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98-594: The River Meavy is a river in the southwest part of Dartmoor in Devon in south-west England. It runs entirely in the national park and connects Burrator Reservoir to the River Plym . The river is 15.9 kilometres (9.9 mi) long, and flows in a generally south-westerly direction throughout its course. It rises at Meavy Head, close to Princetown , and flows across Walkhampton Common, through Norsworthy and adjacent plantations, and into Burrator Reservoir . It leaves

196-400: A (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called cross-bedding . Cross-bedding is characteristic of deposition by a flowing medium (wind or water). The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. Differences in laminations are generally caused by cyclic changes in

294-424: A diagenetic structure common in carbonate rocks is a stylolite . Stylolites are irregular planes where material was dissolved into the pore fluids in the rock. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions . Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be

392-514: A more complete list see List of Dartmoor tors and hills . The high ground of Dartmoor forms the catchment area for many of Devon's rivers. As well as shaping the landscape, these have traditionally provided a source of power for moor industries such as tin mining and quarrying. The moor takes its name from the River Dart , which starts as the East Dart and West Dart and then becomes

490-647: A mysterious pack of " spectral hounds ", and a large black dog , among others. During the Great Thunderstorm of 1638, the moorland village of Widecombe-in-the-Moor was said to have been visited by the Devil . Sedimentary rock The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but sedimentary rock is estimated to be only 8% of

588-658: A number do not. However, this does not appear to relate to whether or not there is an outcrop of rock on their summit. The tors are the focus of an annual event known as the Ten Tors Challenge , when around 2,400 people aged between 14 and 19 walk for distances of 56, 72 or 88 km (35, 45 or 55 mi) between ten tors on many differing routes. The highest points on Dartmoor are on the northern moor: High Willhays , 621 m (2,037 ft), ( grid reference SX 580892 ) and Yes Tor , 619 m (2,031 ft), ( grid reference SX 581901 ). The highest points on

686-477: A number of migrant species, like the pied flycatcher , the wood warbler or the common redstart . Mammals found here include otters, hazel dormice and nearly all of the UK's 16 bat species. Three rare species: the barbastelle , and the greater and lesser horseshoe bats are of particular importance. The upper reaches of the rivers are spawning grounds for Salmon and trout ; Palmate newts , frogs and toads breed in

784-463: A particular sedimentary environment. Examples of bed forms include dunes and ripple marks . Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition. Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where

882-472: A person's feet. Quakers result from sphagnum moss growing over the water that accumulates in the hollows in the granite. The vegetation of the bogs depends on the type and location. Blanket bog , which forms on the highest land where the rainfall exceeds 2,000 millimetres (79 in) a year, consists mainly of cotton-grass ( Eriophorum species), sedges ( Carex and Rhynchospora ), Bog Asphodel and Common Tormentil , with Sphagnum thriving in

980-465: A red colour does not necessarily mean the rock formed in a continental environment or arid climate. The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at

1078-489: A rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes. The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of

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1176-465: A sediment after its initial deposition. This includes compaction and lithification of the sediments. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and is characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Some biochemical processes, like

1274-431: A sedimentary rock may have been present in the original sediments or may formed by precipitation during diagenesis. In the second case, a mineral precipitate may have grown over an older generation of cement. A complex diagenetic history can be established by optical mineralogy , using a petrographic microscope . Carbonate rocks predominantly consist of carbonate minerals such as calcite, aragonite or dolomite . Both

1372-918: A single river at Dartmeet . It leaves the moor at Buckfastleigh , flowing through Totnes below where it opens up into a long ria , reaching the sea at Dartmouth . Other rivers flowing from Dartmoor include the Teign , the Taw , the Tavy , the Avon , and the Lyd . Some of the rivers in Dartmoor have been dammed to create reservoirs for drinking water , including the River Avon and the South Teign River (Fernworthy Reservoir). Much more rain falls on Dartmoor than in

1470-516: A small-scale property of a rock, but determines many of its large-scale properties, such as the density , porosity or permeability . The 3D orientation of the clasts is called the fabric of the rock. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water. The grain size of

