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35-683: Epsom Downs is an area of chalk upland near Epsom , Surrey ; in the North Downs . Part of the area is taken up by the racecourse , the gallops are part of the land purchased by Stanly Wootton in 1925 and are open to users such as ramblers , model aircraft flyers, golfers and cyclists . Since January 2006 model aircraft flyers on the Downs have been required to be members of the Epsom Downs Model Aircraft Club. There are over 20 km of routeways for hack riders. There are bylaws for

70-449: A lacustrine environment or in a restricted marine environment, although not all rifts contain such sequences. Reservoir rocks may be developed in pre-rift, syn-rift and post-rift sequences. Effective regional seals may be present within the post-rift sequence if mudstones or evaporites are deposited. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under

105-492: A 20% loss in that period and an assessment of chalk grassland in Dorset found that over 50% had been lost between the mid-1950s and the early 1990s. Much remaining chalk downland has been protected against future development to preserve its unique biodiversity . Rift In geology , a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics . Typical rift features are

140-431: A central linear downfaulted depression, called a graben , or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley , which may be filled by water forming a rift lake . The axis of the rift area may contain volcanic rocks , and active volcanism is a part of many, but not all, active rift systems. Major rifts occur along

175-462: A high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of the same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect

210-403: A kind of orogeneses in extensional settings, which is referred as to rifting orogeny. Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence. The amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor (initial crustal thickness divided by final crustal thickness), but

245-495: A matrix of micrite mud. Small amounts of silica were also deposited, mainly from sponge spicules , which moved during diagenesis and accumulated to form flints . The Chalk Group either directly overlies the impermeable uppermost Lower Cretaceous Gault Clay or permeable Upper Greensand Formation above the Gault Clay. Since its deposition, the chalk in southern England has been uplifted, faulted , fractured and folded by

280-438: A mid-oceanic ridge and a set of conjugate margins separated by an oceanic basin. Rifting may be active, and controlled by mantle convection . It may also be passive, and driven by far-field tectonic forces that stretch the lithosphere. Margin architecture develops due to spatial and temporal relationships between extensional deformation phases. Margin segmentation eventually leads to the formation of rift domains with variations of

315-426: A ribbed pattern of grass covered horizontal steps a foot or two high. Although subsequently emphasised by cattle and sheep walking along them, these terracettes (commonly known as sheep tracks) were formed by the movement of soil downhill, a process known as soil creep . The dominant habitat in chalk downland is typically calcareous grassland , formed by grazing from both livestock and wild animals. Chalk downland

350-401: A shallow dark humus rich surface layer which grades through a lighter brown hillwash containing small pellets of chalk, to the white of the chalk itself. This is largely because of the purity of the chalk, which is about 98% calcium carbonate , and the consequent absence of soil-building clay minerals which are abundant, for example, in valley floors. Steep slopes on chalk downland develop

385-402: Is also affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water. The simple 'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence. This has generally been replaced by

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420-487: Is often unsuitable for intensive agriculture , horticulture , or development because of the nutrient-poor, shallow soil and difficult slopes. For this reason downland often survived uncultivated when other, more easily worked land was ploughed or reseeded. This shallow soil structure makes downland ecosystems extremely fragile and easy to destroy. With modern machinery and fertilising techniques, it has become possible to use some previously uncultivated downland for farming, and

455-811: Is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from the upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30 °C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to

490-659: The Hampshire Downs onto the Isle of Wight . To the northeast, downlands continue along the Berkshire Downs and Chiltern Hills through parts of Berkshire , Oxfordshire , Buckinghamshire , Hertfordshire , Bedfordshire and into Cambridgeshire . To the east downlands are found north of the Weald in Surrey , Kent and part of Greater London , forming the North Downs . To the southeast

525-423: The Moho topography, including proximal domain with fault-rotated crustal blocks, necking zone with thinning of crustal basement , distal domain with deep sag basins, ocean-continent transition and oceanic domain. Deformation and magmatism interact during rift evolution. Magma-rich and magma-poor rifted margins may be formed. Magma-rich margins include major volcanic features. Globally, volcanic margins represent

560-532: The Vale of White Horse . In many chalk downland areas there is no surface water at all other than artificially created dewponds . The soil profile of chalk downland in England is a thin soil overlaying the parent chalk . Weathering of the chalk has created a characteristic soil known as rendzina . Unlike many soils in which there are easily distinguished layers or soil horizons , a chalk rendzina soil consists of only

595-645: The 'flexural cantilever model', which takes into account the geometry of the rift faults and the flexural isostasy of the upper part of the crust. Some rifts show a complex and prolonged history of rifting, with several distinct phases. The North Sea rift shows evidence of several separate rift phases from the Permian through to the Earliest Cretaceous , a period of over 100 million years. Rifting may lead to continental breakup and formation of oceanic basins. Successful rifting leads to seafloor spreading along

630-454: The Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in some cases the polarity (the dip direction), of the main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at

