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41-656: South Foreland is a chalk headland on the Kent coast of southeast England . It presents a bold cliff to the sea, and commands views over the Strait of Dover . It is centred 3 miles (4.8 km) northeast of Dover and 15 miles south of North Foreland . It includes the closest point on the Island of Britain to the European mainland at a distance of 20.6 miles (33.2 km). This proximity gives it military significance and during

82-447: 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 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

123-429: 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

164-422: 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

205-468: 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

246-428: 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

287-642: 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

328-510: 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

369-469: 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 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

410-428: 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

451-518: 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,

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492-515: 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 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

533-415: 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

574-582: 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

615-577: 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

656-595: 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 the Cap Blanc Nez on

697-466: 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

738-767: The Second World War its coastal battery was built, along with a radar station. Lying between the busy Port of Dover and the remote and hazardous Goodwin Sands sandbank , its two lighthouses were important for navigation before their disuse. Much of the area is now owned by the National Trust and is open to the public; it is traversed by the Saxon Shore Way , the Kent coastal walk. LB&SCR H2 class 4-4-2 no. 421 (later no. B421, 2421, and 32421)

779-409: 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 is so common in Cretaceous marine beds that

820-732: 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

861-697: The Strait of Dover. During the Second World War South Foreland carried a Chain Home radar station similar to the still-extant towers at Swingate , east of Dover. This was a coastal artillery battery with four Mark X 9.2 inch guns and a network of bunkers and ammunition stores, northeast of the lighthouse on the road to St Margaret's. The site was cleared after World War II leaving traces, although since these have become heavily overgrown. The heavily reinforced positions were hewn into

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902-473: 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 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

943-619: The cliff in the three months from 28 December 1940. The first gun arrived on 25 March 1941. No. 4 gun was test fired on 28 November 1941. On 12 February 1942, the light battleships Gneisenau , Scharnhorst and the heavy cruiser Prinz Eugen attempted the Channel Dash from Brest, France to Germany. The Foreland's K-band radar started to track the ships of the Brest Group coming up the Channel towards Cap Gris Nez . At 12:19,

984-414: 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

1025-487: The first salvo was fired; maximum visibility was five miles, there was no observation of fall of shot by sight. The blips of the radar showed zig-zagging of the ships prompting full battery salvo firing. 33 rounds were fired at the German ships, which were moving out of range at 30 kn (35 mph; 56 km/h). The Germans revealed that all had missed. By the end of the war the four guns had fired 2,248 shells, most in

1066-450: 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

1107-422: 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

1148-512: 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 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 ,

1189-540: 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

1230-453: 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

1271-405: 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

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1312-651: 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 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

1353-528: 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

1394-481: The months before and after the Normandy landings . 28 enemy ships were sunk by the coastal batteries around Dover, which loss deterred use of the Channel Dash by the German surface fleet. 51°8′37″N 1°22′25″E  /  51.14361°N 1.37361°E  / 51.14361; 1.37361 Chalk Chalk is a soft, white, porous , sedimentary carbonate rock . It is a form of limestone composed of

1435-406: 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 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

1476-562: 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 —

1517-593: 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

1558-587: The northern extremity of the Boulonnais in the French département of Pas-de-Calais . The two are the landward ends of the fiercely cleft Strait of Dover land bridge and their chalk geological stratum dictates the route of the Channel Tunnel . Geologists have theorised that much of the erosion was river erosion from the extended Rhine and numerous southern North Sea channels discharging through

1599-667: 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

1640-436: 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

1681-401: Was named South Foreland after this landmark. Two lighthouses are on South Foreland: the lower light disused since 1910 and the upper light, a National Trust asset, disused since 1988. South Foreland marks the south-western limit of St Margaret's Bay (named after the village of St Margaret's at Cliffe ). It is the geological counterpart of Cap Blanc Nez ( lit. Cape White Nose), at

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