μm (0.79 mils) and because sparite stands out under a hand lens or in thin section as white or transparent crystals. Sparite is distinguished from carbonate grains by its lack of internal structure and its characteristic crystal shapes. Geologists are careful to distinguish between sparite deposited as cement and sparite formed by recrystallization of micrite or carbonate grains. Sparite cement
5203-488: Is evidence of mining in the Mendips dating back to the late Bronze Age, which increased after the Roman invasion, particularly for lead and silver around Charterhouse . The difficult conditions in the area were noted by William Wilberforce in 1789, which inspired Hannah More to begin her work improving the conditions of the Mendip miners and agricultural workers. In the 18th and early 19th centuries 7,300 ha (18,000 acres) of
5324-469: Is located on Warrens Hill Road, on the rim of Batts Combe quarry between Shipham and Charterhouse. Thomas Hardy described the Mendips as "a range of limestone rocks stretching from the shores of the Bristol Channel into the middle of Somersetshire", and several of his books refer to the Mendips or sites on the hills. According to legend, Augustus Montague Toplady was inspired to write the words of
5445-566: Is often called a semi-wild cave. Access may involve anything between an easy stroll and dangerous climbing . Most cave accidents happen in this kind of cave, as visitors underestimate the difficulties and dangers. The oldest known show cave in the world is probably Reed Flute Cave in China with inscriptions from 792 in the time of the Tang dynasty . Other old show caves are Postojna Cave in Slovenia , with
5566-560: Is one of the first minerals to precipitate in marine evaporites. Most limestone is formed by the activities of living organisms near reefs, but the organisms responsible for reef formation have changed over geologic time. For example, stromatolites are mound-shaped structures in ancient limestones, interpreted as colonies of cyanobacteria that accumulated carbonate sediments, but stromatolites are rare in younger limestones. Organisms precipitate limestone both directly as part of their skeletons, and indirectly by removing carbon dioxide from
5687-473: Is otherwise chemically fairly pure, with clastic sediments (mainly fine-grained quartz and clay minerals ) making up less than 5% to 10% of the composition. Organic matter typically makes up around 0.2% of a limestone and rarely exceeds 1%. Limestone often contains variable amounts of silica in the form of chert or siliceous skeletal fragments (such as sponge spicules, diatoms , or radiolarians ). Fossils are also common in limestone. Limestone
5808-472: Is produced by decaying organic matter settling into the deep ocean that is not removed by photosynthesis in the dark depths. As a result, there is a fairly sharp transition from water saturated with calcium carbonate to water unsaturated with calcium carbonate, the lysocline , which occurs at the calcite compensation depth of 4,000 to 7,000 m (13,000 to 23,000 feet). Below this depth, foraminifera tests and other skeletal particles rapidly dissolve, and
5929-501: Is quarried at several sites. Ash – maple woodland, calcareous grassland and mesotrophic grassland which can be found across the Mendip Hills provide nationally important semi-natural habitats. With their temperate climate these support a range of flora and fauna including birds, butterflies and small mammals. The dry stone walls that divide the pasture into fields are of botanical importance as they support important populations of
6050-478: Is required to visit the vast majority of the caves, but Cheddar Gorge and Wookey Hole Caves are two show caves which are easily accessible to the public. The active Mendip Caving Group and other local caving organisations organise trips and continue to discover new caverns. The Hills conceal the largest underground river system in Britain; attempts to move from one cave to another through the underground rivers led to
6171-514: Is secondary dolomite, formed by chemical alteration of limestone. Limestone is exposed over large regions of the Earth's surface, and because limestone is slightly soluble in rainwater, these exposures often are eroded to become karst landscapes. Most cave systems are found in limestone bedrock. Limestone has numerous uses: as a chemical feedstock for the production of lime used for cement (an essential component of concrete ), as aggregate for
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#17328442977556292-624: Is set in the Mendip Hills and is one of the largest outdoor activity centres in the South West. It is a key part of the local tourism community bringing significant revenue and visitor numbers each year, welcoming 10,000s of visitors. They have a thriving outdoor education program with schools, delivering school residentials. Families and individuals take part in activities including skiing and snowboarding, climbing and abseiling, kayaking and paddle boarding, caving, archery, air rifle, tobogganing and disc golf. Their pop-up campsite, Mendip Basecamp, opened to
