SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
Desert varnish. View of a sandstone cliff face in a canyon with streaks of desert varnish. It is thought to form when very fine local clay particles are blown against a damp cliff face. The clay forms a film on the rock, and the water slowly moves outward through it. The water carries traces of iron and manganese from the clay which eventually precipitate as the water evaporates. Photographed at the Canyon de Chelly National Monument, Arizona USA.
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