Birnessite and hematite have been identified as the Mn- and Fe-oxide phases in desert varnish by IR spectroscopic and chemical techniques. These oxides are in intimate physical association with the mixed-layer illite—montmorillonite clay minerals of varnish. An abrupt change in chemistry, mineralogy and morphology exists at the varnish—rock interface. The origin of the material appears to be external to the rock which it coats. The oxides appear to be transported by water, and it is most likely that the clays are transported by wind or water. Clay minerals are necessary for varnish formation, probably through their influence on oxide concentration or deposition.