Soft, fine-grained, easily pulverized, white-to-grayish variety of limestone, composed of the shells of minute marine organisms. The purest varieties contain up to 99percnt calcium carbonate in the form of the mineral calcite. Extensive deposits occur in western Europe south of Sweden and in England, notably in the chalk cliffs of Dover along the English Channel. Other extensive deposits occur in the U.S. from South Dakota to Texas and eastward to Alabama. Chalk is used for making lime and portland cement and as a soil additive. Finely ground and purified chalk is known as whiting and is used as a filler, extender, or pigment in a wide variety of materials, including ceramics, putty, cosmetics, crayons, plastics, rubber, paper, paints, and linoleum. The chalk commonly used in classrooms is a manufactured substance rather than natural chalk.
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Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find flint and chert nodules embedded in chalk.
Chalk is relatively resistant to erosion and slumping compared to the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.
The Chalk Formation is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage.