Desert varnish is a thin coating (patina) of manganese, iron and clays on the surface of sun-baked boulders. It is formed by colonies of microscopic bacteria living on the rock surface for thousands of years.
These bacteria absorb trace amounts of manganese and iron from the atmosphere resulting in a black layer of manganese oxide or reddish oxide on the rock surfaces.
For thousands of years native Indians have used desert varnish for their rock carvings (petroglyphs) by etching into the dark coating to reveal the light colored granite below. Various forms such as spirals, wavy lines, footprints, men, deer and desert bighorn sheep have been identified.