6413-509: Is that the name is cognate with Mened ( Welsh mynydd ), a Brythonic term for upland moorland. The suffix may be a contraction of the Old English hop , meaning a valley. Possible further meanings have been identified. The first is 'the stone pit' from the Celtic meyn and dyppa in reference to the collapsed cave systems of Cheddar . The second is "mighty and awesome" from
6534-615: Is the dullest month and June the sunniest. The south-west of England enjoys a favoured location, particularly in summer, when the Azores High extends its influence north-eastwards towards the UK. Cumulus cloud often forms inland, especially near hills, and reduces exposure to sunshine. The average annual sunshine is about 1,600 hours. Rainfall tends to be associated with Atlantic depressions or with convection. In summer, convection caused by solar surface heating sometimes forms shower clouds, and
6655-405: Is the only Mendip breeding site for downy emerald dragonflies. In 2007 the first confirmed sighting of a red kite ( Milvus milvus ) on the Mendips was made at Charterhouse. A range of important small mammals are found in the area, including the hazel dormouse ( Muscardinus avellanarius ) and bats. The hazel dormouse is restricted largely to coppice woodland and scrub, while the bats, including
6776-699: The A38 . Further east, and running almost north to south, are the A37 , A39 , A36 and the A361 . During the late 19th and early 20th century, the Bristol and North Somerset Railway ran roughly parallel to the A37. Further south and west, the Cheddar Valley Line and Wrington Vale Light Railway , branches of the Bristol and Exeter Railway , served towns and villages from Cheddar to Wells. In
6897-440: The Bristol Channel . The hills gave their name to the rare mineral mendipite , an oxohalide of lead with chlorine with formula Pb 3 Cl 2 O 2 which was first described in the area. A sample of mendipite was found at the head of Ebbor Gorge . Along with the rest of South West England , the Mendip Hills have a temperate climate that is generally wetter and milder than the rest of England. The annual mean temperature
7018-543: The Dartford warbler ( Sylvia undata ), which can be found at Black Down and Crook Peak . In Britain, this species is usually associated with lowland heath. The woodlands at Stock Hill are a breeding site for nightjars ( Caprimulgus europaeus ) and long-eared owls ( Asio otus ). The Waldegrave Pool, part of Priddy Mineries , is an important site for dragonflies , including downy emerald ( Cordulia aenea ) and four-spotted chaser ( Libellula quadrimaculata ). Waldegrave Pool
7139-686: The Harz in Germany though this cave was intensively visited much earlier. The development of electric lighting enabled the illumination of show caves. Early experiments with electric light in caves were carried out by Lieutenant Edward Cracknel in 1880 at Chifley Cave , Jenolan Caves , Australia . In 1881, Sloupsko-Šošůvské Jeskyně , Czech Republic , became the first cave in the world with electric arc light. This light did not use light bulbs, but electric arc lamps with carbon electrodes, which burned down and had to be replaced after some time. The first cave in
7260-434: The Mendip Way and Limestone Link . Several explanations for the name "Mendip" have been suggested. Its earliest known form is Mendepe in 1185. One suggestion is that it is derived from the medieval term Myne-deepes . Others suggest it derives from Celtic monith , meaning mountain or hill, with an uncertain second element, perhaps Old English yppe in the sense of upland or plateau. A third explanation
7381-618: The Priddy Nine Barrows and Ashen Hill Barrow Cemeteries . The Historic England Archive holds over 1,200 entries for the area, and there are over 600 listed buildings , in addition to over 200 scheduled ancient monuments . These protected monuments range from prehistoric barrows and hillforts to the Black Down bombing decoy from the Second World War. Settlement on the Mendip Hills appears to fall into two types. The first, apparent in
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#17328442977557502-611: The barrows and forts around Priddy and at Dolebury Warren . The caves of Cheddar Gorge have yielded many archaeological remains, as flood waters have washed artefacts and bones into the caves and preserved them in silt. The Cheddar Man , Britain's oldest complete skeleton, was found in Gough's Cave , part of the Cheddar Complex . Within the Mendip Hills AONB, good evidence exists for 286 definite examples of round barrows, including
7623-496: The Earth's history. Limestone may have been deposited by microorganisms in the Precambrian , prior to 540 million years ago, but inorganic processes were probably more important and likely took place in an ocean more highly oversaturated in calcium carbonate than the modern ocean. Diagenesis is the process in which sediments are compacted and turned into solid rock . During diagenesis of carbonate sediments, significant chemical and textural changes take place. For example, aragonite
