Figure 1    Map showing location of the study sites in the drylands of western USA (A) and map of the studied alluvial fans in Death Valley (B). Stars and a rectangle in (B) denote rock varnish sampling sites. Abbreviations used are Death Valley (DV), Cima Volcanic Field (CVF), Silurian Valley (SV), Yucca Mountain (YM), Badwater (BW), Mormon Point (MP), Galena Canyon fan (GC), Hanaupah Canyon fan (HC), Six Spring Canyon fan (SC), and Warm Spring Canyon fan (WC).