Chalk is a soft, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3, it forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells shed from micro-organisms called coccolithophores. Flint is common as bands parallel to the bedding or as nodules embedded in chalk, it is derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is deposited around larger fossils such as Echinoidea which may be silicified. Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have few obvious bedding planes unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones; this indicates stable conditions over tens of millions of years. Chalk has greater resistance to weathering and slumping than the clays with which it is associated, thus forming tall, steep cliffs where chalk ridges meet the sea.
Chalk hills, known as chalk downland form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water through dry seasons. Chalk is mined from chalk deposits both above underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks; some abandoned chalk mines remain tourist destinations due to their massive expanse and natural beauty. The Chalk Group is a European stratigraphic unit, it forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is 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 – both once formed a single island.
Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Chalk was one of the earliest rocks made up of microscopic particles to be studied under the microscope, when it was found to be composed entirely of coccoliths, their shells were made of calcite extracted from the rich seawater. As they died, a substantial layer built up over millions of years and, through the weight of overlying sediments became consolidated into rock. Earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level; the chemical composition of chalk is calcium carbonate, with minor amounts of clay. It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, made of mineral chalk, since it crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be erased.
Blackboard chalk manufacture now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum. While gypsum-based blackboard chalk is the lowest cost to produce, thus used in the developing world, calcium-based chalk can be made where the crumbling particles are larger and thus produce less dust, is marketed as "dustless chalk". Colored chalks, pastel chalks, sidewalk chalk, used to draw on sidewalks and driveways, are made of gypsum. Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. In southeast England, deneholes are a notable example of ancient chalk pits; such bell pits may 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 the most famous is the extensive complex at Grimes Graves in Norfolk. Woodworking joints may be fitted by chalking one of the mating surfaces. A trial fit will leave a chalk mark on the high spots of the corresponding surface.
Chalk transferring to cover the complete surface indicates a good fit. Builder's putty mainly contains chalk as a filler in linseed oil. Chalk may be used for its properties as a base. In agriculture, chalk is used for raising pH in soils with high acidity; the most common forms are CaCO3 and CaO. Small doses of chalk can be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for polishing. For example, toothpaste contains small amounts of chalk, which serves as a mild abrasive. Polishing chalk is chalk prepared with a controlled grain size, for fine polishing of metals. Chalk can be used as fingerprint powder. Several traditional uses of chalk have been replaced by other substances, although the word "chalk" is still applied to the usual replacements. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth by tailors, it is now made of talc. 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 p
The Needles is a row of three distinctive stacks of chalk that rise about 30m out of the sea off the western extremity of the Isle of Wight, United Kingdom, close to Alum Bay, part of Totland, the westernmost civil parish of the Isle of Wight. The Needles Lighthouse stands at the western end of the formation. Built in 1859, it has been automated since 1994; the waters and adjoining seabed form part of the Needles Marine Conservation Zone and the Needles along with the shore and heath above are part of the Headon Warren and West High Down Site of Special Scientific Interest. The formation takes its name from a fourth needle-shaped pillar called Lot's Wife, that collapsed in a storm in 1764; the remaining rocks are not at all needle-like. The Needles were featured on the BBC Two TV programme Seven Natural Wonders as one of the wonders of Southern England; the Needles lie just to the southwest of Alum Bay, are a tourist draw. Scenic boat trips operate from Alum Bay; the rocks and lighthouse have become icons of the Isle of Wight photographed by visitors, are featured on many of the souvenirs sold throughout the island.
The main tourist attractions of the headland itself are the two gun batteries, the experimental rocket testing station, the four Coastguard cottages owned by the National Trust. A branch of the National Coastwatch Institution is based at the Needles, sited near the New Battery and Rocket Testing Site on High Down; the Needles – Landmark Attraction situated at the top of the cliff at Alum Bay is a small amusement park. A Chairlift operates between the beach; the Needles were a site of a long-standing artillery battery, from the 1860s to 1954, decommissioned. A nearby site on High Down was employed in the testing of rockets for the British ICBM programme; the headland at High Down was used for Black Knight and Black Arrow rocket engine tests from 1956–71. During the peak of activity in the early 1960s some 240 people worked at the complex, while the rockets were built in nearby East Cowes; these rockets were used to launch the Prospero X-3 satellite. The site is now owned by the National Trust, is open to the public.