7744-478: The Iron Age onward the ownership of land took on increasing importance, with large landholdings based on the mines or on stock grazing, denying settlers access to the plateau or forcing them off the hills. There is evidence of mining dating back to the late Bronze Age, when there were technological changes in metal-working indicating the use of lead. The Roman invasion, and possibly the preceding period of involvement in
7865-534: The Mendip Hills has, since 1972, been designated an Area of Outstanding Natural Beauty (AONB) under the National Parks and Access to the Countryside Act 1949 . The Mendip Society, which was formed in 1965, helps to raise awareness of this designation and protect the area. The society runs a programme of guided walks and educational presentations. The society also has a small grants fund to assist communities with
7986-537: The Mendip Hills is Charterhouse Cave with a vertical range of 220 m (722 ft). Many caves in the Mendip area were excavated for archaeological and natural history studies by pioneer explorer Herbert E. Balch and were photographed by the early caver Harry Savory at the start of the 20th century using cameras , glass plates and flash powder . Several sites on the Mendips are designated as open access land , and there are many footpaths and bridleways that are generally clearly marked. The Limestone Link
8107-451: The Mendips is open calcareous grassland, supporting a wide variety of flowering plants and insects. Some parts are deciduous ancient woodland , and some have been used intensively for arable agriculture, particularly since World War I. As the demand for arable land in Britain declined, some areas were returned to grassland, but the use of fertilisers and herbicides has reduced biodiversity. Grazing by rabbits, sheep and cattle maintains
8228-685: The Mendips the Carboniferous Limestone layers are found in the subsurface and are exposed in Avon Gorge , and are overlain by younger strata in Dundry Hill and the Cotswolds , where oolitic limestone of Jurassic age is found at the surface. West of the main Mendip plateau the Carboniferous Limestone continues in Bleadon Hill and Brean Down , and on the islands of Steep Holm and Flat Holm in
8349-487: The Mendips, many based on the particular geology of the area. The hills are recognised as a national centre for caving and cave diving , as well as being popular with climbers, hillwalkers and natural historians. Wookey Hole Caves and some of the caves in Cheddar Gorge are open as show caves ; however, many of the caves of the Mendip Hills are only accessible to members of caving clubs. Long-distance footpaths include
8470-411: The Mendips, of which eleven represent faunal remains and lithic artefacts recovered from caves. The remaining eight sites refer to surface lithic discoveries, and the artefacts found include points, scrapers, and handaxes. Twenty-seven Mesolithic finds are represented by flint and chert lithics. Large numbers of artefacts have been found near Neolithic , Iron Age , and Bronze Age features, such as
8591-454: The Mendips, reflect the arrival of this new road. Much of the high plateau, however, remained uncultivated and unenclosed until the 18th century, resulting in many roads remaining as narrow winding lanes between high banks and hedges or stone walls. Where the tracks had their origins as drovers roads , they typically become open roads with wide verges. The roads tend to follow the line of gorges and valleys, as at Cheddar Gorge. The more major of
8712-591: The Mendips: the Devonian sandstones visible around Blackdown and Downhead and the Carboniferous Limestones, which dominate the hills and surround the older rock formations. There are nine active quarries and a host of disused sites, several of which have been designated as geological Sites of Special Scientific Interest by English Nature. Because of the effect quarrying has on the environment and local communities,
8833-565: The Neolithic and Bronze Age Britain periods, and repeated on a small scale in the Middle Ages and post-medieval era, comprised occupation by self-sufficient groups in small communities or isolated farms. The second was represented in the Iron Age Britain and Roman periods by large sites with specialist functions, existing by virtue of their ability to exert power over lowland producers. From
8954-569: The Old English moen and deop . The rock strata known as the Carboniferous Limestone were laid down during the early Carboniferous Period , about 320–350 million years ago. Subsequently, much of northwestern Europe underwent continental collision throughout the late Paleozoic Era , culminating in the final phases of the Variscan orogeny near the end of the Carboniferous, 300 million years ago. This tectonic activity produced
9075-430: The accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life. About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone. The remaining carbonate rock is mostly dolomite , a closely related rock, which contains
