Fig. 209.—Landslide topography on Badger Mountain, Washington. The slumping material in this case is basalt.
Creep, slumps, and landslides.—When the soil and subsoil on a slope become charged with water they tend to move downward. When the movement is too slow to be sensible it is called creep. The common downward inclination of trees growing in such situations, the result of the more rapid creep of the surface as compared with the deeper part of the soil, is both an expression of the movement and of its slowness. Other factors besides ground-water are involved in creep (see [p. 112]).
When the movement is rapid enough to be sensible the material is said to slump or slide. This may happen when the slope on which water-charged mantle rock lies is steep ([Fig. 207]). Great landslides of this sort have been recorded, and some of them have done great damage. Where a stream’s banks are high, and of unindurated material, such as clay, considerable masses sometimes slump from the bank or bluff into the river, or settle away slowly from their former positions. This is a common phenomenon along streams which have cut valleys in drift, and along shores on which waves are encroaching. The same phenomenon is common on a larger scale on the slopes of steep mountains.[107] Considerable terraces are sometimes developed on their slopes in this way, but they are usually irregular and discontinuous (Figs. [208], and [209]). The loose débris on steep slopes sometimes assumes a sort of flowing motion and descends the slope with some such form and at some such rate as a glacier. Such bodies of débris are sometimes called “talus glaciers” ([Fig. 210]). In many such cases, snow and ice have had some part in their development.
In creep and in landslides gravity is the force involved, and the ground-water only a condition which makes gravity effective. Gravity alone accomplishes similar results, as illustrated by [Fig. 211].
Summary.
All in all, ground-water is to be looked upon as a most important geological agent. When it is remembered that a very large part of all the water which falls on the surface of the earth, either in the form of rain or snow, sinks beneath the surface; that much of it sinks to a great depth; that much of it has a long underground course before it reappears at the surface; that it is everywhere and always active, either in subtracting from the rock through which it passes, in adding to it, in effecting the substitution of one mineral substance for another, or in bringing about new chemical combinations; and when it is remembered that this process has been going on for untold millions of years, it will be seen that the total result accomplished must be stupendous. The rock formations of the earth to the depths to which ground-water penetrates are to be looked upon as a sort of chemical laboratory through which waters are circulating in all directions, charged with all sorts of mineral substances. Some of the substances in solution are deposited beneath the surface, and some are brought to the surface where the waters issue. Much of the material brought to the surface in solution is carried to the sea and utilized by marine organisms in the making of shells. Without the mineral matter brought to the sea by springs and rivers, many shell-bearing animals of great importance, geologically, would perish. Biologically, therefore, as well as geologically, ground-water is of great importance.
PLATE XVII.
U. S. Geol. Surv.
Scale, 1+ miles per inch.