The chalk is laid down

Coccolithophores — single-celled algae armoured in calcite discs — bloom in the warm, high-sea-level Cretaceous oceans and shed their plates continuously to the seafloor. Over tens of millions of years the accumulation builds chalk hundreds of metres thick across northern Europe and the American interior, so pure it is almost entirely biological in origin. The White Cliffs of Dover are the visible face of it, and the Latin creta, chalk, gives the period its name. The same rock is the aquifer under much of southern England and a reservoir under the North Sea.