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39-608: The Berkshire Downs are a range of chalk downland hills in southern England , part of the North Wessex Downs Area of Outstanding Natural Beauty . The Berkshire Downs are wholly within the traditional county of Berkshire , although split between the current ceremonial counties of Berkshire and Oxfordshire . The western parts of the downs are also known as the Lambourn Downs . The Berkshire Downs run east–west, with their scarp slope facing north into

78-429: A 3 by 3 tensor. The tensor is realised using a 3 by 3 matrix being both symmetric and positive definite (SPD matrix): The permeability tensor is always diagonalizable (being both symmetric and positive definite). The eigenvectors will yield the principal directions of flow where flow is parallel to the pressure gradient, and the eigenvalues represent the principal permeabilities. These values do not depend on

117-422: A few are more recent. A mixture of chalk and mercury can be used as fingerprint powder . However, because of the toxicity of the mercury, the use of such mixtures for fingerprinting was abandoned in 1967. Permeability (Earth sciences) Permeability is a property of porous materials that is an indication of the ability for fluids (gas or liquid) to flow through them. Fluids can more easily flow through

156-468: A fraction to several thousand millidarcys. The unit of square centimetre (cm ) is also sometimes used (1 cm = 10 m ≈ 10 d). The concept of permeability is of importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs, and of groundwater in aquifers . For a rock to be considered as an exploitable hydrocarbon reservoir without stimulation, its permeability must be greater than approximately 100 md (depending on

195-428: A material with high permeability than one with low permeability. The permeability of a medium is related to the porosity , but also to the shapes of the pores in the medium and their level of connectedness. Fluid flows can also be influenced in different lithological settings by brittle deformation of rocks in fault zones ; the mechanisms by which this occurs are the subject of fault zone hydrogeology . Permeability

234-525: A number of poems and sonnets. Downland pasture is firm and well drained, suited to grazing sheep and grazing and training horses. Horse racing is a major business in the area, with much of the downs covered with training areas, and stables centred on the village of Lambourn . The Berkshire Downs can be accessed from various cities via the Great Western Main Line and its current single operator runs localised stopping trains as well as

273-642: Is mined from chalk deposits both above ground and underground . Chalk mining boomed during the Industrial Revolution , due to the need for chalk products such as quicklime and bricks . Most people first encounter chalk in school where it refers to blackboard chalk , which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or

312-610: Is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity. It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton , such as foraminifera or coccolithophores . These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as molluscs , echinoderms , or bryozoans . Chalk

351-404: Is a soft, white, porous , sedimentary carbonate rock . It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor . Chalk is common throughout Western Europe , where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as

390-510: Is also affected by the pressure inside a material. The SI unit for permeability is the square metre (m ). A practical unit for permeability is the darcy (d), or more commonly the millidarcy (md) (1 d ≈ 10 m ). The name honors the French Engineer Henry Darcy who first described the flow of water through sand filters for potable water supply. Permeability values for most materials commonly range typically from

429-508: Is also the only form of limestone that commonly shows signs of compaction. Flint (a type of chert ) is very common as bands parallel to the bedding or as nodules in seams , or linings to fractures , embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint). Chalk

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468-428: Is applied to the hands and feet to remove perspiration and reduce slipping. Chalk may also be used as a house construction material instead of brick or wattle and daub : quarried chalk was cut into blocks and used as ashlar , or loose chalk was rammed into blocks and laid in mortar. There are still houses standing which have been constructed using chalk as the main building material. Most are pre- Victorian though

507-518: Is commonly used as a drying agent to obtain better grip by gymnasts and rock climbers. Glazing putty mainly contains chalk as a filler in linseed oil . Chalk and other forms of limestone may be used for their properties as a base . Chalk is a source of quicklime by thermal decomposition , or slaked lime following quenching of quicklime with water. In agriculture , chalk is used for raising pH in soils with high acidity . Small doses of chalk can also be used as an antacid . Additionally,