Concrete installations remain, but the buildings that were less durable have either been demolished or were torn down by the elements. In 1982, HRH Prince Charles opened the restored Needles Old Battery facility. Underground rocket testing rooms are being restored for exhibition; the first phase of restoration was completed in 2004. The batteries are accessible by car, foot and bus. Though there is a paved road up to The Old and The New batteries, access is on foot, from a car park; the battery site becomes dangerous in high winds and is closed to the public in winds above force 8. In the spring and summer, the Southern Vectis bus company sends open-top buses along a route called The Needles Breezer; this route approaches the Battery along the cliff edge. The Needles Breezer has stops in Alum Bay, Colwell Bay, Fort Victoria and Freshwater Bay. Breezer buses are the only vehicles allowed on the road from Alum Bay, apart from those owned by National Trust staff or, by prior appointment, vehicles transporting disabled visitors.
This is because the single track road's position close to the cliff edge is considered dangerous for multiple car use. The Isle of Wight Coast Path has its westernmost point at the Coastguard Cottages; the Needles' pointed shape is a result of their unusual geology. The strata have been so folded during the Alpine Orogeny that the chalk is near vertical; this chalk outcrop runs through the centre of the Island from Culver Cliff in the east to the Needles in the west, continues under the sea to the Isle of Purbeck, forming Ballard Cliff, Lulworth Cove and Durdle Door. At Old Harry Rocks these strata lines moving from horizontal to near vertical can be seen from the sea. Just off the end of the Needles formation is the Shingles, a shifting shoal of pebbles just beneath the waves; the Shingles is three miles in length. Many ships have been wrecked on the Shingles; some controversy has been raised about the actual shape of the Lot's Wife stone column, that collapsed in 1764. A drawing of The Needles by Dutch landscape artist Lambert Doomer, made in 1646, depicts a rock formation with much stouter shape than that shown in Isaac Taylor's 1759 "one inch" map of Hampshire.
The Doomer etching is contained in Atlas Blaeu-Van der Hem, in the Austrian National Library in Vienna. It is not clear from this drawings what transpired and whether Doomer was exercising artistic license. Doomer's painting shows three stacks when there should have been four, prior to the collapse of Lot's Wife. Palmerston Forts Palmerston Forts, Isle of Wight Needles Battery tourist website National Trust on The Needles Old Battery Page 1- The Needles, Steve Shafleet, pictures of the Needles, from "Alum Bay and the Needles", Isle of Wight Historic Postcards, 24 June 2007. Pictures of the Needles Rocket Test Site Video of Microlight flight over the needles
White Cliffs of Dover
The White Cliffs of Dover, part of the North Downs formation, is the name given to the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of 350 feet, owes its striking appearance to its composition of chalk accented by streaks of black flint; the cliffs, on both sides of the town of Dover in Kent, stretch for eight miles. A section of coastline encompassing the cliffs was purchased by the National Trust in 2016; the cliffs are part of the Dover to Kingsdown Cliffs Site of Special Scientific Interest and Special Area of Conservation. The cliffs are part of the coastline of Kent in England between 51°06′N 1°14′E and 51°12′N 1°24′E, at the point where Great Britain is closest to continental Europe. On a clear day they are visible from the French coast; the chalk cliffs of the Alabaster Coast of Normandy in France are part of the same geological system. The White Cliffs are at one end of the Kent Downs designated Area of Outstanding Natural Beauty.
In 1999 a sustainable National Trust visitor centre was built in the area. The Gateway building, designed by van Heyningen and Haward Architects, houses a restaurant, an information centre on the work of the National Trust, details of local archaeology and landscape.cheese spread About 70 million years ago Great Britain and much of Europe was submerged under a great sea. The sea bottom was covered with white mud formed from fragments of coccoliths, the skeletons of tiny algae that floated in the surface waters and sank to the bottom during the Cretaceous period and, together with the remains of bottom-living creatures, formed muddy sediments, it is thought that the sediments were deposited slowly half a millimetre a year, equivalent to about 180 coccoliths piled one on top of another. Up to 1,600 feet 500 metres of sediments were deposited in some areas; the weight of overlying sediments caused the deposits to become consolidated into chalk. Subsequent earth movements related to the formation of the Alps raised the sea-floor deposits above sea level.
Until the end of the last glacial period, the British Isles were part of continental Europe, linked by the unbroken Weald-Artois Anticline, a ridge that acted as a natural dam to hold back a large freshwater pro-glacial lake, now submerged under the North Sea. The land masses remained connected until between 450,000 and 180,000 years ago when at least two catastrophic glacial lake outburst floods breached the anticline and destroyed the ridge that connected Britain to Europe. A land connection across the southern North Sea existed intermittently at times when periods of glaciation resulted in lower sea levels. At the end of the last glacial period, around 10,000 years ago, rising sea levels severed the last land connection; the cliffs' chalk face shows horizontal bands of dark-coloured flint, composed of the remains of sea sponges and siliceous planktonic micro-organisms which hardened into the microscopic quartz crystals. Quartz silica filled cavities left by dead marine creatures which are found as flint fossils the internal moulds of Micraster echinoids.
