JOHN MEAD / SCIENCE PHOTO LIBRARY JOHN MEAD / SCIENCE PHOTO LIBRARY
White chalk cliffs. From a grassy summit, these cliffs drop almost vertically to the sea and beach below. White sediments are seen to discolour the water. The chalk cliffs of southern England are composed largely of calcium carbonate, laid down 65 to 100 million years ago during the Upper Cretaceous period. These chalk deposits are in fact chiefly the remains of marine plants: microscopic single-celled algae called coccoliths, and which secrete lime. Apart from coccoliths, these cliffs are a rich source of other fossils too. Photographed at Beachy Head in Sussex, England.
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