Rock varnish and its use to study climatic change in geomorphic settings

The dusky brown to black coating of rock varnish dominates bare rock surfaces of many desert landforms (Oberlander, 1994). Thicknesses less than even 0.020 mm (or 20 micrometres, μ m) are enough to darken light-colored rock types (Fig. 21.1). The gradual pace of change on many desert landforms permits the slow accretion of rock varnish at rates of a few micrometres per thousand years (Dorn, 1998; Liu and Broecker, 2000). Just about any rock type will accumulate varnish, in so long as rock-surface erosion is slow enough to permit varnish accretion.