Several different ocean floor species such as brachiopods, bivalves and sponges be can found in the chalk deposits, as can sharks' teeth. In some areas, layers of a soft, grey chalk known as a hardground complex can be seen. Hardgrounds are thought to reflect disruptions in the steady accumulation of sediment when sedimentation ceased and/or the loose surface sediments were stripped away by currents or slumping, exposing the older hardened chalk sediment. A single hardground may have been exhumed 16 or more times before the sediments were compacted and hardened to form chalk; the cliff face continues to weather at an average rate of 1 centimetre per year, although large pieces will fall. In 2001, a large chunk of the cliff edge, as large as a football pitch, fell into the Channel. Another large section collapsed on 15 March 2012; the chalk grassland environment above the cliffs provides an excellent environment for many species of wild flowers and birds, has been designated a Special Area of Conservation and a Site of Special Scientific Interest.
Rangers and volunteers work to clear invasive plants. A grazing programme involving Exmoor ponies has been established to help to clear faster-growing invasive plants, allowing smaller, less robust native plants to survive; the ponies are managed by the National Trust, Natural England, County Wildlife Trusts to maintain vegetation on nature reserves. The cliffs are the first landing point for many migratory birds flying inland from across the English Channel. After a 120-year absence, in 2009 it was reported. Similar in appearance but smaller, the jackdaw is abundant; the rarest of the birds that live along the cliffs is the peregrine falcon. In recent decline and endangered, the skylark makes its home on the cliffs; the cliffs are home to fulmars, which resemble gulls, to colonies of black-legged kittiwake, a species of gull. Bluebird, as mentioned in the classic World War II song " The White Cliffs of Dover" is an old country name for swallows and house martins, which make an annual migration to continental Europe, many of them crossing the English Channel at least twice a year.
Among the wildflowers are several varieties of orchids, the rarest of, the early spider orchid, which has yellow-green to brownish green petals and looks like the body of a large spider. The oxtongue broomrape is an unusual plant, it has yellow, white, or blue snapdragon-like flowers and about 90 per cent of the UK's population is found on the cliffs. V
The North Downs are a ridge of chalk hills in south east England that stretch from Farnham in Surrey to the White Cliffs of Dover in Kent. Westerham Heights, at the northern edge of the North Downs, near Bromley, South London, is the highest point in London at an elevation of 245 m; the North Downs lie within two Areas of Outstanding Natural Beauty, the Surrey Hills and the Kent Downs. The North Downs Way National Trail runs along the North Downs from Farnham to Dover.'Downs' is from Old English dun, amongst other things, "hill". The word acquired the sense of "elevated rolling grassland" around the 14th century; the name contains "North" to distinguish them from a similar range of hills – the South Downs – which runs parallel to them but some 50 km to the south. The narrow spine of the Hog's Back between Farnham and Guildford forms the western extremity of the North Downs, whilst the cliffs between Folkestone and Deal terminate the ridge in the east. There are two distinct aspects, the steep south-facing escarpment and the gentle north-facing dip slope.
The southern boundary is defined by the foot of the escarpment which gives way to the flat, broad clay lands between the Downs and Greensand Ridge known as the Vale of Holmesdale. The northern boundary is less apparent but occurs where the chalk submerges below the more recent Paleocene deposits; the Downs are highest near the Kent–Surrey border reaching heights in excess of 200 m above sea level at the crest of the escarpment. The highest point is Botley Hill in Surrey at 269 m; the County top of Kent at Betsom's Hill, with a height of 251 m is located nearby, the highest point in Greater London, Westerham Heights, at 245 m is on the northern side of the same hill. East of the Medway Valley the Downs become broader and flatter, extending as far as the Isle of Thanet; the ridge is intersected by the valleys of a series of rivers: the Wey, Darent and Stour rivers. These drain much of the Weald to the south; the western rivers are tributaries of the Thames. In addition to existing rivers, the Downs are crossed by a number of wind gaps – prehistoric river valleys no longer occupied by rivers – including those at Farnham, Caterham and Hawkinge.
