Naturalis Historia. Streaks of desert varnish run down the face of this sandstone cliff. Anyone who has spent time in a desert has probably noticed many dark streaks or patches on the rocks. I expect that most people simply look at these dark streaks and think they are simply stains like a stain on a cement driveway or that line of rust color running down a wall from a rusty nail above. No big deal right?
Desert Varnish: A Crude Marker of the Passage of Time – Naturalis Historia
Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is approximately one micrometer thick and exhibits nanometer -scale layering. Desert varnish forms only on physically stable rock surfaces that are no longer subject to frequent precipitation , fracturing or wind abrasion. The varnish is primarily composed of particles of clay along with iron and manganese oxides. The color of the varnish varies from shades of brown to black. It has been suggested that desert varnish should be investigated as a potential candidate for a " shadow biosphere ". Originally scientists thought that the varnish was made from substances drawn out of the rocks it coats.