Sandstone pillars in a desert. Parts of the pillars are covered by desert varnish (dark areas near top), 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. Photographed in Tagrera, Tassili du Hoggar, southern Algeria.