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Rates of rock-varnish accumulation have never been systematically documented owing to the difficulty in accurately determining both varnish thickness and age. In this study, we quantitatively assess varnish-accumulation rates through thin sectioning and microscopic examination of rock varnish from radiometrically dated geomorphic features of late Quaternary age in the western United States drylands, Our data indicate that rock varnish grows at rates ranging from <1 to 40 mu m/k.y. on subaerially exposed rock surfaces and rarely reaches a thickness exceeding 200 mu m regardless of age. Our data also indicate that varnish-accumulation rates vary greatly from sample to sample at a given site, suggesting that varnish thickness does not correlate with the age of the associated geomorphic feature, invalidating the potential use of varnish thickness as a relative age indicator in geomorphology and archaeology. Nevertheless, being the slowest known accumulating terrestrial sedimentary deposit, rock varnish constitutes a unique long-term microscale sedimentary archive of past environmental changes in deserts.