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41-629: Blue Bell Hill is a chalk hill between Maidstone and Rochester in the English county of Kent . It overlooks the River Medway and is part of the North Downs . Settlements on the hill include the Walderslade suburb of Chatham and the villages of Blue Bell Hill and Kit's Coty . During the eighteenth and nineteenth centuries much of the hill was quarried for chalk. The hill is

82-532: A 5-hectare (12-acre) nature reserve which is managed by Kent Wildlife Trust . It is also part of the Kent Downs Area of Outstanding Natural Beauty and Wouldham to Detling Escarpment Sites of Special Scientific Interest and is a Nature Conservation Review site, Grade I. A picnic area serves as a rest point for walkers on the North Downs Way which runs along the top of the hill, whilst

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

328-579: 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 the Dover cliffs on the Kent coast of the English Channel . Chalk

369-510: 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

410-508: 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

451-428: 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

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492-518: 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,

533-494: 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

574-415: 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

615-464: 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 is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity. It

656-582: 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

697-510: Is on the hill, as is the Lower Bell pub. The Upper Bell pub was closed around March 2005 due to the reduced demand caused by the closure of a slip-lane onto the southbound A229 , and then demolished in 2013. A famous feature of the hill is the eastern group of the prehistoric tombs called the Medway megaliths including Kit's Coty House and Countless Stones . The Blue Bell Hill Dolmen

738-554: 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

779-577: 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

820-401: 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

861-752: 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

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902-466: 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

943-732: 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

984-548: 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

1025-494: 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

1066-437: 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

1107-414: 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

1148-450: 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

1189-422: 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

1230-540: 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

1271-453: 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

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1312-415: The larger ones and beneath the standing stones was a large circular pit dug into the natural chalk and filled with many flints . Local people told the investigating antiquarians that many such pits had been found on the hill and that the flints were used as a source of stone to metal new roads. From these fragments it is thought that one, or possibly more, chamber tombs stood on Blue Bell Hill in addition to

1353-405: 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

1394-528: 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

1435-562: 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 —

1476-593: 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

1517-619: The prehistoric trackway of the Pilgrims' Way skirts its foot. A modern crematorium also surmounts the hill. The A229 dual carriageway follows the route of a former Roman road and climbs the hill, today linking the M2 and M20 motorways . High Speed 1 runs beneath the hill, via the North Downs Tunnel , and archaeological work in advance of it uncovered a Neolithic long house on its slopes. The Bluebell Hill transmitting station

1558-436: 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

1599-437: The surviving Medway Megaliths. Remains of a Roman building containing human remains and traces of burnt wood had been excavated near to Kit's Coty. They are claimed to be the ruins of a Roman temple to have been built on the hill later. 51°20′0″N 0°30′0″E  /  51.33333°N 0.50000°E  / 51.33333; 0.50000 Chalk Chalk is a soft, white, porous , sedimentary carbonate rock . It

1640-476: Was a now lost member of the group of Neolithic chamber tombs in the English county of Kent . It is thought to have been one of the Medway Megaliths . Its precise location is unclear but it stood on Blue Bell Hill on the North Downs between Maidstone and Rochester , to the north of Kit's Coty House . Only fragments of antiquarians ' records now remain. It was possibly investigated in 1844 and

1681-503: Was still extant in the early twentieth century. A sketch in Maidstone Museum indicates that three sarsen standing stones survived to heights of 7 feet forming the walls of the burial chamber. A stone that may have formed the capstone lay between them. The tomb was found to have contained the skeleton of a man and fragments of red pottery were found although none of these has since survived. A kerb of smaller stones surrounded

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