Except for the river valleys and wind gaps, the crest of the escarpment is continuous along its length. The dip slope is dissected by many small dry valleys, in the broad eastern part in Kent, by further river valleys such as that of the Little Stour. Leith Hill is sometimes incorrectly referred to as part of the North Downs, but it is located on the parallel Greensand Ridge and does not consist of chalk; the Downland of the North Downs consists of distinct lithostratigraphic units: The more level tops of the Downs are covered by acidic strata including a layer of Clay-with-Flints, a sandy clay with many flints, or various sands and gravels. The Chalk Group, composed entirely of chalk, a kind of soft fine-grained limestone, it is formed of three parts, the Upper Chalk, which has many flints, the Middle Chalk, with fewer flints, the Lower Chalk or Coombe Rock, with few flints. The chalk is most exposed on slopes or as cliffs, where the overlying acidic strata have been quarried or washed away.
The buried upper surface of the chalk beneath the acidic strata is eroded into pipes and pinnacles, sometimes visible in road cuttings and quarries. The Upper Greensand Formation, a whitish, limy sandstone used for building, for which it has been mined from beneath the chalk; the Upper Greensand of the North Downs is a thin bed of one or two metres thickness, it is visible at the surface. The Upper Greensand marks the southern edge of the Downs, being underlain by: The Gault Formation of stiff blue clay; the Lower Greensand Formation of the Lower Cretaceous period, containing greensand, a glauconite sand or sandstone, as well as a certain amount of silts, clays and limestone. The topography of the North Downs consists of the Chalk Group, the rock strata of the Upper Cretaceous period which in certain areas is overlain by superficial deposits of gravels or clay-with-flints. Citing Dr D. T. Aldiss of the British Geological Survey: The Greensand Ridge is separate from the Downs. Again, one has to be aware of the distinction between'greensand' and'the Greensand', a lithostratigraphic term which refers to the Lower Greensand Group.
The Lower Greensand does contain some greensand, but much silt and limestone: most of it is neither green nor sand. It forms a distinct layer below the Gault Formation and the Upper Greensand Formation which directly underlie the Chalk Group. The'Greensand Ridge' refers to one of a series of escarpments formed by the Lower Greensand. In Surrey, the Upper Greensand is thin and is not separately marked by rising ground, but elsewhere it too forms an escarpment; these groups and formations each occur in separate layers. In Surrey these dip northwards at an angle of 2 degrees or less but increasing to as much as 55 degrees in the Hog's Back area, west of Guildford; the North Downs support several important habitats. The most distinctive of these is chalk grassland, limited to steep escarpment and valley slopes; this semi-natural habitat is maintained through sheep and rabbit grazing
St Boniface Down
St Boniface Down is a chalk down on the Isle of Wight, England. It is located close to the town of Ventnor, in the southeast of the Island, rises to 241 metres, the Island's highest point, 1 kilometre north of the town. There is reputed to be a wishing well on its southern slope, which requires the wisher to climb up from the south without looking back. In 1545 a French invasion force attempted this against a force of the Isle of Wight Militia commanded by Sir John Fyssher- which included several women archers- and were routed. In 1940 the radar station was bombed by Ju 87 Stuka dive bombers, reconstructed in the film "The Battle of Britain"; the top is surmounted by a round barrow. At the eastern foot of the down, on the A3055 road between Bonchurch and Luccombe, a path descends into Bonchurch Landslips via a scenic rock cleft, the Devil's Chimney. St Boniface Down is home to the largest cricket within the British Isles, the great green bush cricket; the area includes some unusual plant communities including acid grassland and heathland, resulting in parts of the Down being designated as a Site of Special Scientific Interest.
The gravel capping supports extensive tracts of gorse Ulex europaeus with intervening areas of heathland and acid grassland dominated by heather Calluna vulgaris, bell heather Erica cinerea, purple moor-grass Molinia caerulea, bristle bent Agrostis curtisii and locally bilberry Vaccinium myrtillus. The occurrence of heathland on deep gravel overlying chalk, the naturalised holm oak woodland and the juxtaposition of heath and chalkland vegetation are all unusual biological features in Britain. St Boniface Down is the name and was the inspiration of a 1956 work by the English composer, Trevor Duncan. 50.60346°N 1.19873°W / 50.60346.
In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, ice, air, plants and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind erosion, zoogenic erosion, anthropogenic erosion; the particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres. Natural rates of erosion are controlled by the action of geological weathering geomorphic drivers, such as rainfall; the rates at which such processes act control. Physical erosion proceeds fastest on steeply sloping surfaces, rates may be sensitive to some climatically-controlled properties including amounts of water supplied, wind speed, wave fetch, or atmospheric temperature.
Feedbacks are possible between rates of erosion and the amount of eroded material, carried by, for example, a river or glacier. Processes of erosion that produce sediment or solutes from a place contrast with those of deposition, which control the arrival and emplacement of material at a new location. While erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally. At well-known agriculture sites such as the Appalachian Mountains, intensive farming practices have caused erosion up to 100x the speed of the natural rate of erosion in the region. Excessive erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the eventual end result is desertification. Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses.
