Abstract

Galena Creek rock glacier (44°38'30″ N., 109°47'30″ W., elevation 2,680 to 3,110 m, length 1.6 km) originates in a north-facing cirque. Although this rock glacier morphologically resembles others described elsewhere, its upvalley two-thirds is composed of a continuous layer of debris 1 to 1.5 m thick over relatively clean glacier ice and has a maximum measured surface velocity of 80 cm/yr. The downvalley one-third is mantled by 2 to 3 m of debris (measured by seismic refraction) over ice of unknown debris content; it has a maximum measured velocity of 14 cm/yr. The transition zone between these two regions has several large (6-m-high, 90-m-wide) lobes that override one another at a maximum measured velocity of 6 cm/yr.

Accumulation occurs primarily as wind-drifted snow in a narrow lens-shaped area against the cirque headwall. Most of the coarse debris is not incorporated in the ice, but is carried past the steep (13° to 33°) snow accumulation area beneath the cirque headwall by snow avalanche and rockfall to form the debris mantle. The debris mantle is sorted, with coarse fragments dominant at the surface and a zone of fines just above the debris-ice contact. The ice beneath the debris mantle contains a maximum of 10 to 12 percent debris by volume, except in probable longitudinal septa downglacier from large debris concentrations in the source area.

Intersecting ridges and furrows on the up-valley portion of the rock glacier probably differ in age, according to lichen sizes and ridge sharpness, and are probably formed by compression below steep reaches of the glacier and by collapse into crevasses.

Ice-cored rock glaciers uniquely have a very low ratio of accumulation area to ablation area (1:7 in this case). This is mainly the result of an ablation rate beneath the debris mantle that is estimated to be about two orders of magnitude less than that of clean ice. The slow rate of addition of ice makes the glacier thin and thus slow-moving.

Because of the debris cover, rock glaciers are not nearly so sensitive to climate as are clean glaciers. The lag effect between retreat of clean glaciers and deactivation of rock glaciers may be several thousand years, and therefore mountain glacier moraines should be correlated with rock glaciers only with extreme care.

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