A chalk is a soft, friable variety of limestone consisting of poorly lithified calcareous ooze, produced by the accumulation of planktonic organisms in a pelagic (open sea) environment. Chalk is largely made of shells of single-celled marine organisms, such as foraminifera and coccoliths, but fragments of shells of bivalves and ostracods might also be present. Organisms with siliceous shells, such as radiolarians or diatoms, might be present within chalk deposits in important quantities, thus producing accumulations of siliceous ooze that might form nodules of cherts during diagenesis. This rock can be classified as a mudstone, according to Dunham classification, and as a micrite, according to Folk classification. Chalks of Cretaceous age are present at planetary scale thanks to the explosion of nannoplancton in the seas that characterized the Cretaceous. For example, the white cliffs of Dover in England, as well as their counterparts in France, Germany, and Denmark, formed during this period.

Distinguishing features
Chalk is soft, porous, poorly lithified limestone. The presence of pores make the reaction with HCl particularly strong. An insoluble clay-rich residuum might form after the reaction with HCl, indicating marly compositions. Chalk often occurs associated with layers or nodules of cherts – that are easily recognizable because they are harder than metal on scratch, show choncoidal fracture, and are more resistant to erosion than chalk.