Because of its wide distribution, the dark stain known as desert varnish, which covers rock surfaces in arid regions, has intriguing potential as a research tool in diverse fields of investigation. Dark coatings on rock surfaces can be seen in nearly all environments, but are most conspicuous in arid regions where coarse rock debris and solid rock outcrops dominate the landscape. The mode of origin of the coating and the enormous enhancement of manganese in it as compared with ambient levels have been topics of controversy for more than 100 years. More recently, there has been concern regarding the influence of desert varnish on the remote sensing of arid lands (e.g. Warner and El-Baz, 1978;Perry, 1979;Elvidge and Collet, 1981).Desert varnish is the arid phase of a wider phenomenon that we have termed 'rock varnish' (Dom and Oberlander, 1980;Dom, 1980). Rock varnish is usually 10 to 30 microns thick, but we have observed varnish depths ranging from under two to over 500 microns. The constituents of all rock varnishes are derived from sources external to the underlying rock. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, detrital particles such as Si02 and CaC03 , and trace elements, among which Mg, Ca, K, Na, Ti, and Cu are most notable. The classic rock varnish, frequently described in the literature on arid landscapes, is a manganese-rich black coating on rocks, but orange to dusky-brown varnishes are also widespread. Iron and manganese are the distinctive elements of rock varnish. Weathering rinds and encrustations of calcium carbonate, sulphate, and other salts are regarded as distinct from the phenomenon of rock varnish.The degree of development of rock varnish has long been used by archaeologists to estimate the relative ages of stone artifacts and prehistoric rock inscriptions. Similarly, the relative development of varnish in arid regions has been used by geomorphologists to differentiate and correlate surface deposits of alluvium, talus, and eolian sand, as well as erosional features such as cliff faces and relict lake shorelines of varying ages. It has long been hoped that some characteristic of varnish might be used to establish absolute chronologies that could be used in both geomorphological and archaeological research. Wide variations in its chemical composition suggest that rock varnish could also be used as a geochemical prospecting tool. Finally, changes in chemical composition and micromorphology through the
