The prying work of frost.—In all countries where winter temperatures range below the freezing point of water, a most potent agent of rock disintegration is the frost which pries at every crevice and cranny of the surface rock. Important in the temperate zones, in the polar regions it becomes almost the sole effective agent of rock weathering. There, as elsewhere, its efficiency as a disintegrating agent is directly dependent upon the nature of the crevices within the rock, so that the omnipresent joints are able to exercise a degree of control over the sculpturing of the surface features which is hardly to be looked for elsewhere (see [plate 10 A]).
Fig. 158.—Talus slope beneath a cliff.
Talus.—Wherever the earth’s surface rises in steep cliffs, the rock fragments derived from frost action, or by other processes of disintegration, as they become detached either fall or slide rapidly downward until arrested upon a flatter slope. Upon the earlier accumulations of this kind, the later ones are deposited, until their surface slopes up to the cliff face as steeply as the material will lie—the angle of repose. Such débris accumulations at the base of a cliff ([Fig. 158]) are known as talus, and the slope is described as a talus slope, or in Scotland as a “scree.”
Fig. 159.—Striped ground from soil flow of chipped rock fragments upon a slope, Snow Hill Island, West Antarctica (after Otto Nordenskiöld).
Soil flow in the continued presence of thaw water.—So soon as the rocks are broken down by the weathering processes, they are easily moved, usually to lower levels. In part this transportation may be accomplished by gravity slowly acting upon the disintegrated rock and causing it to creep down the slope. Yet even in such cases water is usually present in quantity sufficient to fill the spaces between the grains, and so act as a lubricant to facilitate the migration.
Upon a large scale rocks which were either originally incoherent or have been made so by weathering, after they have become saturated with water, may start into sudden motion as great landslides or avalanches, which in the space of a few moments materially change the face of the country, and by burying the bottom lands leave disaster and misery in their wake.
