PPT Slide
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 petroglyphs.
Several genera of bacteria are known to produce desert varnish, including Metallogenium and Pedomicrobium. Varnish bacteria thrive on smooth rock surfaces in arid climates.
http://waynesword.palomar.edu/pljan98.htm#Introduction
The cyanobacterium Gloeocapsa (a blue-green alga) lives in harsh environments where water is intermittently available. Gloeocapsa produces mycosporine-like compounds that serve as a UV sunscreen and that can be used as a chemical marker for presence of the biofilms.