98-704: Leigh Woods is a 2-square-kilometre (0.77 sq mi) area of woodland on the south-west side of the Avon Gorge , close to the Clifton Suspension Bridge , within North Somerset opposite the English city of Bristol and north of the Ashton Court estate, of which it formed a part. Stokeleigh Camp , a hillfort thought to have been occupied from the third century BC to the first century AD and possibly also in
196-429: A Filton -based RAF Vampire jet from 501 Squadron crashed into Leigh Woods. The pilot, Flying officer John Greenwood, was killed after he flew under the deck of the suspension bridge while performing a victory roll . Small mountain biking circuits are present in the woods and the area is a popular walking and rock climbing area for Bristolians. Because of the rare flora and fauna, the woods have been included in
294-455: 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
392-457: 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
490-495: A habitat for Peregrine falcons and other wildlife. The steep walls of the gorge support some rare fauna and flora, including species unique to the gorge. There are a total of 24 rare plant species and two unique trees: the Bristol and Wilmotts's whitebeams . Other notable plants include Bristol rock cress , Bristol onion , spiked speedwell , autumn squill and honewort . Because of its steep sides, there are many parts of
588-434: 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
686-612: 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
784-449: 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
882-949: 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
980-412: 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
1078-483: 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
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#17328521389431176-621: 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
1274-455: 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
1372-490: 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
1470-435: 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
1568-466: 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
1666-525: 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
1764-523: 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
1862-448: 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
1960-661: A popular venue for rock climbing from the 1940s – a time when most UK climbing was centred in mountain areas. The first guide to climbing routes in Avon Gorge was published in 1955 by the University of Bristol Mountaineering Club. The same year, British mountaineer Chris Bonington climbed Main Wall putting up Mercavity. The 2017 edition of The Climbers' Club Guide to Avon Gorge by Martin Crocker lists 400 pages of routes and guidance. Avon Gorge
2058-449: 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
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#17328521389432156-531: A rich variety of trees. Rare trees include Bristol whitebeam ( Sorbus bristoliensis ) and wild service tree ( Sorbus torminalis ). There are multiple species of Sorbus within the woods with at least nine native and four imported species, making it one of the most important sites in Britain for this tree. Birds which live in the woods include the raven ( Corvus ) and peregrine falcon ( Falco peregrinus ). Many butterflies and moths can be seen in summer including
2254-472: A small station with the aim of attracting tourists to the area; Nightingale Valley Halt was not very successful and was only in use for less than five years, from 9 July 1928 until 12 September 1932. The line was reopened for freight traffic as far as Royal Portbury Dock on 21 December 2001 and local councils are proposing to reintroduce a passenger service to Portishead as part of their MetroWest plans, although this would not happen before 2019. In 1957
2352-558: Is a national nature reserve and is included in the Avon Gorge Site of Special Scientific Interest . The woods, which cover an area of 2 square kilometres (0.77 sq mi), are on a ridge which mainly consists of limestone , with some sandstone which runs from Clifton to Clevedon , 10 miles (16 km) away on the Bristol Channel coast. At the southern end of the woods is Nightingale Valley (one of several thus named in
2450-455: Is a 1.5-mile (2.5-kilometre) long gorge on the River Avon in Bristol , England. The gorge runs south to north through a limestone ridge 1.5 miles (2.4 km) west of Bristol city centre, and about 3 miles (5 km) from the mouth of the river at Avonmouth . The gorge forms the boundary between the unitary authorities of North Somerset and Bristol, with the boundary running along
2548-560: 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
2646-573: Is also possible that the site was occupied in the Middle Ages. North of Stokeleigh Camp are a series of limestone and mineral quarries , overlooking the River Avon, which are now disused. In the late 19th and early 20th centuries this had an important celestine quarry and a tramway which linked it to a dock on the Avon; both are now derelict. The area has in recent years been restored as an arboretum . At
2744-482: 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
2842-402: 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
2940-474: 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
3038-416: 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
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3136-420: 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
3234-545: 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
3332-503: 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
3430-460: 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 )
3528-617: 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
3626-462: 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