Water and wind erosion are the two primary causes of land degradation. Intensive agriculture, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall, the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, gully erosion. Splash erosion is seen as the first and least severe stage in the soil erosion process, followed by sheet erosion rill erosion and gully erosion. In splash erosion, the impact of a falling raindrop creates a small crater in the soil, ejecting soil particles; the distance these soil particles travel can be as much as 0.6 m vertically and 1.5 m horizontally on level ground. If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs.
If the runoff has sufficient flow energy, it will transport loosened soil particles down the slope. Sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are of the order of a few centimetres or less and along-channel slopes may be quite steep; this means that rills exhibit hydraulic physics different from water flowing through the deeper, wider channels of streams and rivers. Gully erosion occurs when runoff water accumulates and flows in narrow channels during or after heavy rains or melting snow, removing soil to a considerable depth. Valley or stream erosion occurs with continued water flow along a linear feature; the erosion is both downward, deepening the valley, headward, extending the valley into the hillside, creating head cuts and steep banks.
In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain; the stream gradient becomes nearly flat, lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone
Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz, metal oxides and organic matter. Geologic clay deposits are composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to particle size and geometry as well as water content, become hard and non–plastic upon drying or firing. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red. Although many occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, some overlap in particle size and other physical properties; the distinction between silt and clay varies by discipline. Geologists and soil scientists consider the separation to occur at a particle size of 2 µm, sedimentologists use 4–5 μm, colloid chemists use 1 μm.
Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits. ISO 14688 grades clay particles as being smaller than 2 silt particles as being larger. Mixtures of sand and less than 40% clay are called loam. Loam is used as a building material. Clay minerals form over long periods of time as a result of the gradual chemical weathering of rocks silicate-bearing, by low concentrations of carbonic acid and other diluted solvents; these solvents acidic, migrate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed through hydrothermal activity. There are two types of clay deposits: secondary. Primary clays remain at the site of formation. Secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. Clay deposits are associated with low energy depositional environments such as large lakes and marine basins.
Depending on the academic source, there are three or four main groups of clays: kaolinite, montmorillonite-smectite and chlorite. Chlorites are not always considered to be a clay, sometimes being classified as a separate group within the phyllosilicates. There are 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals. Varve is clay with visible annual layers, which are formed by seasonal deposition of those layers and are marked by differences in erosion and organic content; this type of deposit is common in former glacial lakes. When fine sediments are delivered into the calm waters of these glacial lake basins away from the shoreline, they settle to the lake bed; the resulting seasonal layering is preserved in an distribution of clay sediment banding. Quick clay is a unique type of marine clay indigenous to the glaciated terrains of Norway, Northern Ireland, Sweden, it is a sensitive clay, prone to liquefaction, involved in several deadly landslides.
Powder X-ray diffraction can be used to identify clays. The physical and reactive chemical properties can be used to help elucidate the composition of clays. Clays exhibit plasticity. However, when dry, clay becomes firm and when fired in a kiln, permanent physical and chemical changes occur; these changes convert the clay into a ceramic material. Because of these properties, clay is used for making pottery, both utilitarian and decorative, construction products, such as bricks and floor tiles. Different types of clay, when used with different minerals and firing conditions, are used to produce earthenware and porcelain. Prehistoric humans discovered the useful properties of clay; some of the earliest pottery shards recovered are from Japan. They are associated with the Jōmon culture and deposits they were recovered from have been dated to around 14,000 BC. Clay tablets were the first known writing medium. Scribes wrote by inscribing them with cuneiform script using a blunt reed called a stylus. Purpose-made clay balls were used as sling ammunition.
Clays sintered in fire were the first form of ceramic. Bricks, cooking pots, art objects, smoking pipes, musical instruments such as the ocarina can all be shaped from clay before being fired. Clay is used in many industrial processes, such as paper making, cement production, chemical filtering; until the late 20th century, bentonite clay was used as a mold binder in the manufacture of sand castings. Clay, being impermeable to water, is used where natural seals are needed, such as in the cores of dams, or as a barrier in landfills against toxic seepage. Studies in the early 21st century have investigated clay's absorption capacities in various applications, such as the removal of heavy metals from waste water and air purification. Traditional uses of clay as medicine goes back to prehistoric times. An example is Armenian bole, used to soothe an upset stomach; some animals such as parrots and pigs ingest clay for similar reasons. Kaolin clay and attapulgite have been used as anti-diarrheal medicines.
Clay as the defining ingredient of loam is one of the oldest building materials on Earth, among other