Chalk, soft white or whitish form of limestone, composed of the remains of small marine organisms such as foraminifera and coccolithophores; it formed after the shells and skeletons of these organisms were deposited in a thick layer on the sea bottom. Chemically, chalk is almost pure calcium carbonate with traces of other minerals. It ranges in hardness and texture from very soft porous varieties to harder close-grained types. Chalk is particularly common in strata of the Cretaceous period (Latin creta,”chalk”). Large deposits are found in Iowa, Texas, and Arkansas in the United States, and in the British Isles. Cretaceous chalk is exposed in the White Cliffs of Dover on either side of the English Channel.

See also Calcite; Limestone.