Sandstone rock and sand. This sandstone deposit is part of the Tassili n'Ajjer plateau, a mountainous region in the central Sahara. The surface of the rock is coated in desert varnish (dark areas), an accretion composed of clay, manganese 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. Photographed in the Tadrart Canyon, Algeria.