665-535: The Downs and the cost of this is split in the same proportion as the representation. The Conservators were set up in 1984 to give a voice to the users of the Downs. Chalk downland The largest area of downland in southern England is formed by Salisbury Plain , mainly in Wiltshire . To the southwest, downlands continue via Cranborne Chase into Dorset as the Dorset Downs and southwards through Hampshire as

700-422: The central axis of most mid-ocean ridges , where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates . Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot . Two of these evolve to

735-503: The characteristic ridges of the downland landscape. The landscape was further modified during the Quaternary period by the area's proximity to the southern edge of the ice sheets formed during the last ice age . These periglacial effects included significant amounts of dissolution of the chalk and the modification of existing valleys due to a combination of frozen ground and snowmelt . Downland develops when chalk rock becomes exposed at

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770-441: The decline of extensive grazing has meant that many areas of downland, neither cultivated nor grazed, revert to scrub or other less rare habitat, essentially destroying the delicate calcareous grassland. The UK cover of lowland calcareous grassland has suffered a sharp decline in extent since the middle of the twentieth century. There are no comprehensive figures, but a sample of chalk sites in England surveyed in 1966 and 1980 showed

805-705: The distant effects of the Alpine Orogeny . The fracturing has greatly increased the chalk's permeability, such that it is a major aquifer . Sedimentary basins formed by rifting during the Triassic to Early Cretaceous were inverted during the Late Paleogene to Miocene leading to the formation of structures such as the Wealden Anticline and the Portland-Wight Monocline . Later erosion has produced

840-636: The downlands continue into West Sussex and East Sussex as the South Downs . Similar chalk hills are also found further north in Lincolnshire and Yorkshire where they are known as the Wolds . The Chalk Group is a sequence of Upper Cretaceous limestones . The dominant lithology is relatively soft porous white chalk with only poorly-defined bedding. The chalk is classified as a biomicrite , with microscopic coccoliths and other fine-grained fossil debris in

875-476: The downs at the base of the chalk layer, greensand or gault clay comes to the surface and at the interface at the top of the gault a springline can occur where water emerges from the porous chalk or the underlying greensand. Along this line, settlements and farms were often built, as on the higher land no water was available. This is demonstrated very clearly beneath the scarp of the White Horse Hills , above

910-401: The height of the water table in chalk hills rises in winter and falls in summer. This leads to characteristic chalk downland features such as dry valleys or coombes , and seasonally-flowing streams or winterbournes . The practice of extracting water from this aquifer, in order to satisfy the increasing demand for water, may be putting some of these streams under stress. In the valleys below

945-402: The larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along the rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins. During the climax of lithospheric rifting, as the crust

980-567: The majority of passive continental margins. Magma-starved rifted margins are affected by large-scale faulting and crustal hyperextension. As a consequence, upper mantle peridotites and gabbros are commonly exposed and serpentinized along extensional detachments at the seafloor. Many rifts are the sites of at least minor magmatic activity , particularly in the early stages of rifting. Alkali basalts and bimodal volcanism are common products of rift-related magmatism. Recent studies indicate that post-collisional granites in collisional orogens are

1015-411: The onset of rifting, the upper part of the lithosphere starts to extend on a series of initially unconnected normal faults , leading to the development of isolated basins. In subaerial rifts, for example, drainage at the onset of rifting is generally internal, with no element of through drainage. As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form

1050-456: The point of seafloor spreading, while the third ultimately fails, becoming an aulacogen . Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by a single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as

1085-404: The product of rifting magmatism at converged plate margins. The sedimentary rocks associated with continental rifts host important deposits of both minerals and hydrocarbons . SedEx mineral deposits are found mainly in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed. Continental rifts are

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1120-838: The rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift. Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are typically about 70 km wide. Contrary to what was previously thought, elevated passive continental margins (EPCM) such as the Brazilian Highlands , the Scandinavian Mountains and India's Western Ghats , are not rift shoulders. The formation of rift basins and strain localization reflects rift maturity. At

1155-510: The sites of significant oil and gas accumulations, such as the Viking Graben and the Gulf of Suez Rift . Thirty percent of giant oil and gas fields are found within such a setting. In 1999 it was estimated that there were 200 billion barrels of recoverable oil reserves hosted in rifts. Source rocks are often developed within the sediments filling the active rift ( syn-rift ), forming either in

1190-580: The surface. The chalk slowly erodes to form characteristic rolling hills and valleys. As the Cretaceous chalk layer in southern England is typically tilted, chalk downland hills often have a marked scarp slope on one side, which is very steep, and a much gentler dip slope on the other. Where the downs meet the sea, characteristic white chalk cliffs form, such as the White Cliffs of Dover and Beachy Head . Chalk deposits are generally very permeable, so

1225-540: The use of the Downs. There are panoramic views of London to the north from the Downs. The area is served by Epsom Downs and Tattenham Corner railway stations. Epsom Downs is managed by the Epsom and Walton Downs Conservators , which consists of six borough councillors, three representatives of the Epsom Downs Racecourse and one from the racehorse trainers. They are responsible for the maintenance and control of

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