Rocks coated with desert varnish were translocated from Scottsdale, Arizona, and Panamint Valley, California, to a fenced-in plot near Biosphere 2 where they were allowed to accumulate a new crop of the 53 day half life, cosmic ray-produced 7Be. Those exposed to precipitation accumulated several times more of this isotope than those shielded from precipitation. No significant difference in 7Be accumulation was observed between a set which was UV irradiated (in an attempt to kill resident bacteria) and a set which received no UV irradiation. This experiment suggests that 62 ¿ 10 percent of the beryllium accumulated on the varnish was supplied by precipitation and 38 ¿ 10 percent by some combination of dew, dust, and aerosols. If bacteria are, as has been proposed, responsible for varnish growth, then either our UV irradiation was inadequate to squelch their activity or the 7Be we measured had not yet been built into the varnish. ¿ 2001 American Geophysical Union