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The precise tectonic role of the left-lateral Garlock fault in southern California has been controversial. Three proposed tectonic models yield significantly different predictions for the slip rate, history, orientation, and total bedrock offset as a function of distance along strike. In an effort to test these models, we present the first slip-rate estimate for the western Garlock fault that is constrained by radiocarbon dating. A channel (referred to here as Clark Wash) incised into a Latest Pleistocene alluvial fan has been left-laterally offset at least 66 +/- 6 m and no more than 100 m across the western Garlock fault, indicating a left-lateral slip rate of 7.6 mm/yr (95% confidence interval of 5.3-10.7 mm/yr) using dendrochronologically calibrated radiocarbon dates. The timing of aggradational events on the Clark Wash fan corresponds closely to what has been documented elsewhere in the Mojave Desert, suggesting that much of this activity has been climatically controlled. The range-front fault, located a few hundred meters northwest of the Garlock fault, has probably acted primarily as a normal fault, with a Holocene rate of dip-slip of 0.4-0.7 mm/yr. The record of prehistoric earthquakes on the Garlock fault at this site, though quite possibly incomplete, suggests a longer interseismic interval (1200-2700 yr) for the western Garlock fault than for the central Garlock fault. The relatively high slip rate determined here indicates that the western and central segments of the Garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data. The high rate of motion on the western Garlock fault is most consistent with a model in which the western Garlock fault acts as a conjugate shear to the San Andreas fault. Other mechanisms, involving extension north of the Garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault, but they cannot explain a high rate of slip on the western Garlock fault.