A clue regarding the origin of rock varnish

137 Cs and excess 210 Pb have been found in rock varnish samples from several localities in the desert regions of the western United States. As 137 Cs was absent in the environment prior to the atmospheric testing of nuclear weapons, and as 210 Pb has a half life of only 22 years, these isotopes must have been added to this slow growing patina during the last half of the present century, demonstrating that, at least in part, the elements constituting varnish are supplied from the atmosphere. The discovery opens a window into the mechanism by which this mysterious substance forms. Not only radioisotopes but also anthropogenic metals can be harnessed to assess the ratio of rate of accumulation of a given element on varnish to its delivery rate from the atmosphere. Based on measurements reported here, for 210 Pb this ratio is about 0.1 and for 137 Cs about 0.01.