3724-594: Is found in Leigh Woods, as is the parasitic plant yellow bird's-nest (Monotropa hypopitys) . Lady orchid (Orchis purpurea) was discovered here in 1990, in Nightingale Valley; there is doubt as to whether this was a wild plant or an introduction. Green-flowered helleborine (Epipactis phyllanthes) is found on the western side of the gorge, in a wooded area next to the towpath below Leigh Woods. Avon Gorge The Avon Gorge ( grid reference ST560743 )
3822-544: Is now in place to reopen the rest of the line and reintroduce passenger services to Portishead. Between 1893 and 1934, the Clifton Rocks Railway linked the passenger steamer pier at Hotwells with Clifton on the rim of the gorge. A footpath and National Cycle Network cycleway run alongside the Portishead Railway and along the old towpath. The gorge's proximity to the urban population of Bristol made it
3920-423: Is often criticised as being so over-climbed many of the routes are "polished". The formation of the Avon Gorge is the subject of mediaeval mythology. The myths tell tales of two giant brothers, Goram and Vincent , who constructed the gorge. One variation holds that Vincent and Goram were constructing the gorge together and Goram fell asleep, to be accidentally killed by Vincent's pickaxe. Another variation tells of
4018-560: 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
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4116-473: 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
4214-472: 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
4312-514: 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
4410-638: The Leigh Woods suburb. Areas not owned by the National Trust have since been taken over by Forestry England . Rare trees include multiple species of Sorbus with at least nine native and four imported species. Bristol rockcress ( Arabis scabra ) which is unique to the Avon Gorge can be seen flowering in April; various species of orchids and western spiked speedwell ( Veronica spicata ) are common in June and July. It
4508-518: The Middle Ages , lies within the reserve on the edge of the Nightingale Valley. On the bank of the Avon, within the reserve, are quarries for limestone and celestine which were worked in the 18th and 19th centuries are now derelict. In 1909 part of the woodland was donated to the National Trust by George Alfred Wills , to prevent development of the city beside the gorge following the building of
4606-453: The white-letter hairstreak ( Satyrium w-album ). On the steep grassy slopes above the River Avon , Bristol rockcress ( Arabis scabra ) which is unique to the Avon Gorge can be seen flowering in April; orchids and western spiked speedwell ( Veronica spicata ) are common in June and July. In autumn the woodland hosts over 300 species of fungi. Bilberry , a scarce plant in the Bristol area,
4704-636: The 3rd century was for a formal garrison or whether it was just used by "squatters" or as a place of refuge in times of crisis. Stokeleigh might have been connected with the Wansdyke , a series of defensive linear earthworks , consisting of a ditch and an embankment running at least from Maes Knoll in Somerset , to the Savernake Forest near Marlborough in Wiltshire ; however, there is little evidence for this. It
4802-401: The Avon Gorge Site of Special Scientific Interest , which received the designation in 1952, and 140 hectares (350 acres) has been designated as a national nature reserve . The south part of the woods is an area of former pasture woodland with old pollards , mainly oak and some small-leaved lime . To the north, the area comprises ancient woodland of old coppice with standards and contains
4900-789: The Blaise Castle estate, and threw himself into the Bristol Channel, turning to stone and leaving his head and shoulder above water as the islands of Flat Holm and Steep Holm . 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
4998-496: 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
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#17328521389435096-430: 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
5194-462: The area), a dry valley which is cut into the side of the gorge. Its area slopes from the highest point nearly opposite the north gate of Ashton Court to the River Avon beside the western buttress of the Clifton Suspension Bridge . The woods are on the Monarch's Way long-distance footpath. Within Leigh Woods is Stokeleigh Camp , a hill fort thought to have been occupied from the third century BC to
5292-434: 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 ,
5390-657: 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
5488-489: The bridge is perhaps the best known landmark in Bristol. Throughout Bristol's history the gorge has been an important transport route, carrying the River Avon, major roads and two railways. It is the gateway to Bristol Harbour , and provided protection against storms or attack. The Bristol Channel and Avon estuary have a very high tidal range of 15 metres (49 ft), second only to Bay of Fundy in Eastern Canada; and
5586-507: The brothers falling for Avona, a girl from Wiltshire , who instructs the giants to drain a lake which stretches from Rownham Hill to Bradford-on-Avon (i.e. the Avon valley). Goram began digging the nearby Hazel Brook Gorge in Blaise Castle estate, but consumed too much beer and fell asleep. Vincent dug the Avon Gorge and drained the lake, winning the affection of Avona. Upon waking Goram stamped his foot, creating "The Giant's Footprint" in
5684-462: 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
5782-469: 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
5880-406: 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
