Figure 5    Optical varnish microstratigraphies from late Quaternary alluvial fan surfaces in Death Valley. Note that similar layering patterns are replicated in different varnish microbasins on the same boulder (C and D; E and F) and on boulders from different fan surfaces (A and B), suggesting that microlaminations in varnish record regional environmental fluctuations, especially climate changes. Also note that varnish layering sequences in (E) and (F) are from a quartzite boulder on the oldest fan unit (Qu8) of Six Spring Canyon fan. This same varnished boulder has been cosmogenically ¹⁰Be dated at 297 ± 11 ka, thus making these layering sequences the oldest varnish climate record yet uncovered and dated in the western USA drylands. See Figs. 1 and 8 and Table 1 for locations of these varnish samples.