Petroglyph (rock engraving) of elephants in the Tadrart-Acacus region of Libya. The oldest petroglyphs are dated to approximately the Neolithic and late Upper Paleolithic boundary, about 10, 000 - 12, 000 years ago during a period when the Sahara was in a wet, savannah phase. In this image the figures of the elephants have been incised on rock coated with 'desert varnish', a dark veneer found on exposed rock surfaces in arid environments. 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. There is also a host of trace elements and almost always some organic matter. The colour of the varnish varies from shades of brown to black. Here the varnish has been etched away to reveal the natural rock colour below.