5978-476: The first century AD and also in the Middle Ages. It is situated on a promontory, bounding the north flank of the Nightingale Valley and occupying around 7.5 acres (3.0 ha). Stokeleigh Camp is thought to have been occupied from the late pre-Roman Iron Age, when it was in the area controlled by the Dobunni . Archaeological investigations suggest during the 1st century Belgae tribes may have been present with some of
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#17328521389436076-399: 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
6174-449: The following year, John Beale was hanged at Taunton for the crime in January 1858. Another violent death occurred in 1948 when George Henry Chinnock, who had been living in the woods, was found with head injuries. In 1909 part of the woodland was donated to the National Trust by the tobacco company owner George Alfred Wills . He did this to prevent housing development on the western side of
6272-458: The following years. On warm days a strong uplift forms in the gorge, on which birds of prey soar while hunting. The gorge also houses large populations of Jackdaw and horseshoe bats , both of which find homes in the caves and bridge buttresses. Due to its geology and ecology, an area of 155.4 hectares (384.0 acres) of the gorge and surrounding woodland has been protected as a biological and geological Site of Special Scientific Interest (SSSI),
6370-572: 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;
6468-429: 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
6566-533: The gorge as Bristol grew in size and population. In 1974 the Ministry of Agriculture, Fisheries and Food bought the woodland associated with the Leigh Court estate. Areas not owned by the National Trust have since been taken over by Forestry England . The Portishead Railway runs close to the river at the bottom of the woods. It was opened on 18 April 1867, but closed on 3 April 1981. The Great Western Railway built
6664-501: The gorge involves balancing the need to protect its ecology with recreational uses such as rock climbing. Green-flowered helleborine is found on the western side of the gorge, in a wooded area next to the towpath below Leigh Woods. lady orchid was discovered here in 1990, in Nightingale Valley on the west side of the Gorge; there is some doubt as to whether this was a wild plant or an introduction. fly orchid and bee orchid are found in
6762-515: The gorge is relatively narrow and meandering, making it notoriously difficult to navigate. Several vessels have grounded in the gorge including the SS Demerara soon after her launch in 1851, the schooner Gipsy in 1878, the steam tug Black Eagle in 1861 and the Llandaff City . The phrase " ship shape and Bristol fashion " arises from when the main harbour in Bristol was tidal, the bottom of which
6860-413: The gorge on which trees cannot grow, making way for smaller plants. The gorge is also home to rare invertebrate species. The gorge has a microclimate around 1 degree warmer than the surrounding land. The steep south-west facing sides receive the afternoon sunlight, but are partially sheltered from the prevailing winds. When winds come from the Bristol Channel in the north west they may be funnelled into
6958-453: The gorge, along with their hybrid. A single plant of lesser meadow-rue is present in the gorge. Bristol rock-cress occurs here, and at nearby Penpole Point ; in the gorge, there are about 3,000 plants on the Bristol side, and about 2,000 below Leigh Woods. Hutchinsia is found on both sides of the gorge. Bloody crane's-bill grows on the Bristol side of the gorge, where it is believed to be native. Little robin occurs on both sides of
7056-494: The gorge, creating harsh and wet conditions. The steep gorge walls make an ideal habitat for peregrine falcons , with a plentiful supply of food nearby in the form of pigeons and gulls . Peregrines have a history of nesting in the gorge, but having become rare in the British Isles they did not breed and were rarely seen in the gorge after the 1930s. In 1990 Peregrines returned to the gorge, and have successfully bred in most of
7154-417: The gorge. 51°27′18″N 2°37′40″W / 51.4549°N 2.6279°W / 51.4549; -2.6279 The gorge cuts through a ridge mainly of limestone , with some sandstone . This particular ridge runs from Clifton to Clevedon , 10 miles (16 km) away on the Bristol Channel coast, although limestone is found throughout the Bristol area. The fossil shells and corals indicate that
7252-456: The gorge. Spiked speedwell grows on both sides of the gorge: the first British record of this plant was from the gorge, in 1641. The gorge area was inhabited at least as early as the Iron Age , probably by the Dobunni tribe. In Leigh Woods above Nightingale Valley, a steep dry valley beside the suspension bridge, is Stokeleigh Camp , one of three Iron Age hill forts in the area. Stokeleigh
7350-644: The gorge. On the east side the Severn Beach Line to Avonmouth and Severn Beach uses the remaining part of the Bristol Port Railway through part of the gorge, and through a tunnel under the Downs. On the west side the Portishead Railway was closed by the Beeching Axe in the 1960s, but has since been reopened for freight traffic as far as Royal Portbury Dock , 2.5 miles (4 km) downstream, and funding
7448-604: The limestone formed in shallow tropical seas in the Carboniferous , 350 million years ago. For a long time it was unclear what caused the Avon to cut through the limestone ridge, rather than run southwest through the Ashton Vale towards Weston-super-Mare . However, Bristol was at the southern edge of glaciation during the Anglian ice age , and it has been suggested that ice blocked the river's natural route through Ashton Vale to
7546-565: The northern end of the woods is Paradise Bottom. This is included in the Leigh Court Estate and was part of the ground laid out by Humphry Repton for Philip John Miles . Some of the first plantings of the giant redwood (Sequoiadendron giganteum) and the Weymouth pine (Pinus strobus) , among other "exotics" imported to the UK, were made here by Sir William Miles in the 1860s. To the south of
