The white cliffs of Dover are composed of a variety of protist fossil shells, including coccolithophores (a type of algae) and foraminiferans. Their process of formation took millions of years: After these protists died, their shells were deposited on the bottom of the ocean in a fine gray mud; after time, layer upon layer of sediment deposited above compressed the mud. After even more time, the mud hardened, forming a type of limestone we call chalk. Eventually, geologic processes—such as the uplift of the land and erosion by water or ice—exposed the outcrop of white limestone.