Abstract

Some 23 rock glaciers have been identified in the central portion of the Mosquito Range. Surface features and characteristic forms correlate well with those reported for rock glaciers in Alaska and elsewhere in Colorado, New Mexico, and Utah. On the basis of air-photo interpretation, about two-thirds of the rock glaciers are lobate in form, and one-third are tongue-shaped. Only 5 of the 23 rock glaciers are believed to be active.

A detailed stability analysis of two rock glaciers in Iowa Gulch indicates that landsliding of unstable morainal deposits played a major role in their formation. Landslide debris provided loading necessary to initiate ice creep in frozen till on the lower valley slopes. Ice creep continued, eventually bringing the rock glaciers to their present form. With the supply of landslide debris exhausted and with insufficient ice wedging and rock exposure to generate large talus cones, stresses generated by the superincumbent load were reduced by creep-induced spreading, and motion ceased. Internal ice, in the form of both interstitial ice and ice layers, still remains.

Techniques developed to evaluate activity of rock glaciers in Alaska have been applied to the two rock glaciers in Iowa Gulch. These techniques, based principally on steepness and form of the rock glacier front as well as on calculated values of basal shear stress, indicate both rock glaciers to be inactive. Re-survey of a mining claim corner and inspection of a shaft show that no movement has occurred over the period of record and that the rock glaciers are indeed inactive, substantiating the usefulness and validity of the above-described techniques for assessment of rock glacier activity elsewhere.

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