1568-437: A species found nowhere else in the world. The South West Peatland Project aims to restore around 300 hectares of Dartmoor's peatland through collaboration with Dartmoor National Park . Preserving these peatlands will help mitigate climate change through sequestering carbon from the atmosphere. More generally, Dartmoor aims to be carbon negative by 2045. The majority of the prehistoric remains on Dartmoor date back to

1666-952: A term for a fissile mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock. Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include: Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate . Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals, such as halite (rock salt), sylvite , baryte and gypsum . This fourth miscellaneous category includes volcanic tuff and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events . Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy: Sedimentary rocks are formed when sediment

1764-417: A texture, only the average size of the crystals and the fabric are necessary. Most sedimentary rocks contain either quartz ( siliciclastic rocks) or calcite ( carbonate rocks ). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in

1862-425: A valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult. Secondary structures can also form by diagenesis or the formation of a soil ( pedogenesis ) when a sediment is exposed above the water level. An example of

1960-598: Is High Willhays , 621 m (2,037 ft) above sea level. The entire area is rich in antiquities and archaeological artefacts . Dartmoor National Park is managed by the Dartmoor National Park Authority , whose 22 members are drawn from Devon County Council , local district councils and Government. The Dartmoor Commoners' Council exists to create and enforce regulations regarding commoners' rights. Parts of Dartmoor have been used as military firing ranges for over 200 years. The public

2058-503: Is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension . This sediment is often formed when weathering and erosion break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the hinterland (the source area of the sediment). However, some sedimentary rocks, such as evaporites , are composed of material that form at

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2156-421: Is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of turbidity currents . The surface of a particular bed, called the bedform , can also be indicative of

2254-540: Is an upland area in southern Devon , South West England . The moorland and surrounding land has been protected by National Park status since 1951. Dartmoor National Park covers 954 km (368 sq mi). The granite that forms the uplands dates from the Carboniferous Period of geological history. The landscape consists of moorland capped with many exposed granite hilltops known as tors , providing habitats for wildlife. The highest point

2352-419: Is associated with the Dartmoor pluton as with other such plutons . Measurement of the anomaly has helped to determine the likely shape and extent of the rock mass at depth. Dartmoor is known for its tors – hills topped with outcrops of bedrock, which in granite country such as this are usually rounded boulder-like formations. More than 160 of the hills of Dartmoor have the word tor in their name but quite

2450-524: Is at least five times as frequent. On the highest ground, in the north of the moor, the growing season is less than 175 days – this contrasts with some 300 days along most of the south coast of the county. Rainfall tends to be associated with Atlantic depressions or with convection. In summer, convection caused by solar surface heating sometimes forms shower clouds and a large proportion of rainfall comes from showers and thunderstorms at this time of year. The wettest months are November and December and on

2548-430: Is believed that they would have had a conical roof, supported by timbers and covered in turf or thatch. There are also numerous kistvaens , Neolithic stone box-like tombs. The climate became wetter and cooler over the course of a thousand years from around 1000 BC, resulting in much of high Dartmoor being largely abandoned by its early inhabitants. The earliest surviving farms, still in operation today, are known as

2646-409: Is called bedding . Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called lamination . Laminae are usually less than a few centimetres thick. Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at

2744-467: Is granted extensive land access rights on Dartmoor (including restricted access to the firing ranges) and it is a popular tourist destination. Dartmoor includes the largest area of granite in Britain, with about 625 km (241 sq mi) at the surface, though most of it is under superficial peat deposits. The granite (or more specifically adamellite ) was intruded at depth as a pluton into

2842-429: Is higher when the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity occurs) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and

2940-491: Is mirrored by the broad categories of rudites , arenites , and lutites , respectively, in older literature. The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks). Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary widely, but most geologists have adopted

3038-460: Is now a Grade II listed building . In 2015, the property was put on the market. The river is popular for canoeing south of Burrator Reservoir, and is considered an easier course than the upper sections of the River Plym, which are some of the most challenging in Dartmoor. Despite the presence of the dam at Burrator, there is generally a sufficient flow of water. Dartmoor Dartmoor

