Chalk is a type of sedimentary rock composed mainly of the microscopic skeletons of single-celled marine plankton called coccoliths. It is typically white or light gray in color and is composed of the minerals calcite (calcium carbonate) and small amounts of other minerals such as clay and silt. Chalk is formed in shallow, warm, and clear marine environments where coccoliths accumulate on the seafloor and are then compacted and cemented into rock over time.
Chalk deposits are often extensive and can form thick layers, known as chalk formations, that can reach hundreds of meters in thickness. The most famous chalk formation is the White Cliffs of Dover in England, which is part of the larger Chalk Group that covers much of southern England.
Chalk has been used for a variety of purposes throughout history, including as a writing material, as a source of lime for agriculture and building, and as an oil reservoir rock in some areas. The high purity and fine grain size of chalk make it a valuable raw material for many industrial processes, including the production of cement, glass, and paper.
In terms of geology, chalk is an important rock unit in many regions, as it is often part of the rock record of the late Cretaceous period and provides important information about past climate, sea level, and biological conditions on Earth.