Abstract

Field studies and examination of aerial photographs of approximately 200 rock glaciers in the Healy (1:250,000) quadrangle in the central Alaska Range showed that there are three types of rock glacier in plan: lobate, in which the length is less than the width (200–3500 feet long and 300–10,000 feet wide); tongue-shaped, in which the length is greater than the width (500–5000 feet long and 200–2500 feet wide); and spatulate, tongue-shaped but with an enlargement at the front. Lobate rock glaciers line cliffs and cirque walls and probably represent an initial stage; the other two move down valley axes and represent more mature stages.

The rock glaciers are composed of coarse, blocky debris that is cemented by ice a few feet below the surface. The top quarter of the thickness is coarse rubble, below which is coarse rubble mixed with silt, sand, and fine gravel. Fronts of active (moving) rock glaciers are bare of vegetation, are generally at the angle of repose, and make a sharp angle with the upper surface. Fronts of inactive (stationary) rock glaciers are covered with lichens or other vegetation, have gentle slopes, and are rounded at the top. Active rock glaciers average 150 feet in thickness, inactive rock glaciers, 70 feet.

The upper surface of most rock glaciers is clothed with turf or lichens. Sets of parallel rounded ridges and V-shaped furrows—longitudinal near the heads of some rock glaciers and transverse, bowed downstream, on the lower parts of others—and conical pits, crevasses, and lobes mark the upper surfaces of many rock glaciers.

The upper surface of a rock glacier at the head of Clear Creek moved 2.4 feet per year between 1949 and 1957, and the front advanced 1.6 feet per year.

Heights of the upper edges of the talus aprons along the fronts of rock glaciers average 45 per cent of the heights of the fronts. Each of these observations implies that motion is not confined to thin surface layers but is distributed throughout the interiors of the rock glaciers, which in this permafrost region are probably frozen. “Viscosity” has been calculated for rock glaciers at between 1014 and 1015 poises; for glacial ice it has been estimated at between 1012 and 1014 poises. Maximum average shear stresses within active rock glaciers range from 1 to 2 bars; these values are much larger than those calculated for solifluction and creep features.

Rock glaciers occur on blocky fracturing rocks which form talus that has large interconnected voids in which ice can accumulate. They are rare on platy or schistose rocks whose talus moves rapidly by solifluction.

The rock glaciers lie in an altitudinal zone about 2000 feet thick, centered on the lower limit of existing glaciers1. Although the firn lines on glaciers rise 1200 feet in a distance of 25 miles northward across the Alaska Range, the lower limit of active rock glaciers rises only 800 feet. The firn line on southward-facing glaciers is 2000 feet higher than that on northward-facing glaciers, yet in any given area southward-facing rock glaciers average only 200 feet higher than northward-facing rock glaciers. Insulation by the debris cover is believed responsible for the difference in altitudinal ranges between rock glaciers and glaciers.

It is concluded that rock glaciers move as a result of the flow of interstitial ice and that they require for their formation steep cliffs, a near-glacial climate cold enough for the ground to be perennially frozen, and bedrock that is broken by frost action into coarse blocky debris with large interconnected voids. The longitudinal furrows are thought to result from the accumulation of ice-rich bands in the swales between talus cones at the head of the rock glaciers and the subsequent melting of this ice as the rock glacier moves down-valley. The transverse ridges are thought to result from shearing within the rock glacier that would occur where the thickness increases or the velocity decreases downstream.

An average of 30 feet of bedrock was removed from source areas to form the present rock glaciers, indicating an average rate of erosion of 1–3 feet per year when they are active.

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