This is a very pure limestone formed of calcite and containing only small amounts of silt or mud. It consists mainly of the tests of microorganisms, such as coccoliths and foraminiferans, which cannot be seen without the aid of a microscope. Macrofossils, which can be seen with the naked eye, are often present, and these include ammonites and bivalves, brachiopods, and echinoderms. Chalk may contain detrital material, mainly quartz, as well as other mineral fragments.

TEXTURE A very fine-grained, powdery, soft rock. It effervesces strongly when in contact with cold, dilute hydrochloric acid.

ORIGIN Formed in marine conditions during the Cretaceous period. During this period, the continental shelves, where the chalk was deposited, were below a much greater depth of seawater than today. The small amount of detrital material suggests that nearby continental areas were low-lying and arid.

Grain size: Fine

Classification: Organic

Fossils: Invertebrates, Vertebrates

Grain shape: Rounded, Angular