7644-410: 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
7742-411: The opposite side of the gorge, in what is now observatory green. Archaeology, plus the configuration of the three forts, suggest they played a role in defending the gorge. During the Middle Ages and Industrial Revolution the area which now forms The Downs was used as common grazing land. It was mined for lead, calamine , iron and limestone, and became home to a windmill which produced snuff from
7840-572: The original notification taking place in 1952. The site may in future be protected as a Special Area of Conservation under the European Commission Habitats Directive (92/43/EEC). The Leigh Woods side of the gorge is largely owned by the National Trust. The Downs on the city side of the gorge are owned by Bristol City Council and managed as a large public park. The gorge side is protected in partnership with Bristol Zoo , WWF and English Nature . The council's management of
7938-512: The pottery showing the influence of the Durotriges . There may have been a break in occupation before reuse in the middle to late 2nd century. In addition to the pottery recovered a possible coin of Gallienus dating from his reign between 253 and 268 has been recovered. An iron- involuted brooch of the La Tène II type has also been found. It is unclear whether the occupation of Stokeleigh Camp in
8036-426: 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
8134-482: 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
8232-460: 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
8330-479: 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
8428-662: 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
8526-474: The south bank. As Bristol was an important port, the gorge formed a defensive gateway to the city. On the east of the gorge is the Bristol suburb of Clifton , and The Downs , a large public park. To the west of the gorge is Leigh Woods , the name of both a village and the National Trust forest it is situated in. There are three Iron Age hill forts overlooking the gorge, as well as an observatory . The Clifton Suspension Bridge , an icon of Bristol, crosses
8624-404: The tobacco which had become one of the city's principal imports. In 1777 the windmill burnt out in a storm, and the building was converted into the observatory , which houses a camera obscura . In the 18th and 19th centuries Bristol's economy boomed and Clifton became a desirable place to live. Mansion houses were built overlooking the gorge, but after grazing was stopped, trees grew and obscured
8722-455: The view from these mansions. In the Victorian era, with houses creeping further onto the Downs, an Act of Parliament was passed to protect them as a park for the people of Bristol. In 1754 a bridge to span the gorge was proposed, but it was nearly 80 years before work began on Isambard Kingdom Brunel 's Clifton Suspension Bridge , and a further 30 years before it was completed. Today
8820-533: 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
8918-553: 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
9016-459: The west. At the Clifton Suspension Bridge the Gorge is more than 700 feet (213 m) wide and 300 feet (91 m) deep. In the 18th century the gorge was quarried to produce building stone for the city. Stone was taken by boat into the floating harbour . In the 19th century celestine was discovered in Leigh Court estate and the Miles family authorised quarrying. Between 1880 and 1920 Bristol
9114-469: The woods is an affluent suburb of Bristol also known as Leigh Woods . It is situated at the western end of the Clifton Suspension Bridge , which opened in 1864, making the development of Leigh Woods as an upmarket residential area practicable. Houses in varying styles were built from the mid-1860s until the First World War . In 1857 the mutilated body of a murdered woman was found in Nightingale Valley;
9212-583: Was closed in 1922, when construction of a major road through the gorge, the Portway , was started. The Portway was opened in 1926. The road is now part of the A4 road , linking Bristol city centre to the M5 motorway , which bypasses the city near Avonmouth . In the late 1990s the wide pavement through nearly all the Avon Gorge was designated to be legally usable by both cyclists and the fewer pedestrians. Two railways still run through
9310-416: 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
9408-399: Was occupied from 3BC to 1AD, and was also used in the Middle Ages. The camp was protected on two sides by the cliff faces of the gorge and Nightingale Valley, and was also protected by earthworks, and is now a scheduled ancient monument . A second hill fort was situated across Nightingale Valley, but has since been built on, and bridge road cuts through it. The third hill fort was situated on
9506-452: Was producing 90% of the world's celestine, but the enterprise did not last long into the 20th century. Bristol Diamonds , brilliant quartz crystals found in geodes in dolomitic conglomerate in the gorge, were popular souvenirs for visitors to the Hotwells spa in the eighteenth and nineteenth centuries. Quarries on the Bristol side of the gorge are now popular with climbers and are
9604-467: Was rocky. If ships were not of stout construction then they would simply break up as the tide receded, hence the phrase. A railway, the Bristol Port Railway , was built through the gorge on the east side from Hotwells to Avonmouth between 1863 and 1865. The Portishead Railway was opened on the west side in 1867. The section of the Bristol Port Railway between Hotwells and Sneyd Park junction
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