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3136-603: Is possibly a surviving fragment from the earliest Neolithic woodland clearances. It is home to exceptional epiphytic mosses, liverworts and lichens. Nearly 50 species of moss and liverwort are found in the wood along with 120 types of lichen , including Smith's horsehair lichen, speckled sea-storm lichen and pendulous wing-moss. Over 60 species of lichens grow on the exposed surfaces of the granite tors, including granite-speck rim-lichen, purple rock lichen, brown cobblestone lichen and goldspot lichen and many rare lichen grow on rocks exposed by mining that are rich in heavy metals . On

3234-401: Is reduced. Sediments are typically saturated with groundwater or seawater when originally deposited, and as pore space is reduced, much of these connate fluids are expelled. In addition to this physical compaction, chemical compaction may take place via pressure solution . Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of

3332-450: Is remote from many sources of light pollution . However, in recent years, LED street lights have caused more light pollution to be spread onto Dartmoor. Along with the rest of South West England , Dartmoor has a temperate climate , which is generally wetter and milder than locations at similar height in the rest of England. At Princetown , near the centre of the moor at a height of 453 metres (1,486 ft), January and February are

3430-493: Is the most stable, followed by feldspar , micas , and finally other less stable minerals that are only present when little weathering has occurred. The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been weathered to clay minerals like kaolinite , illite or smectite . Among

3528-503: The Ancient Tenements . Most of these date back to the 14th century, and some even earlier. It was not until the early Mediaeval period that the weather again became warmer, and settlers moved back onto the moors. Like their ancient predecessors, they used the natural granite to build their homes, preferring a style known as the longhouse ; some of theses are still inhabited today, although they have been clearly adapted over

3626-483: The Dartmoor Commons Act 1985 much of it has been designated as access land , which, although it remains privately owned, has no restrictions on where walkers can roam. In addition to this access land, there are about 730 km (450 mi) of public rights of way on Dartmoor, and many kilometres of permitted footpaths and bridleways where the owners allow access. Because of the 1985 Act, Dartmoor

3724-412: The history of life . The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology . Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences , such as pedology , geomorphology , geochemistry and structural geology . Sedimentary rocks can be subdivided into four groups based on

3822-427: The marsh fritillary butterfly, southern damselfly, narrow-bordered bee hawkmoth and bog hoverfly . Areas of bracken are home to the high brown fritillary and pearl-bordered fritillary . Insects found in the heathlands include the emperor moth , green hairstreak and the bilberry bumblebee . The old oak woodlands have a distinctive group of insects including the blue ground beetle and Heckford's pygmy moth ,

3920-479: The moors topped with granite tors are the most iconic part of Dartmoor's landscape, only about half of Dartmoor is actually moorland. Equally important for wildlife are the blanket bogs , upland heaths and the oak woodlands, which are all of global importance. Dartmoor is a Special Area of Conservation (SAC) with four habitats ( Northern Atlantic wet heaths with Erica tetralix ; European dry heaths ; Blanket bog s and Old sessile oak woods with Ilex and Blechnum in

4018-682: The organic material of a dead organism undergoes chemical reactions in which volatiles such as water and carbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form, graphite . This form of fossilisation is called carbonisation . It is particularly important for plant fossils. The same process is responsible for the formation of fossil fuels like lignite or coal. Structures in sedimentary rocks can be divided into primary structures (formed during deposition) and secondary structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in

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4116-464: The British Isles ) being listed as primary reasons for the selection of Dartmoor as a SAC. In addition the area has a population of the southern damselfly , which is also a primary reason for its selection along with populations of Atlantic salmon and otter being qualifying reasons. Wistman's Wood is one of the old sessile oak woods that contribute to the listing of Dartmoor as a SAC and

4214-501: The Dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains. Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between

4312-524: The MoD does not notify the public of this in advance. Some "challenge" and charitable events take place with assistance of the military on Dartmoor, including the long established Ten Tors event and the more recent Dartmoor Beast. The disused Rippon Tor Rifle Range was built to train soldiers during the Second World War, and remained in use until its closure in 1977. Throughout human history,

