MICHAEL MARTEN / SCIENCE PHOTO LIBRARY MICHAEL MARTEN / SCIENCE PHOTO LIBRARY
Chalk cliffs. Lines of flint embedded in chalk cliffs on the coast of southern England. The English chalk cliffs are of Cretaceceous origin and are made of the calcareous shells of billions of microscopic marine creatures. The flints, which follow the line of the chalk bedding, are of organic origin, being derived from the silica skeletons of radiolarians and sponges. Photographed near Durdle Door on the coast of Dorset, southern England.
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