Within the soil the movement of water is in different directions according to circumstances. In climates of considerable rainfall which have no long dry season, the movement is chiefly downward, due to gravity, but if there are long periods of drought, evaporation from the surface causes an upward movement of the water by capillarity; and in the tropics this upward movement produces certain important and characteristic effects.

At a depth below the surface, which varies greatly at different times and places, the soil and rocks are saturated with water, which is called the ground water. Near the sea, or other bodies of surface water, the ground water may be very little below the surface of the ground, while in arid regions, with irregular topography, it may sink to great depths. In the eastern United States the ground water is encountered at depths of 1-100 feet, as is shown by the countless wells which are supplied by it. In the limestone plateau of eastern Kentucky and Tennessee the ground water is from 200-300 feet below the surface and is determined by the level at which the surface streams flow, that is, the drainage level of the region. In the plateau of the Colorado River, which is dissected by profoundly deep canons, the ground water is, in places, nearly 3500 feet from the surface.

The level of the ground water is thus highly irregular and depends upon the amount of precipitation and upon the topographical features. As a general rule, the level of ground water is at that of the streams and rises toward the divides, but less steeply than the surface of the ground. Similarly, the ground water level fluctuates with the rainfall, rising in wet seasons and sinking in dry, as is shown by the failure of wells after a long drought.

It is usual to regard the ground water as everywhere penetrating to great depths, and, from this point of view, it is frequently called the "sea of ground water," but there is reason for much hesitation in accepting this belief. In a large number of very deep mining shafts in various parts of the world, and in both humid and arid regions, water is found only in the upper levels, within 2500 feet or less of the surface, while below the mines are dry, even dusty. Such shafts frequently encounter water in the lower levels, when they intersect large fissures, and this indicates that water descends to great depths principally through such fissures. The character of the rocks themselves has a great effect upon the depths to which water can penetrate, - some rocks being porous and with such open joints as to permit a free passage of water, while others are almost impervious. "It is probable that the universal presence of ground water is characteristic of a comparatively shallow surface belt, below which the water which has not been again drawn off at the surface, at a lower level, or has not been used up in hydration processes, is concentrated into the larger fissures." (Spurr).

Aside from the extremely slow movement of water through the mass of porous rock, underground waters follow the larger openings, such as joint-cracks, bedding planes, etc. The inclination of the stratified rocks, the alternation of porous and impervious beds, and the character of the joints and fissures are thus the factors which determine the direction of flow, especially in the shell of weathering, where the rocks are not saturated with water. In soluble rocks, such as limestones, the water may dissolve out its own channels. Surface topography has but a subordinate effect upon the course of underground waters, and it often happens that, for considerable distances, the surface and subterranean movements of water are in exactly opposite directions.

The factors which determine the movement of underground waters are of great practical importance in all problems of drainage and water supply. Serious evils have followed from carelessly taking for granted that the underground flow would be in the same (direction as that on the surface.

As the movement of underground waters is almost always excessively slow, their mechanical work is trifling, but chemically they bring about important changes. The water, making its way downward through the joints and bedding planes of the rocks, exerts its solvent and decomposing action upon the walls of these crevices, in the manner already described in connection with the work of rain. Down to the level of the ground water, or in the shell of weathering, percolating waters are the great agent of decomposition and therefore always contain more or less mineral matter in solution, the nature and quantity of which depend upon the character of the rocks traversed. Below the ground water level in the shell of cementation, the effects are more reconstructive than destructive, though solution and alteration of minerals continue at these lower levels.

In passing through limestones in the shell of weathering, percolating waters dissolve channels, great and small, through the rock. Pipes and sink-holes are dissolved downward from the surface, and in the mass of the rock great caverns are formed by the solvent power of the carbonated waters. Such caverns, as the Mammoth Cave of Kentucky, for example, are often many miles in extent and have considerable rivers flowing in them. Indeed, in limestone regions the smaller streams generally have a longer or shorter underground course. The lower level of the caverns is determined by the general drainage level at which the surface streams flow. In the shell of cementation the movement of water is very much slower and its solvent effects are much lessened. The beds of rock-salt, which would long ago have been dissolved away by moving waters, are found at depths which may be reached by mining or boring.

Fig. 47. - Sink-hole in limestone, near Cambria, Wyoming. (U. S. G. S).

When underground waters become highly heated through contact with hot volcanic masses, or by descending to great depths along channels which permit a return to higher levels, their solvent efficiency is greatly increased. Rocks penetrated by such thermal waters are profoundly altered in character and composition. The complex minerals of the igneous rocks are decomposed; the felspars become opaque from the formation of kaolin, or are altered to hydrated micas; minerals containing magnesia and iron give rise to talc, chlorite, serpentine, and the like, while the lime compounds are converted into the bicarbonate and carried away in solution. Some of the minerals are altered in place, and others are deposited in the crevices of the rocks. Thermal waters also alter minerals by bringing in new material in solution. In the Yellowstone Park the lavas of the great volcanic plateau, which has been deeply trenched by the Yellowstone River, are profoundly decomposed and altered by the hot waters which traverse it.

Fig. 48. - Canon and lower falls of the Yellowstone River. (U. S. G. S).

Fig. 49. - Profile of Turtle Mt., showing the amount of material removed in the Frank rock-slide. (Brock).

Except in caverns, underground waters flow too slowly to accomplish direct mechanical erosion, but indirectly they may bring about important mechanical changes. Masses of soil or talus, lying on steep slopes, saturated by long-continued, heavy rains, may have their weight so increased and their friction so reduced, as to glide downward in land-slips, which are sometimes disastrous. Of this kind was the great land-slip of 1826 in the White Mountains of New Hampshire.

Fig. 50. - Rock-slide of 1903 at Frank, Alberta. The lake in the foreground due to the damming of a stream by the mass of debris..

(Brock).

Rock-slides occur when the rocks forming a slope become saturated with water, until they can no longer support themselves. The movement is much facilitated by underlying beds of clay, or clay rocks, which become very slippery when lubricated with water. Mountain valleys in all parts of the world show plain evidence of such rock-slides, and often a vast quantity of rock is thus displaced. At Elm, Switzerland, in 1881, more than 12,000,000 cubic yards of rock were carried down for a distance of 2000 feet. In 1903 a great rock-slide occurred at Frank in the Canadian province of Alberta, when the entire face of Turtle Mountain fell and rushed across the valley in a huge avalanche of rock fragments, estimated at 40,000,000 cubic yards. The causes of this great rock-slide were several, but an unusual amount of ground water and a severe frost following warm weather were the chief agents.