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.
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]).
Fig. 162.—A large glacial bowlder split by a growing tree near East Lansing, Michigan (after a photograph by Bertha Thompson).
The rock mantle and its shield in the mat of vegetation.—Through the action of weathering, the rocks, as we have seen, lose their integrity within a surface layer, which, though it may be as much as a hundred feet or more in thickness, must still be accounted a mere film above the underlying bed rock. The mechanical agents of the breakdown operate only within a few feet of the surface, and the agents of rock decomposition, derived as they are from the atmosphere, become inert before they have descended to any considerable depth. The surface layer of incoherent rock is usually referred to as the rock mantle ([Fig. 163]). Where the rock mantle is relatively deep, as it is in the states south of the Ohio in the eastern United States, there is found, deep below the outer layer of soil, a partially decomposed and disintegrated rock, of which the unaltered minerals lie unchanged in position but separated by the new minerals which have resulted from the breakdown of their more susceptible associates. While thus in a certain sense possessing the original structure, this altered material is essentially incoherent and easily succumbs to attack by the pick and spade, so that it is only at considerably greater depths that the unaltered rock is encountered.