9196-524: The area closed in the 1960s. Although the Roman Fosse Way crossed the hills, the main roads generally avoid the higher areas and run along the bottom of the scarp slope on the north and south of the hills. The western end of the hills is crossed by the M5 motorway and A38 . Further east, and running almost north to south, are the A37 and A39 . A wide range of outdoor sports and leisure activities take place in
9317-434: The base of roads, as white pigment or filler in products such as toothpaste or paint, as a soil conditioner , and as a popular decorative addition to rock gardens . Limestone formations contain about 30% of the world's petroleum reservoirs . Limestone is composed mostly of the minerals calcite and aragonite , which are different crystal forms of calcium carbonate ( CaCO 3 ). Dolomite , CaMg(CO 3 ) 2 ,
9438-657: The beds. This may include the formation of vugs , which are crystal-lined cavities within the limestone. Diagenesis may include conversion of limestone to dolomite by magnesium-rich fluids. There is considerable evidence of replacement of limestone by dolomite, including sharp replacement boundaries that cut across bedding. The process of dolomitization remains an area of active research, but possible mechanisms include exposure to concentrated brines in hot environments ( evaporative reflux ) or exposure to diluted seawater in delta or estuary environments ( Dorag dolomitization ). However, Dorag dolomitization has fallen into disfavor as
9559-462: The bottom with the concave face downwards. This traps a void space that can later be filled by sparite. Geologists use geopetal structures to determine which direction was up at the time of deposition, which is not always obvious with highly deformed limestone formations. The cyanobacterium Hyella balani can bore through limestone; as can the green alga Eugamantia sacculata and the fungus Ostracolaba implexa . Show cave A show cave
9680-448: The common heathland on the hills were enclosed . In World War II a bombing decoy was constructed on top of Black Down at Beacon Batch. More recently, the mast of the Mendip transmitting station , micro-hydroelectric turbines and a wind turbine have been installed. There are several quarries on the Mendip Hills . Some of the stone is still carried by Mendip Rail ; the other railways in
9801-467: The conservation and enhancement of the landscape and to encourage its enjoyment and celebration. As their landscapes have similar scenic qualities, AONBs may be compared to the national parks of England and Wales . In contrast to national parks, which have their own authorities and legal power to prevent unsympathetic development, very few statutory duties are imposed on the local authorities within an AONB. However, further regulation and protection of AONBs
9922-549: The current roads often started as turnpikes in the 16th century. These avoid the highest areas of the hills. To the north of the western part of the Mendips, the A368 separates the hills from the Chew Valley , while on the southern edge the A371 similarly runs along the bottom of the scarp slope between the hills and the Somerset Levels . The western end of the hills is crossed by the M5 motorway with access at junctions 21 and 22, along with
10043-462: The depositional fabric of carbonate rocks. Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles, based on criteria such as whether the grains were originally in mutual contact, and therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of
10164-469: The deposits are highly porous, so that they have a spongelike texture, they are typically described as tufa . Secondary calcite deposited by supersaturated meteoric waters ( groundwater ) in caves is also sometimes described as travertine. This produces speleothems , such as stalagmites and stalactites . Coquina is a poorly consolidated limestone composed of abraded pieces of coral , shells , or other fossil debris. When better consolidated, it
10285-427: The development of cave diving in Britain. The first cave dive was attempted at Swildon's Hole in 1934, and the first successful dive was achieved the following year at Wookey Hole Caves, which has the deepest sump in Britain at 76 m (250 ft). The cave complexes at St. Dunstan's Well Catchment , Lamb Leer , and Priddy Caves have been identified as Sites of Special Scientific Interest. The deepest cave in
10406-406: The earth's oceans are oversaturated with CaCO 3 by a factor of more than six. The failure of CaCO 3 to rapidly precipitate out of these waters is likely due to interference by dissolved magnesium ions with nucleation of calcite crystals, the necessary first step in precipitation. Precipitation of aragonite may be suppressed by the presence of naturally occurring organic phosphates in
10527-525: The east, the Somerset and Dorset Joint Railway ran south from Bath into Dorset , and also served Wells. These have all now closed, but Mendip Rail has freight lines to carry limestone from the quarries of the Mendip Hills . There is also the East Somerset Railway which is an operational heritage railway . The Somerset Coal Canal reached some of the pits of the Somerset Coalfield in
