Analysis of the Factors Controlling Cliff Burial by Talus within Blanca Massif, Southern Colorado, U.S.A.

Abstract

Consideration of the factors influencing talus accumulation leads to an hypothesis that slope morphometry, bedrock fracture density, and topoclimatic situation combine to determine degree of cliff-to-talus transformation within recently deglaciated terrain. A statistical analysis of 27 cirque walls from a portion of the Sangre de Cristo Mountains, southern Colorado, provides indirect support for the research hypothesis. Differences in potential insolation, altitude, windward/leeward aspect, and bedrock fracturing explain 66% of the observed variability in degree of postglacial cliff-to-talus transformation within Blanca Massif. Free-face fracture density has the strongest statistical effect on degree of postglacial slope modification and this may be indicative of a functional relationship between bedrock weathering susceptibility and rate of cliff retreat. Average rockwall weathering rates (estimates from a subsample of slopes with similar topoclimatic situation) range between 0.05 and 0.82 mm yr-1 and the estimates are greatest for slopes eroded into densely fractured lithology. Potential insolation and altitude interact multiplicatively in their control over degree of postglacial cliff burial, implying that the effect of increasing insolation exposure decreases with elevation above the Pleistocene glaciation limit. Degree of cliff-to-talus transformation is about 11% greater at windward aspects than leeward aspects when the effects of the other environmental factors are statistically controlled. The results of this study suggest that postglacial talus accumulation is maximized at sites that are unfavorably situated for glacier and rock glacier facies. However, a broad range of topoclimatic situations are acceptable for the production of fragmented rock debris so that talus facies may form almost anywhere along the walls of a recently deglaciated cirque.