4410-537: The National Park are Ashburton (the largest, with a population of about 3,500), Buckfastleigh , Moretonhampstead , Princetown , Yelverton , Horrabridge , South Brent , Christow , and Chagford . For a full list, expand the Settlements of Dartmoor navigational box at the bottom of this page. Dartmoor is known for its myths and legends. It is reputedly the haunt of pixies , a headless horseman ,

4508-784: The acidification of the soil and the accumulation of peat and bogs . After a few thousand years, the mild climate deteriorated. This left these areas uninhabited, and, consequently, relatively undisturbed to the present day. The highly acidic soil has ensured that no organic remains have survived, but the durability of the granite has meant that the remains of buildings, enclosures and monuments have survived well, as have flint tools. A number of remains were "restored" by enthusiastic Victorians and, in some cases, they have placed their own interpretation on how an area may have looked. Numerous prehistoric menhirs (more usually referred to locally as standing stones or longstones), stone circles , kistvaens , cairns and stone rows are to be found on

4606-470: The activity of bacteria , can affect minerals in a rock and are therefore seen as part of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasing overburden (lithostatic) pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals (such as mica ) are deformed, and pore space

4704-399: The bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales. The size , form and orientation of clasts (the original pieces of rock) in a sediment is called its texture . The texture is

4802-525: The buildings themselves and its physical location. The Dartmoor landscape is scattered with the marks left by the many generations who have lived and worked there over the centuries – such as the remains of the Dartmoor tin-mining industry, and farmhouses long since abandoned. See Industrial archaeology of Dartmoor . Over half of Dartmoor National Park (57.3%) is private land; the Forest of Dartmoor being

4900-504: The cement and the clasts (including fossils and ooids ) of a carbonate sedimentary rock usually consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral. The resistance of rock-forming minerals to weathering is expressed by the Goldich dissolution series . In this series, quartz

4998-421: The cement to produce secondary porosity . At sufficiently high temperature and pressure, the realm of diagenesis makes way for metamorphism , the process that forms metamorphic rock . The color of a sedimentary rock is often mostly determined by iron , an element with two major oxides: iron(II) oxide and iron(III) oxide . Iron(II) oxide (FeO) only forms under low oxygen ( anoxic ) circumstances and gives

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5096-474: The centre of Dartmoor and features several rock tors near the river. The river is believed to named after an Old English word, meaning "lively stream". The village of Meavy is in turn named after the river. Drake's Leat was a watercourse constructed in the 16th century to transport drinking water from the River Meavy towards Plymouth . Burrator Lodge was built in 1896 for the reservoir owner. It

5194-511: The centuries. Many are now used as farm buildings, while others have been abandoned and fallen into ruin. Some way into the moor stands the town of Princetown , the site of Dartmoor Prison , which was originally built by Isbell Rowe & Company, Plymouth, for prisoners of war from the Napoleonic Wars and the War of 1812 . The prison has a reputation for being escape-proof, due to both

5292-402: The chancellor of the high court, ruled: "In my judgment, on the first issue set out at [14] above, the claimants are entitled to the declaration they seek that, on its true construction, section 10(1) of the 1985 Act does not confer on the public any right to pitch tents or otherwise make camp overnight on Dartmoor Commons. Any such camping requires the consent of the landowner." A protest against

5390-407: The coldest months with mean minimum temperatures around 1 °C (34 °F). July and August are the warmest months with mean daily maxima not reaching 18 °C (64 °F). Compared with Teignmouth , which is on the coast about 22 miles (35 km) to the east, the average maximum and minimum temperatures are 3.0 °C (5.4 °F) and 2.6 °C (4.7 °F) lower respectively, and frost

5488-572: The current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current. Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats. Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers. Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within

5586-663: The dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (1/16 to 2 mm diameter), and mud (<1/16 mm diameter). Mud is further divided into silt (1/16 to 1/256 mm diameter) and clay (<1/256 mm diameter). The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand , and mudrocks are made mostly of mud. This tripartite subdivision

