Chalk cliffs. View of Seven Sisters chalk cliffs in East Sussex, England. Grassland covers the tops of these near vertical cliffs. The chalk that now forms the cliffs was laid down about 80 million years ago during the late Cretaceous period. At this time high sea levels meant that a shallow ocean covered most of Southern Britain. Microscopic algae called coccolithophorids that lived in this ocean formed this chalk. Chalk is the compressed calcium carbonate skeletons of these algae. Seven Sisters chalk cliffs are a Site of Special Scientific Interest (SSSI). A SSSI is a UK site that is being conserved for scientific reasons.