The area of an alluvial fan is approximately proportional to the 0.9 power of the area of the drainage basin discharging to the fan. The constant of proportionality is, in part, a function of the erodibility of the bedrock in the source area. By using multiple linear-regression techniques to analyze this functional relationship in three groups of fans in Death Valley, California, and vicinity, we determined coefficients of relative erodibility for some of the dominant rock types. Comparison of these coefficients with weathering characteristics observed in the field and with changes during weathering experiments in the laboratory suggests that fracture density is more important than mineralogy in determining rock erodibility.

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