SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY
Weathered desert varnish at the base of a sandstone pillar. Desert varnish is a coating that forms on rocks in arid environments. Rock varnish is composed of clay minerals, manganese, iron and trace elements. It forms when wind-blown particles stick to wet rocks and then react in the heat of the sun, forming an accretion on the surface of the rock. This coating then becomes enriched in manganese by biochemical processes. Here the varnish has been weathered due to the temperature extremes in the desert. Photographed in Tagrera, Tassili du Hoggar, southern Algeria.
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