25

Many explanations have been put forward to account for the forma- tion and behaviour of rock glaciers (Capps, 1910; Sharpe, 1937; Kesseli, 1941). The variety of hypotheses may in itself be an indication that rock glaciers are the result of a combination of several processes, and that different rock glaciers may have had considerably different origins. In general, rock glaciers appear to have been regarded as either: (1) a phase of normal glaciers in which the rock fragments greatly predominate over interstitial ice, developed during the wasting stages of a glacier’s history; these rock glaciers could be considered an end member of a series of rock- and-ice mixtures whose other extreme is represented by glaciers of pure ice; or (2) ‘runaway’ talus, felsenmeer, or moraine deposits that have accumu- lated to sufficient size, or have acquired sufficient interstitial lubricant, to render them unstable and capable of flowing more or less as a unit.

Most of the rock glaciers in Aiken Lake map-area do not seem to be merely moraine-choked dying glaciers. The area contains many dying glaciers, in the form of thin sheets of ice, heavily coated with moraine, on cirque floors; as small ice patches hanging in sheltered recesses in the cirque headwall; and as “fan” glaciers at the feet of couloirs, nourished chiefly by avalanche snow and containing many rock fragments. No single instance was noted of a transition between one of these dying glaciers and a rock glacier, and no rock glacier was observed in such a position as to suggest that it developed directly from such a glacier. Rock glaciers appear invariably to head on steep slopes, whose gradient is controlled by the angle of repose of coarse dry talus, whereas most dying ice glaciers containing abundant moraine are found far back in the cirque basins, where the gradient is so low that the thin sheet of ice becomes stagnant long before its embedded rock load becomes concentrated to the proportions found in active rock glaciers. The few ice patches found on steep slopes rarely contain an appreciable rock load. Ice glaciers and rock glaciers are found side by side. A compound cirque at the head of Tenakihi Creek contains two dying glaciers, one of which is nearly buried in talus. A well-developed rock glacier, conspicuously distinct from the surrounding talus, descends beside one of these glaciers and flows along the valley floor below one of the terminal moraines of the existing glacier. There appears to be no tendency for the moraine of the remnant glacier, developed on nearly the same slope and composed of the same type of rock as the rock glacier, to flow.

Most rock glaciers in the map-area are in cirques, because most of the area above timber-line is composed of cirques. The best-developed rock glaciers are not confined to the heads of cirques; and, indeed, are rarely found there. More commonly, the rock glaciers head high on one wall of a cirque or hanging valley, flow down to the valley floor, and turn along it (Plate VI A). Streams in the valley floor pass under the tongues of these rock glaciers. Other rock glaciers are not found in cirques but are flowing down open valley slopes, with south, as well as north and east, exposures. Some active, low-level rock glaciers are found near the bottom of valleys that have been long vacated by ice. In general, the rock glaciers do not head in natural drainage basins or channels, but begin on relatively smooth, steep slopes directly from a ridge crest or beneath a cliff.

78609—34