5684-489: The field. Sedimentary structures can indicate something about the sedimentary environment or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers. Sedimentary rocks are laid down in layers called beds or strata . A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks

5782-404: The flow calms and the particles settle out of suspension . Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles. Most authors use " shale " as

5880-485: The grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. The increased pressure and temperature stimulate further chemical reactions, such as the reactions by which organic material becomes lignite or coal. Lithification follows closely on compaction, as increased temperatures at depth hasten the precipitation of cement that binds the grains together. Pressure solution contributes to this process of cementation , as

5978-510: The grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc. Although the Dott classification scheme is widely used by sedimentologists, common names like greywacke , arkose , and quartz sandstone are still widely used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as

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6076-596: The highest parts of the moor the average annual total rainfall is over 2,000 millimetres (79 in). This compares with less than 800 millimetres (31 in) in the lower land to the east around the Exe Estuary , which is in the rain shadow of the moor. Due to the influence of the Gulf Stream snowfall is not common, though due to its high altitude it is more vulnerable to snowfall than surrounding regions. Between 1961 and 1990 Met Office data shows that there

6174-587: The host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels , vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralization . The most common minerals involved in permineralization are various forms of amorphous silica ( chalcedony , flint , chert ), carbonates (especially calcite), and pyrite . At high pressure and temperature,

6272-506: The integrity of the moor; however, the cost of compensating companies for these licences, which may not have been granted in today's political climate, could prove prohibitive. The military use of the moor has been another source of controversy, as when training was extended in January 2003. The national park authority received 1,700 objections before making the decision. Objectors said that Dartmoor should be an area for recreation, and that

6370-631: The landscape has been exploited for industrial purposes. In recent years, controversy has surrounded the work of industrial conglomerates Imerys and Sibelco (formerly Watts Blake Bearne), who have used parts of the moor for china clay mining. Licences were granted by the British Government but were recently renounced after sustained public pressure from bodies such as the Dartmoor Preservation Association . The British government has made promises to protect

6468-512: The late Neolithic and early Bronze Age . Indeed, Dartmoor contains the largest concentration of Bronze Age remains in the United Kingdom, which suggests that this was when a larger population moved onto the hills of Dartmoor. The large systems of Bronze Age fields, divided by reaves , cover an area of over 10,000 hectares (39 sq mi) of the lower moors. The climate at the time was warmer than today, and much of today's moorland

6566-435: The lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes . The same process can form mud volcanoes on

6664-647: The major part of this, owned by the Duke of Cornwall . The Ministry of Defence owns 14% (see below), 3.8% is owned by water companies (see Dartmoor reservoirs ), 3.7% by the National Trust , 1.8% by the Forestry Commission and 1.4% by Dartmoor's national park authority . About 37% of Dartmoor is common land . Dartmoor differs from some other National Parks in England and Wales , in that since

6762-450: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. This further reduces porosity and makes the rock more compact and competent . Unroofing of buried sedimentary rock is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sedimentary rock, such as leaching of some of

6860-502: The moor. The most significant sites include: There are also an estimated 5,000 hut circles still surviving although many have been raided over the centuries by the builders of the traditional dry stone walls. These are the remnants of Bronze Age houses. The smallest are around 1.8 m (6 ft) in diameter, and the largest may be up to five times this size. Some have L-shaped porches to protect against wind and rain; some particularly good examples are to be found at Grimspound . It

6958-525: The new restrictions, led by local storyteller Martin Shaw, was held in January 2023. However, on 31 July the ban was overturned by the Court of Appeal. There is a history of military usage of Dartmoor dating back to the Napoleonic Wars . Today, a large British Army training camp remains at Okehampton — also the site of an airbase during the Second World War. The Ministry of Defence (MoD) uses three areas of

7056-448: The northern moor for manoeuvres and live-firing exercises, totalling 108.71 km (41.97 sq mi), or just over 11% of the National Park. Red and white posts mark the boundaries of these military areas (shown on Ordnance Survey 1:25,000 scale maps). Flagpoles on many tors in and around the ranges fly red flags when firing is taking place. At other times, members of the public are allowed access. Blank rounds may also be used, but

