May 22, 2003
This photo clearly shows the interrelationship between the living environment and the geological environment. The vertical stripes are desert varnish, and the rock beneath it is sandstone (possibly Dakota formation?). Unlike moist environments where lichens thrive, dry environments favor the growth of desert varnish. While varnish can contain lichen (a symbiotic form of algae and fungi) it's mainly a type of manganese-oxidizing bacterial colony, combined with a protective layer of locally derived clay. Some desert varnish in the U.S. Southwest can be up to 10,000 years old and has been important in archeological dating.
(Please note the corrected links below - Ed.)