Desert Varnish

Desert varnish is but one of more than a dozen rock coatings (Table 1) that drastically alter the appearance of rock surfaces. The better term is rock varnish because this coating occurs in virtually all environments, including alpine, antarctic, arctic, desert, periglacial, stream, temperate, and tropical settings. This paper-thin accretion is characterized by extremely high concentrations, typically more than 10%, of manganese oxides that give it a characteristic black to dark brown appearance. Clay minerals, however, make up the bulk of rock varnish along with iron oxides.

There are four general explanations for how rock varnish accretes on top of rock surfaces, but all these models reject the old idea that the constituents of varnish derive from the underlying rock. The model that has not yet ...