10648-591: The east. Many of the villages on the Mendips have their own parish councils , which have some responsibility for local issues. Local people also elect councillors to district councils or to unitary authorities. The 198 km (76 sq mi) of the AONB are split across four districts: Mendip District Council 87.67 km (33.8 sq mi), Sedgemoor District Council 34.03 km (13.1 sq mi), Bath and North East Somerset Council 36.95 km (14.3 sq mi), and North Somerset Council 39.35 km (15.2 sq mi). The population on
10769-536: The eastern end of the Mendips. In recent centuries the Mendips, like the Cotswolds to the north, have been quarried for stone to build the cities of Bath and Bristol, as well as smaller towns in Somerset. The quarries are now major suppliers of road stone to southern England, among them producing around twelve million tonnes of limestone every year, employing over two thousand people, and turning over approximately £150 million per annum. There are two main rock types on
10890-399: The first refers to the grains and the second to the cement. For example, a limestone consisting mainly of ooids, with a crystalline matrix, would be termed an oosparite. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample. Robert J. Dunham published his system for limestone in 1962. It focuses on
11011-572: The formation of distinctive minerals from the silica and clay present in the original limestone. Two major classification schemes, the Folk and Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks . Based on composition, there are three main components: allochems (grains), matrix (mostly micrite), and cement (sparite). The Folk system uses two-part names;
11132-429: The geologic record are called bioherms . Many are rich in fossils, but most lack any connected organic framework like that seen in modern reefs. The fossil remains are present as separate fragments embedded in ample mud matrix. Much of the sedimentation shows indications of occurring in the intertidal or supratidal zones, suggesting sediments rapidly fill available accommodation space in the shelf or platform. Deposition
11253-401: The grassland habitat. Of the many bird species found in the Mendips, the peregrine falcon ( Falco peregrinus ), which has gradually recolonised the area since the 1980s, is particularly significant. It breeds on sea and inland cliffs and on the faces of active and disused quarries. The upland heaths of the west Mendips have recently increased in ornithological importance due to colonisation by
11374-402: The higher plateau is widely dispersed in small farms and hamlets, although rather than working in agriculture or forestry, most people now commute to employment in surrounding cities and towns. The largest village on the plateau on the western Mendips is Priddy , which had a population of 624 at the 2011 census along with the smaller hamlet of Charterhouse . The larger villages and towns are on
11495-453: The hills were enclosed, mainly with dry stone walls that today form a key part of the landscape. In 2006 funding was obtained to maintain and improve the walls, which had steadily deteriorated over the years. In World War II, a bombing decoy was constructed on top of Black Down at Beacon Batch in an attempt to confuse bombers aiming to damage the city of Bristol, and piles of stones (known as cairns ) were created to prevent enemy aircraft using
11616-494: The hilltop as a landing site. In the 1960s, the tallest mast in the region at 293 metres (961 ft) above ground level, the Mendip UHF television transmitter , was installed on Pen Hill near Wells , one of the highest points of the Mendips. The transmitter's antenna rises to almost 589 metres (1,932 ft) above sea level. Since 2003, arguments have raged over plans to erect a wind turbine near Chewton Mendip. The proposal
11737-695: The hymn " Rock of Ages " while sheltering under a rock in Burrington Combe during a thunderstorm in 1763; there is a metal plaque marking the site. Limestone Limestone ( calcium carbonate CaCO 3 ) is a type of carbonate sedimentary rock which is the main source of the material lime . It is composed mostly of the minerals calcite and aragonite , which are different crystal forms of CaCO 3 . Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as
11858-509: The internal affairs of the south of England, was inspired, in part, by the mineral wealth of the Mendips. William Wilberforce 's visit to Cheddar in 1789, during which he saw the poor circumstances of the locals, inspired Hannah More to begin her work improving the conditions of the Mendip miners and agricultural workers. Under her influence, schools were built and children were formally instructed in reading and Christian doctrine. Between 1770 and 1813 some 7,300 ha (18,000 acres) of land on
11979-408: The limestone produced many of the gorges including Cheddar Gorge and Burrington Combe . Springs are a common feature of the eastern part of the hills, a number of which have associated tufa deposits. Black Down is a moorland area, with its steeper slopes covered in bracken ( Pteridium ) and its flatter summit in heather ( Calluna ) and grasses rather than the pasture which covers much of
