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Geology is the Way


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.

Sample of chalk from the Upper Chalk formation (Upper Cretaceous, Cliffs of Dover, England). Photo by James St. John.
Outcrop of chalk (Smoky Hill Chalk Member, Niobrara Formation, Upper Cretaceous of Kansas, U.S.A.). Photo by James St. John.
Badlands of eroded chalk, containing abundant microfossils of coccoliths (Niobrara Formation, Upper Cretaceous of Kansas, U.S.A.). Photo by James St. John.
The white cliffs of Etretat (Normandy, France) consist of white chalk interbedded with nodules of chert. Photo: Samuele Papeschi/Geology is the Way

Hancock, J. M. (1975). The petrology of the Chalk. Proceedings of the Geologists’ Association86(4), 499-535.
Scholle, P. A. (1977). Chalk diagenesis and its relation to petroleum exploration: oil from chalks, a modern miracle?. AAPG Bulletin61(7), 982-1009.
Stanley, S. M., Ries, J. B., & Hardie, L. A. (2005). Seawater chemistry, coccolithophore population growth, and the origin of Cretaceous chalk. Geologia33(7), 593-596.


Detrital and Authigenic Minerals
Sedimentary Structures
Sedimentary Rocks


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