546-708: Is generally thought that in Anglo-Saxon times the downs were known as Æscesdūn or Ashdown, and that it was here that the Battle of Ashdown was fought in 871. In 1915, after a brief stint as a hospital orderly at the British hospital for French soldiers in Haute-Marne , John Masefield moved to his country retreat at Lollingdon Farm . The setting at the foot of the Downs - Masefield's "Lollingdon Downs" would inspire

585-415: Is important in petroleum engineering , when considering the optimal extraction of gas from unconventional sources such as shale gas , tight gas , or coalbed methane . To model permeability in anisotropic media, a permeability tensor is needed. Pressure can be applied in three directions, and for each direction, permeability can be measured (via Darcy's law in 3D) in three directions, thus leading to

624-582: Is now usually made of talc (magnesium silicate). Chalk beds form important petroleum reservoirs in the North Sea and along the Gulf Coast of North America. In southeast England, deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but perhaps

663-604: Is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits. The name Cretaceous was derived from Latin creta , meaning chalk . Some deposits of chalk were formed after the Cretaceous. The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent , England, as well as their counterparts of

702-577: Is the scalar hydraulic permeability, and 1 is the identity tensor . Permeability is typically determined in the lab by application of Darcy's law under steady state conditions or, more generally, by application of various solutions to the diffusion equation for unsteady flow conditions. Permeability needs to be measured, either directly (using Darcy's law), or through estimation using empirically derived formulas. However, for some simple models of porous media, permeability can be calculated (e.g., random close packing of identical spheres ). Based on

741-401: Is typically almost pure calcite, CaCO 3 , with just 2% to 4% of other minerals. These are usually quartz and clay minerals , though collophane (cryptocrystalline apatite , a phosphate mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted to dolomite , CaMg(CO 3 ) 2 , and in a few cases

780-752: The Cap Blanc Nez on the other side of the Dover Strait . The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage . Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark . Chalk deposits are also found in Cretaceous beds on other continents, such as the Austin Chalk , Selma Group , and Niobrara Formations of

819-536: The Dover cliffs on the Kent coast of the English Channel . Chalk is mined for use in industry, such as for quicklime , bricks and builder's putty , and in agriculture , for raising pH in soils with high acidity . It is also used for " blackboard chalk " for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, or gypsum . Chalk

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858-466: The Hagen–Poiseuille equation for viscous flow in a pipe, permeability can be expressed as: where: Absolute permeability denotes the permeability in a porous medium that is 100% saturated with a single-phase fluid. This may also be called the intrinsic permeability or specific permeability. These terms refer to the quality that the permeability value in question is an intensive property of

897-834: The Vale of White Horse and their dip slope bounded by the course of the River Kennet . Geologically they are continuous with the Marlborough Downs to the west and the Chilterns to the east. In the east they are divided from the Chilterns by Goring Gap on the River Thames . In the west their boundary is generally taken to be the border between Berkshire and Wiltshire , although the downs in Wiltshire between

936-783: The Berkshire border and the valley of the River Og are sometimes considered to be part of the Berkshire Downs. English downland has attracted human habitation since prehistoric times. The ancient track known as the Ridgeway runs along the Berkshire Downs. Prehistoric sites in the Downs include Wayland's Smithy ( Neolithic ), numerous tumuli (Neolithic or Bronze Age ), Uffington White Horse (Bronze Age), Liddington Castle and Uffington Castle (Bronze Age and Iron Age ), and Segsbury Camp and Grim's Ditch (Iron Age). It

975-732: The North American interior. Chalk is also found in western Egypt (Khoman Formation) and western Australia ( Miria Formation ). Chalk of Oligocene to Neogene age has been found in drill cores of rock under the Pacific Ocean at Stewart Arch in the Solomon Islands . There are layers of chalk, containing Globorotalia , in the Nicosia Formation of Cyprus , which formed during the Pliocene . Chalk

1014-548: The chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, resulting in the early cementation of such limestones. In chalk, absence of this calcium carbonate conversion process prevented early cementation, which partially accounts for chalk's high porosity. Chalk

1053-494: The dolomitized chalk has been dedolomitized back to calcite. Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent. While it is similar in appearance to both gypsum and diatomite , chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact). In Western Europe, chalk was formed in the Late Cretaceous Epoch and