12100-502: The lower slopes of the western hills, often in river valleys. Axbridge , with a population of 2,057, and Cheddar (5,755), both within the Sedgemoor district, together with the Mendip town of Shepton Mallet (10,369) and the city of Wells (10,636) are along the southern border of the hills. The North Somerset parishes of Blagdon (1,116), and the parishes of Compton Martin (508), East Harptree (644) and West Harptree (439), lie along
12221-550: The nationally rare lesser ( Rhinolophus hipposideros ) and greater horseshoe bats ( Rhinolophus ferrumequinum ), have a number of colonies in buildings, caves, and mines. A rare and endangered species, the greater horseshoe bat is protected under the Wildlife and Countryside Act 1981 and is listed in Annex ;II of the 1992 European Community Habitats Directive . Amphibians such as the great crested newt ( Triturus cristatus ) have
12342-401: The nationally scarce wall whitlowgrass ( Draba muralis ). The origin of the name "Mendip" is unclear, but it is known that there has been human habitation since Palaeolithic and Mesolithic times with a range of artefacts being recovered from caves. Neolithic , Iron Age , and Bronze Age features such as barrows are numerous with over 200 scheduled ancient monuments recorded. There
12463-504: The northern edge. Further east are the towns of Midsomer Norton and Radstock and the village of Paulton (population 5,302) within the unitary authority of Bath and North East Somerset. In the middle of the 1st century, ancient tracks across the hills were superseded by the Roman Fosse Way , from Bath to Ilchester , a branch of which served the Charterhouse lead mines. Stratton-on-the-Fosse and Lydford-on-Fosse , two villages of
12584-410: The ocean basins, but limestone is rarely preserved in continental slope and deep sea environments. The best environments for deposition are warm waters, which have both a high organic productivity and increased saturation of calcium carbonate due to lower concentrations of dissolved carbon dioxide. Modern limestone deposits are almost always in areas with very little silica-rich sedimentation, reflected in
12705-418: The past and this is reflected in areas of contaminated rough ground known locally as "gruffy". The word "gruffy" is thought to derive from the grooves that were formed where the lead ore was extracted from veins near the surface. Other commodities obtained included calamine ( zinc ore), manganese , iron, copper and baryte . The eastern area reaches into parts of the Somerset Coalfield . North and east of
12826-416: The plateau. The main body of the range is an extended plateau, 6–8 km (4–5 miles) wide and generally about 240 metres (800 ft) above sea level. In some places lead and zinc ores have mineralised the limestone and the dolomitic conglomerate . From the time of Roman Britain until 1908, the hills were an important source of lead. These areas were the centre of a major mining industry in
12947-560: The presumed first record of a cave tour in 1213. Other early show caves are Jasov Cave in Slovakia with inscriptions from 1452, the Sontheimer Höhle in Germany which was reportedly visited by Herzog Ulrich von Württemberg on 20 May 1516 and Vilenica Cave in Slovenia where entrance fees were taken from 1633 on. In 1649, the first "authorized" cave guide started guiding Baumannshöhle in
13068-486: The public have temples, monasteries and religious statues or monuments. Some caves are visited by millions of tourists annually. The term is used inconsistently between nations: many countries tend to call all caves which are open to the public show caves or public caves in contrary to all other caves which are not allowed to enter. However, there are many such caves which are not developed with trails, light and tours, which are visited by very many people. This kind of cave
13189-498: The public in 2020 and in 2023 was named Gold Campsite at the Bristol, Bath and Somerset Tourism Awards. Large areas of limestone on the Mendips have been worn away by water, making the hills a national centre for caving . Some of the caves have been known about since the establishment of the Mendip lead mining industry in Roman times. However, many have been discovered or explored only in the 20th century. Specialist equipment and knowledge
13310-426: The reaction: Fossils are often preserved in exquisite detail as chert. Cementing takes place rapidly in carbonate sediments, typically within less than a million years of deposition. Some cementing occurs while the sediments are still under water, forming hardgrounds . Cementing accelerates after the retreat of the sea from the depositional environment, as rainwater infiltrates the sediment beds, often within just
