Chalk
Chalk is a type of limestone. It was made by calcareous (CaCO3) skeletons of tiny planktonic algae called coccoliths. They live in the sea in huge numbers. They make chalk (CO2) as a by-product of their photosynthesis. The chalk cliffs of Dover and elsewhere were made when England was below the equator in a tropical sea. It was in the Upper Cretaceous, when the Earth was even warmer than now. There are coccoliths today, but not the same species as formed the white cliffs.