Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Streams are subject to more disastrous floods in some stages of their development than in others. Floods resulting from heavy rains are likely to be greatest where the slopes above the drainage lines are on the whole greatest, for this is the condition under which the water is most quickly gathered into the drainage channels. The most disastrous floods, humanly speaking, are those which affect wide-bottomed valleys, where the flats are settled. In such cases a relatively slight rise may flood very extensive areas. In such valleys the most disastrous floods are generally in the spring, when the waters from the melting snows of the preceding winter are being discharged.[43] Many other considerations enter into the problem of floods. The presence of forests and other forms of vegetation on the slopes retards the flow of water into the valleys, and so tends to prevent floods, or at any rate to make them less severe. Porous soil and subsoil, or in their absence porous rock, absorb the rainfall, and prevent its prompt descent into the valleys and so tends to prevent or diminish floods.

The acreage of arable land within a given area stands in some relation to its drainage development. At an early stage in its erosion history, before an upland has been dissected by valleys, nearly all of it may be arable. Later, when drainage is at its maturity, and when hillsides and ridge slopes constitute a large part of the area, there is probably the least acreage of arable land. This is especially true if the slopes are so steep as to allow the soil to be readily washed away. At a still later stage, when the valley bottoms have become wide and the slopes of the ridges and hills so reduced as to be available, the area of cultivable land is again increased.

Marshes, ponds, and lakes have some bearing on the resources and industries of a region, and they stand in a more or less definite relation to the stage of erosion in which a region finds itself. In its youth ponds and lakes may occupy much of the surface; in its maturity they will have been largely drained.

These suggestions are sufficient to show that the topography of a region, even in so far as shaped by erosion, touches human interests at many points.

ANALYSIS OF EROSION.[44]

Erosion is the term applied to all the processes by which earthy matter or rock is loosened and removed from one place to another. It consists of three sub-processes, namely, weathering, transportation, and corrasion.

Weathering.

The term weathering is applied to nearly all those natural processes which tend to loosen or change the exposed surfaces of rock. The lettering of inscriptions on exposed marble becomes fainter and fainter as time goes by, and finally disappears, because the rock in which the letters were cut has weathered away. Some of it has crumbled off as the result of the expansion and contraction induced by changes of temperature, and some of it has been dissolved by the rain which has fallen upon it. In this case the weathering is effected partly by the atmosphere and partly by water. These are the chief, but not the only agents concerned in the general processes of weathering. Those phases of weathering which are the result of the activities of the atmosphere, whether physical or chemical, have been discussed in connection with the atmosphere (pp. [42] and [54]).

The rain which falls upon the surface of exposed rock, and that which sinks through the soil to the solid rock below, dissolves, even if slowly, some of the rock constituents. Each constituent of a rock composed of several minerals may be looked upon as a binding material for the others. When one is dissolved the rock crumbles, much as mortar does when the lime which cements the sand is dissolved.

The solution of mineral matter by ground water, as well as the other chemical changes it effects, is greatly augmented by the impurities, especially carbonic and other organic gases, dissolved by the water from the atmosphere and the soil. The commonest chemical changes effected by the joint action of water and air, oxidation and carbonation, have been referred to in [Chapter II]. Hydration is more exclusively the work of water, and is one of the commonest processes of rock change, and often of rock disintegration. Numerous other less simple chemical changes resulting from the activities of ground water are constantly in progress, and in so far as they lead to the disintegration of rock are processes of weathering. Many chemical changes involve notable changes in volume of the mineral matter concerned. Merrill has calculated that in the conversion of the granitic rock of the vicinity of Washington, D. C., into soil, its volume has been increased 88 percent., largely as the result of hydration.[45] Even when the chemical changes do not themselves directly involve the disintegration of the rock, the accompanying increase of volume is sometimes sufficient to cause its physical disruption. This also may be regarded as a phase of weathering.