13431-482: The relative purity of most limestones. Reef organisms are destroyed by muddy, brackish river water, and carbonate grains are ground down by much harder silicate grains. Unlike clastic sedimentary rock, limestone is produced almost entirely from sediments originating at or near the place of deposition. Limestone formations tend to show abrupt changes in thickness. Large moundlike features in a limestone formation are interpreted as ancient reefs , which when they appear in
13552-460: The rock. The Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright (1992). It adds some diagenetic patterns to the classification scheme. Travertine is a term applied to calcium carbonate deposits formed in freshwater environments, particularly waterfalls , cascades and hot springs . Such deposits are typically massive, dense, and banded. When
13673-593: The route taken by Charles II after his defeat at the Battle of Worcester in 1651. The route enters Somerset near Chewton Mendip and crosses the Mendip Hills heading for Wells. A shorter local path, the 72-kilometre (45 mi) long Mendip Pub Trail, connects six pubs owned by Butcombe Brewery. The trail runs from Hinton Blewett through Priddy, Axbridge, Bleadon , Rowberrow , and Compton Martin. Various forms of oval short-track racing, including F1 and F2 stock cars , have taken place at Mendips Raceway since 1969. The track
13794-479: The sediments increases. Chemical compaction takes place by pressure solution of the sediments. This process dissolves minerals from points of contact between grains and redeposits it in pore space, reducing the porosity of the limestone from an initial high value of 40% to 80% to less than 10%. Pressure solution produces distinctive stylolites , irregular surfaces within the limestone at which silica-rich sediments accumulate. These may reflect dissolution and loss of
13915-662: The sediments of the ocean floor abruptly transition from carbonate ooze rich in foraminifera and coccolith remains ( Globigerina ooze) to silicic mud lacking carbonates. In rare cases, turbidites or other silica-rich sediments bury and preserve benthic (deep ocean) carbonate deposits. Ancient benthic limestones are microcrystalline and are identified by their tectonic setting. Fossils typically are foraminifera and coccoliths. No pre-Jurassic benthic limestones are known, probably because carbonate-shelled plankton had not yet evolved. Limestones also form in freshwater environments. These limestones are not unlike marine limestone, but have
14036-481: The value of this resource and between 1846 and 1853 created a series of tunnels, pipes, and aqueducts called the "Line of Works", which still carry approximately 18,200 m (4 million imperial gallons) of water a day to Barrow Gurney Reservoirs for filtration and then on to Bristol and the surrounding areas. This collection and conveyance of water from the Chewton Mendip and East and West Harptree areas
14157-533: The water by photosynthesis and thereby decreasing the solubility of calcium carbonate. Limestone shows the same range of sedimentary structures found in other sedimentary rocks. However, finer structures, such as lamination , are often destroyed by the burrowing activities of organisms ( bioturbation ). Fine lamination is characteristic of limestone formed in playa lakes , which lack the burrowing organisms. Limestones also show distinctive features such as geopetal structures , which form when curved shells settle to
14278-553: The water. Although ooids likely form through purely inorganic processes, the bulk of CaCO 3 precipitation in the oceans is the result of biological activity. Much of this takes place on carbonate platforms . The origin of carbonate mud, and the processes by which it is converted to micrite, continue to be a subject of research. Modern carbonate mud is composed mostly of aragonite needles around 5 μm (0.20 mils) in length. Needles of this shape and composition are produced by calcareous algae such as Penicillus , making this
14399-464: Was added by the Countryside and Rights of Way Act 2000 . The Mendip Hills Partnership, which performs an administrative role, includes the five local authorities that cover the AONB, statutory bodies such as Natural England , together with parish councils and other organisations and groups that have an interest in the conservation and care of the area. The Mendip Hills AONB staff unit of the partnership
14520-409: Was initially rejected by Mendip District Council, with the support of a range of local groups and organisations, on the grounds that the environmental impact on the edge of the AONB outweighed the amount of electricity which would be generated. In April 2006, however, a planning enquiry gave Ecotricity permission to build a 102 m (335 ft) turbine during the following year. The western end of
14641-416: Was likely deposited in pore space between grains, suggesting a high-energy depositional environment that removed carbonate mud. Recrystallized sparite is not diagnostic of depositional environment. Limestone outcrops are recognized in the field by their softness (calcite and aragonite both have a Mohs hardness of less than 4, well below common silicate minerals) and because limestone bubbles vigorously when
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