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. [Illustration: FIG. 160.—Pavement of horizontal surface due to soil flow, Spitzbergen (after Otto Nordenskiöld).] Within the subpolar regions, where a large part of the surface is for much of the year covered with snow, the underlying rocks are for long periods saturated with thaw water, and in alternation are repeatedly frozen and thawed. Essentially similar conditions are met with in the high, snow-capped mountains of temperate or torrid regions. For the subpolar regions particularly it is now generally recognized that somewhat special processes of soil flow, described under the name _solifluction_, are characteristic. The exact nature of these processes is as yet imperfectly understood, but there can be little doubt concerning the large rôle which they have played in the transportation of surface materials. Such soil flow is clearly manifested under different aspects, and it is likely that by this comprehensive term distinct processes have been brought together. [Illustration: FIG. 161.—Tree roots entering fissured rock and prying its sections apart.] Possibly the most striking aspect of the soil flow in subpolar regions is furnished by the remarkable “stone rivers” and “rock glaciers”; though the more generally characteristic are peculiar stripings or other markings which appear upon the surface of the ground and thus betray the movements of the underlying materials. Upon slopes it is not uncommon for the surface to be composed of angular rock fragments riven by the frost and crossed by broad parallel furrows as though a gigantic plow had gone over it (Fig. 159). The direction of the furrows is always up and down the slope, and the striping is marked in proportion as the slope is steep. Where the bottom is reached, the furrows are replaced by a sort of mosaic pavement of hexagonal repeating figures, each of which may be an area of the surface six feet or more across (Fig. 160, and Fig. 390, p. 368). The depressions which separate the “blocks” of the pavement are often filled with clay, while the inclosed surfaces are made up of coarsely chipped stone. =The splitting wedges of roots and trees.=—In the mechanical breakdown of the rocks within humid regions a not unimportant part is sometimes taken by the trees, which insinuate the tenuous extremities of their rootlets into the smallest cracks and by continued growth slowly wedge even the firmer rocks apart (Fig. 161). In a similar manner the small tree trunk growing within a crevice of the rock may in time split its parts asunder (Fig. 162). [Illustration: