As electrons flow along the membranes of our mitochondria, molecules of ATP are generated. However, in varnish bacteria the electrons come from the oxidation of manganese and iron rather than glucose. Herein lies the marvelous adaptive advantage for producing a layer of black and red varnish on desert boulders. The sun-baked boulders of the Alabama Hills in Owens Valley, California look like they were blackened by ancient campfires. They are actually coated with a black layer of clay and manganese oxide precipitated by colonies of remarkable bacteria living on the rock surface for countless centuries.
Varnish bacteria thrive on smooth rock surfaces in arid climates.
According to Ronald Dorn, perhaps 10,000 years are required for a complete varnish coating to form in the deserts of the southwestern United States. In fact, dating of varnished surfaces is of enormous importance to the study of desert landforms and to the study of early humans in America, since many artifacts lying on the ground become coated with desert varnish. The thin layer of reddish iron oxide varnish on this rock surface has been etched to reveal the lighter granodiorite beneath. Several Indian tribes utilized desert varnish to create their marvelous petroglyphs. Throughout northern Owens Valley, there are acres of elaborate petroglyphs carved into desert varnish and Bishop tuff, including spirals, circles, wavy lines, footprints, men, deer and desert bighorn sheep.