Rock Glaciers of the Miller Mountain area, Absaroka Mountains, Montana.

Kirby Bean

Department of Geology, Whitman College, Walla Walla, WA 99362

The east and north facing slopes of miller mountain, just northwest of Cooke City, Montana have experienced glacial, periglacial, and mass-wasting processes. At the base of the Miller Mountain aretes are several fossil rock glaciers (elevation 2865 to 2965 meters) and one active rock glacier (approximate elevation 3030 meters). The rock supply for these valley-wall or lobate rock glaciers is talus from fractured Eocene Absaroka volcanic rocks. From lower to higher elevations there is a transition from morainal ridges to rock glaciers to talus slopes. Different portions of the rock glaciers are characterized by different morphologies, clastsizes, types of vegetation, and degrees of soil development. The fossil rock glaciers have complex surface morphology, including conical pits, and both transverse and longitudinal ridges and furrows. In general, slope angles are less than the angle of repose, but locally furrows have slopes of up to 37 degrees. These rock glaciers probably originated during deglaciation of the area; as the glaciers diminished, reduced lateral support on valley sides allowed rockfalls to bury the remaining stagnant ice and morainal debris. Although interstitial ice is locally present, most of the rock glaciers are not active because of insufficient thickness and slope.

The exception is a small rock glacier just northwest of the summit of Miller Mountain. The unstable front of this active rock glacier has a slope as high as 43 degrees.


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