chalk

Soil developed on a bed of chalk. Soils develop as a result of weathering of rocks, breakdown of vegetation, and the effect of water. Chalk is an unusual rock. When it is weathered the chalk is changed to calcium bicarbonate. This is soluble and removed by running water. As a result, chalk normally does not have much material on the top of it. Chalk soils consist of a thin top soil composed of decayed vegetation.

Waves on the Isle of Purbeck, Dorset, pound away at the coast, and as the chalk is worn away steep cliffs are formed. The lack of rubble at the base of the chalk is due to its removal by high-energy waves. This area of coastline was initially eroded because these were low-lying valleys; a remnant of the valley is seen here, as a low point on the chalk valley. Today it is a dry valley - a valley without a river - and is known as Scratchey Bottom.

Soft, fine-grained, whitish sedimentary rock composed of calcium carbonate, CaCO₃, extensively quarried for use in cement, lime, and mortar, and in the manufacture of cosmetics and toothpaste. Blackboard chalk in fact consists of gypsum (calcium sulphate, CaSO₄.2H₂O).

Chalk was once thought to derive from the remains of microscopic animals or foraminifera. In 1953, however, it was seen under the electron microscope to be composed chiefly of coccolithophores, unicellular lime-secreting algae, and hence primarily of plant origin. It is formed from deposits of deep-sea sediments called oozes.