Figure 2    Electron microprobe maps of elements (256×256 µm) in rock varnish on a sample from Galena Canyon fan in Death Valley. The image in the lower right corner shows the optical microstratigraphy seen in varnish ultrathin section under polarized light. The element imaged in each panel is shown in the black bar at the bottom of the panel and the color scale of elemental concentration is shown on the right side of the panel. A depth profile of Mn (with ~ 2 µm focused beam) was taken along the vertical blue line marked on the inset image. As seen from these probe maps, silica (Si), aluminum (Al), potassium (K), and magnesium (Mg) achieve their highest concentrations in the yellow/orange layers in the middle of the varnish microstratigraphy, which are interpreted to represent the last interglaciation (the Holocene surface yellow layer is absent in this sample because of surface spalling). In contrast, manganese (Mn), barium (Ba), and calcium (Ca) achieve their highest concentrations in the dark layers, which are interpreted to represent the last and penultimate glaciations. This varnish contains a layering sequence of LU-1/…/LU-5/LU-6 (WP12), suggesting a minimum VML age estimate of ~ 165 ka for the initiation of varnish formation (see text for discussion).