7154-450: The numerous small pools. Two shrimp species can be found on Dartmoor: fairy shrimp that can be found in temporary pools, and, in underground streams, very rare cave shrimp. The world's largest land slug, the Ash black , is also found. Reptiles include common lizards and adders . The farmland in the wet valleys around the edge of the moors is the most important habitat for insects, including:

7252-438: The place of deposition. The nature of a sedimentary rock, therefore, not only depends on the sediment supply, but also on the sedimentary depositional environment in which it formed. As sediments accumulate in a depositional environment, older sediments are buried by younger sediments, and they undergo diagenesis. Diagenesis includes all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by

7350-627: The processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, volcanism , and other minor processes. Clastic sedimentary rocks are composed of rock fragments ( clasts ) that have been cemented together. The clasts are commonly individual grains of quartz , feldspar , clay minerals , or mica . However, any type of mineral may be present. Clasts may also be lithic fragments composed of more than one mineral. Clastic sedimentary rocks are subdivided according to

7448-409: The reservoir at the main dam, and flows past the village of Meavy before joining the River Plym at the upper end of Bickleigh Vale . The entirety of the river is inside Dartmoor National Park. Most of the valley surrounding the river's southern section is wooded, and has been described as a semi-neutral ancient woodland. The north section, beyond Burrator Reservoir, is less covered as it is towards

7546-480: The result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots. In carbonate rocks such as limestone or chalk , chert or flint concretions are common, while terrestrial sandstones sometimes contain iron concretions. Calcite concretions in clay containing angular cavities or cracks are called septarian concretions . After deposition, physical processes can deform

7644-454: The rock a grey or greenish colour. Iron(III) oxide (Fe 2 O 3 ) in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. Thick sequences of red sedimentary rocks formed in arid climates are called red beds . However,

7742-469: The same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'. The form of the clasts can reflect the origin of the rock. For example, coquina , a rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters: Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. To describe such

7840-433: The sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar to tree rings ) are called varves . Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term laminite . When sedimentary rocks have no lamination at all, their structural character is called massive bedding. Graded bedding

7938-402: The sediment, producing a third class of secondary structures. Density contrasts between different sedimentary layers, such as between sand and clay, can result in flame structures or load casts , formed by inverted diapirism . While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer. Sometimes, density contrasts occur or are enhanced when one of

8036-443: The sediment. They can be indicators of circumstances after deposition. Some can be used as way up criteria . Organic materials in a sediment can leave more traces than just fossils. Preserved tracks and burrows are examples of trace fossils (also called ichnofossils). Such traces are relatively rare. Most trace fossils are burrows of molluscs or arthropods . This burrowing is called bioturbation by sedimentologists. It can be

8134-441: The southern moor are Ryder's Hill , 515 m (1,690 ft), ( grid reference SX 660690 ), Snowdon 495 m (1,624 ft), ( grid reference SX 668684 ), and an unnamed point, 493 m (1,617 ft) at ( grid reference SX 603645 ), between Langcombe Hill and Shell Top. The best-known tor on Dartmoor is Haytor (called Hey Tor by William Crossing), 457 m (1,499 ft), ( grid reference SX 757771 ). For

8232-464: The story of a man who was making his way through Aune Mire at the head of the River Avon when he came upon a top-hat brim down on the surface of the mire. He kicked it, whereupon a voice called out: "What be you a-doin' to my 'at?" The man replied, "Be there now a chap under'n?" "Ees, I reckon," was the reply, "and a hoss under me likewise." Dartmoor is popular with local astronomers because it

8330-453: The subsurface that is useful for civil engineering , for example in the construction of roads , houses , tunnels , canals or other structures. Sedimentary rocks are also important sources of natural resources including coal , fossil fuels , drinking water and ores . The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history , including palaeogeography , paleoclimatology and

