This photo was taken just outside Moab, Utah. It is called Desert Varnish, and it only forms on hard, erosion resistant rock, as it takes a long time to form. It is made up of windblown clay, manganese, and iron oxide, along with organic material, that sticks to the surface of the rock.

The high concentration of manganese was a puzzle, since it is more than 50 times higher than the rocks or surrounding soils. Recent discoveries indicate that it is the result of chemical processes by bacteria, depositing manganese into the clay.