1092-437: The early Palaeocene Epoch (between 100 and 61 million years ago). It was deposited on extensive continental shelves at depths between 100 and 600 metres (330 and 1,970 ft), during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which

1131-414: The effect of temperature on the viscosity of the fluid flowing though the porous medium and to address other fluids than pure water, e.g. , concentrated brines , petroleum , or organic solvents . Given the value of hydraulic conductivity for a studied system, the permeability can be calculated as follows: Tissue such as brain, liver, muscle, etc can be treated as a heterogeneous porous medium. Describing

1170-450: The flow of biofluids (blood, cerebrospinal fluid, etc.) within such a medium requires a full 3-dimensional anisotropic treatment of the tissue. In this case the scalar hydraulic permeability is replaced with the hydraulic permeability tensor so that Darcy's Law reads Connecting this expression to the isotropic case, κ = k 1 {\displaystyle {\boldsymbol {\kappa }}=k\mathbb {1} } , where k

1209-422: The flow of water through a porous medium is called the hydraulic conductivity ( K , unit: m/s). Permeability, or intrinsic permeability, ( k , unit: m ) is a part of this, and is a specific property characteristic of the solid skeleton and the microstructure of the porous medium itself, independently of the nature and properties of the fluid flowing through the pores of the medium. This allows to take into account

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1248-540: The ground conditions of a site are suitable for construction. Permeability is part of the proportionality constant in Darcy's law which relates discharge (flow rate) and fluid physical properties (e.g. dynamic viscosity ), to a pressure gradient applied to the porous media: Therefore: where: In naturally occurring materials, the permeability values range over many orders of magnitude (see table below for an example of this range). The global proportionality constant for

1287-734: The high-speed trains along the Vale of White Horse calling at major stops Swindon and Didcot Parkway . From Reading to Newbury trains run along the Reading to Taunton Line in the River Kennet Valley to reach Devon on the quickest route from London. From Reading there are the scenic Thames Valley stations of Pangbourne , Goring & Streatley and Cholsey (linked to the Cholsey and Wallingford Railway ). 51°34′30″N 1°34′12″W  /  51.575°N 1.570°W  / 51.575; -1.570 Chalk Chalk

1326-453: The interface with the solid when the gas mean free path is comparable to the pore size (about 0.01 to 0.1 μm at standard temperature and pressure). See also Knudsen diffusion and constrictivity . For example, measurement of permeability through sandstones and shales yielded values from 9.0×10 m to 2.4×10  m for water and between 1.7×10  m to 2.6×10  m for nitrogen gas. Gas permeability of reservoir rock and source rock

1365-405: The medium, not a spatial average of a heterogeneous block of material equation 2.28 ; and that it is a function of the material structure only (and not of the fluid). They explicitly distinguish the value from that of relative permeability . Sometimes permeability to gases can be somewhat different than those for liquids in the same media. One difference is attributable to "slippage" of gas at

1404-528: The mineral gypsum ( calcium sulfate ). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world , use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as "dustless chalk". Coloured chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks and often coloured), used to draw on sidewalks , streets, and driveways , are primarily made of gypsum rather than calcium carbonate chalk. Magnesium carbonate chalk

1443-562: The most famous is the extensive complex at Grimes Graves in Norfolk . Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court. If a ball hits the line, a cloud of chalk or pigment dust will be visible. In recent years, powdered chalk has been replaced with titanium dioxide . In gymnastics, rock-climbing, weightlifting and tug of war , chalk — now usually magnesium carbonate —

1482-593: The nature of the hydrocarbon – gas reservoirs with lower permeabilities are still exploitable because of the lower viscosity of gas with respect to oil). Rocks with permeabilities significantly lower than 100 md can form efficient seals (see petroleum geology ). Unconsolidated sands may have permeabilities of over 5000 md. The concept also has many practical applications outside of geology, for example in chemical engineering (e.g., filtration ), as well as in Civil Engineering when determining whether

1521-436: The small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mild abrasive . Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals. French chalk (also known as tailor's chalk) is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors . It

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