Chalk is very distinctive variety of limestone that is soft, whitish, and powdery. Chalk is composed of calcite (CaCO3), and will bubble in acid. The most spectacular chalk locality on Earth is the White Cliffs of Dover, along the southern shores of Britain. The rocks there are Cretaceous in age (“creta” means “chalk”).
Chalk is a biogenic sedimentary rock, but it is not at all obvious how this white powdery material represents the remains of once-living organisms. When examined under a scanning electron microscope, chalk powder is seen to be composed of immense numbers of exceedingly small microfossils, principally coccoliths. Coccoliths are calcitic plates that once covered a living cell (see example photo). The cell was an entire organism called a coccolithophorid (Kingdom Protista, Phylum Chrysophyta, Class Coccolithophorida). Coccolithophorids are unicellular, photosynthetic organisms. They are often called "algae", but they’re better called photosynthetic protists. When they die, the cell degrades, and the numerous hard calcitic plates covering the cell fall to the seafloor.
Chalk generally forms in moderately deep marine environments (but not in the deepest ocean depths), where high numbers of coccolith plates can accumulate as sediments, without calcite dissolution, and undiluted by muddy or sandy sediments washed in from the continents.
Chalk from the "Upper Chalk" Formation (Cenomanian Stage, lower Upper Creatceous) at Dover Cliffs (White Cliffs of Dover), southeastern coast of Kent, southeastern-most England.