I have developed a relative age index for alluvial surfaces using the degree of rubification on the under sides of clasts, corroborated with other surfical parameters such as soil development and percent varnish coverage. The central section of the Garlock fault in the Mojave Desert of southern California provides an ideal setting for developing and testing this index, because the ages of the surfaces can be independently estimated using their offset and the known slip rate of the fault. This rubification index may be widely applicable to other Quaternary studies in arid regions. Using Munsell color notation, I quantified rubification on the undersides of clasts from 15 different surfaces ranging in age from 0.5 ka to about 25 ka. On each surface I studied 10 - 25 clasts from each of 10 different lithologies, and assigned point values for color data. My data indicate that for progressively older surfaces, the hue reddens and the chroma intensifies consistently. Maximum colors saturate at 10 R 4/8 on the oldest surfaces that we studied, indicating that this index will be most useful for latest Quaternary surfaces. The rapid rate at which the undersides of clasts redden is surprising because of the slow rates of weathering expected in arid environments. To study this phenomenon, I deployed five temperature probes and two humidity sensors to measure the physical conditions surrounding the clasts under study. I found that although relatively humidity rarely reached 100%, the undersides of clasts condensed moisture nightly for a period of several days to over a week after each rain episode. During the same period, relative humidity dropped below 100% during the day. Absorption of solar energy and elevated clast temperatures during the day causes evaporation of moisture from beneath the clasts. During the nightly cooling periods, soil moisture was observed to recondense on the clast bottoms. I infer that these numerous wet/dry cycles accelerate the rate of clast rubification, thereby leading to rapid Holocene clast reddening.