In such portions of the temperate regions as are favored by a humid climate, the mat of vegetation holds down a layer of soil, and mat and soil in coöperation are effective in preventing any such large measure of frostwork as is characteristic of the subpolar regions or of high levels in the arid lands. In humid regions the rocks become a prey especially to the processes of solution and accompanying chemical decomposition, and these processes, although guided by the course of the percolating ground water along the fracture planes, do not afford such striking examples of the control of surface relief.
Those limestones which slowly pass into solution in the percolating water do, however, quite generally indicate a localization of the solution along the joint channels ([Fig. 239] and [plate 6 B]). Though in other rocks not so apparent, yet solutions generally take their courses along the same channels, and upon them is localized the development of the newly formed hydrated and carbonate minerals, as is well illustrated by the phenomenon of spheroidal weathering ([Fig. 155], [p. 150]).
Fig. 239.—Outcrop of flaggy limestone which shows the effects of solution along neighboring joints in a sagging of the upper beds (after Gilbert, U. S. G. S.).
The fracture control of the drainage lines.—The etching out of the earth’s architectural plan in the surface relief, which we have seen begun in the processes of weathering, is continued after the transporting agents have become effective. It is often easy to see that a river has taken its course in rectangular zigzags like the elbows of a jointed stove pipe, and that its walls are formed of joint planes from which an occasional squared buttress projects into the channel. This structure is rendered in the plan of the Abisko Cañon of northern Lapland ([Fig. 240]). We are later to learn that another great transporting agent, the water wave, makes a selective attack upon the lithosphere along the fractures of the joint system ([Fig. 250], [p. 233] and [Fig. 254], [p. 235]).
Fig. 240.—Map of the joint-controlled Abisko Cañon in northern Lapland (after Otto Sjögren).
Where the scale of the example is large, as in the cases which have been above cited, the actual position and directions of the joint wall are easily compared with the near-by elements of the river’s course, so that the connection of the drainage lines with the underlying structure is at once apparent. In many examples where the scale is small, the evidence for the controlling influence of the rock structure in determining the courses of streams may be found in the peculiar character of the drainage plan. To illustrate: the course of the Zambesi River, within the gorge below the famous Victoria Falls, not only makes repeated turnings at a right angle, but its tributary streams, instead of making the usual sharp angle where they join the main stream, also affect the right angle in their junctions ([Fig. 241]).
Fig. 241.—Map of the gorge of the Zambesi River below the Victoria Falls (after Lamplugh).