8428-416: The surrounding lowlands. As much of the national park is covered in thick layers of peat (decaying vegetation), the rain is usually absorbed quickly and distributed slowly, so the moor is rarely dry. In areas where water accumulates, dangerous bogs or mires can result. Some of these, topped with bright green moss, are known to locals as "feather beds" or "quakers", because they can shift (or 'quake') beneath

8526-480: The surrounding sedimentary rocks during the Carboniferous period, probably about 309 million years ago. It is generally accepted that the present surface is not far below the original top of the pluton; evidence for this includes partly digested shale xenoliths , contamination of the granite and the existence of two patches of altered sedimentary rock on top of the granite. A considerable gravity anomaly

8624-527: The three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification . Dead organisms in nature are usually quickly removed by scavengers , bacteria , rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilisation. The chance of fossilisation

8722-531: The training disturbs the peace. Those who objected included the Open Spaces Society and the Dartmoor Preservation Association . During her lifetime, Sylvia Sayer was another outspoken critic of the damage that she perceived that the army was doing to the moor. The designated Dartmoor National Park area has a resident population of about 33,000, which swells considerably during holiday periods with incoming tourists. The largest settlements within

8820-438: The upland heaths heather ( ling ) and bell heather are common along with western gorse . In dry grassy areas tormentil , heath bedstraw and heath milkwort are all common. Cross-leaved heath and purple moor grass grow in wetter spots and in the boggy areas many different species of sphagnum and other mosses can be found, along with liverworts , Hare's-tail Cotton-grass , round-leaved sundew and bog asphodel , and in

8918-420: The valley bottoms, many different sedges , bogbean and pale butterwort all grow. A large variety of bird species can be found on Dartmoor including ones that have declined elsewhere in the UK, such as skylark and common snipe ; some are even rare nationally, such as the ring ouzel and the cuckoo . There are internationally important populations of meadow pipit and stonechat . Woodland birds include

9016-441: The volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks . Sedimentary rocks are deposited in layers as strata , forming a structure called bedding . Sedimentary rocks are often deposited in large structures called sedimentary basins . Sedimentary rocks have also been found on Mars . The study of sedimentary rocks and rock strata provides information about

9114-561: The wettest patches. The valley bogs have lush growth of rushes , with sphagnum, cross-leaved heath , sundews and several other species. Some of the bogs on Dartmoor have achieved notoriety. Fox Tor Mires was supposedly the inspiration for Great Grimpen Mire in Conan Doyle's novel The Hound of the Baskervilles , although there is a waymarked footpath across it. Sabine Baring-Gould , in his Book of Dartmoor (1900) related

9212-404: The woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils , examples of which are burrows , footprints , etc. As a part of a sedimentary rock, fossils undergo the same diagenetic processes as does

9310-409: Was an average of 20 days when snow fell on the moor, and over 40 days a year with hail, which is as high as anywhere else in the country. This results when cold polar maritime air that has travelled over a large expanse of warmer ocean is forced to rise over high country. When average temperatures at Princetown between 1961 and 2000 are compared, the average annual temperature in the decade 1990–2000

9408-401: Was covered with trees. The prehistoric settlers began clearing the forest, and established the first farming communities. Fire was the main method of clearing land, creating pasture and swidden types of fire-fallow farmland. Areas less suited for farming tended to be burned for livestock grazing. Over the centuries these Neolithic practices greatly expanded the upland moors, and contributed to

9506-474: Was largely unaffected by the Countryside and Rights of Way Act 2000 , which established similar rights in other rural parts of the country, until 2006, when the 2000 Act opened up much of the remaining restricted land to walkers. In January 2023, in a high court judgement in a case brought by landowner Alexander Darwall , the right for members of the public to wild camp on Dartmoor was lost. Julian Flaux ,

9604-438: Was up by 0.2 °C (0.4 °F) and the late winter temperature increased by 0.5 °C (0.9 °F). Because of Dartmoor's height and granite geology, it experiences strong winds and has acidic soils. In consequence it has been subject to very little intensive farming , and all these factors combine to form the basis of the important ecosystems found here. The landscape is one of granite with peat bogs overlying it. While

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