INTRODUCTION

Desert (rock) varnish is a thin (< 100 to 500 μm thick) coating of ferromanganese oxides, clay minerals, and trace elements, which can form on sedimentary, igneous, or metamorphic rocks with stable surfaces. Microscopic study of varnish shows that it is characterized by many kinds of textures including numerous ultrathin laminae that commonly grade into irregular pods of jumbled material (Krinsley et al., 1990; Krinsley and Dorn, 1991). Thus, varnish is a special kind of microscale sedimentary deposit. It is most common on Quaternary-age rocks, but it has also been reported on Miocene, Triassic, and even Precambrian rocks. Both abiotic and biotic (mostly bacterial) processes may be involved in the formation of varnish.

Rock varnish is most common in arid environments (e.g., Krinsley and Dorn, 1991), but it also forms in more humid environments (e.g., Douglas, 1987). Because the origin of varnish may be related to climatic factors (Dorn, 1986), it is of particular interest to paleoclimatologists. It is also of interest to archeologists and geomorphologists, who have used the cation-ratio (CR) dating technique to determine the age of the varnish (Dorn and Whitley, 1984; Dorn et al., 1988; Pineda et al., 1990), which establishes a minimum age for a surface upon which varnish has formed. Geologists are es-pecially interested in the origin and depositional history of rock varnish and the diagenetic changes that may affect it after initial formation.