Why are the white cliffs near Dover made of chalk?

The white cliffs near Dover are composed of chalk, a soft, fine-grained, and porous variety of limestone that formed during the Cretaceous Period between 145.5 and 65.5 million years ago.1

This chalk is primarily made up of the skeletal remains of minute marine organisms, such as coccoliths, which are tiny calcium carbonate plates secreted by algae. Over millions of years, these microscopic shells accumulated on the ocean floor to form thick layers of sediment that eventually hardened into the rock we see today.231

Geologically, these cliffs are part of a larger chalk formation that was once part of a vast geologic upfold known as the Weald. Over time, the arch of this upfold was eroded, leaving behind the inward-facing chalk escarpments of the North and South Downs. The waters of the English Channel subsequently undermined and eroded these formations, exposing the white chalk layers that now form the iconic cliffs at the Strait of Dover.4