How Mountains are Carved: Valleys.—It would be a mistake to suppose that the form of mountains as we now see them is usually due to the faulting, folding, and crumpling that they have passed hrough. These processes are very slow according to human standards. Hence even while they are in progress the rivers and to a much less extent the glaciers have a chance to carve valleys and carry away enormous amounts of rock. So far has this process gone that among the Rocky Mountains only rarely is it easy to detect the orig inal form due to movements of the earth's crust. Every little stream, and even every tiniest rivulet formed during a shower carries away part of the substance of the mountains and tends to form a valley. No matter how hard the rock may be, a river or even a small stream can eventually carve a valley thousands of feet deep and then with the help of its tributaries can widen that valley and reduce the steepness of its slopes until finally the very mountain tops melt down. Toward the end of the process the mountains become low and rounded like the White Mountains and the Adirondacks. The only parts that still stand high are those where the rock is particularly hard and resistant. Such mountains are called residual, and any one of them may be called a monadnock after a mountain of that name in southern New Hampshire.
During the early stages of their life-history, when the valleys are 3teep-sided and often very deep and precipitous, and when parts of the form due to the original uplift of the crust are still visible, moun tains are spoken of as young. At such times all the influences upon civilization which we shall later describe are at their greatest, as may be seen in many parts of the Andes. When the valleys begin to widen and the slopes become less steep, and the original form due to uplift has disappeared, as has happened in the Rockies, the moun tains are called mature. Their effect on civilization, however, is still very pronounced. Even when they become old with gentle slopes, wide valleys, and no great height this still remains true to a certain extent. In their final stages, however, the mountains are worn so low that they are reduced to a peneplain, that is almost to a plain. They then form a low rolling country with only a few monadnocks rising here and there as in the Piedmont region of the Atlantic slope, and are practically plains with all their advantages. 1,, How Plains are Formed.—Plains are formed by the wearing down of any kind of region to a gentle relief, or else by the deposition of materials brought down from higher regions. Most plains are of this latter sort. Some, such as the "high plains" of Colorado and Texas or the basin plains of Utah and Nevada, have been formed by streams which flow out from the neighboring mountains. When the streams
lose their velocity on reaching the lowlands, they at once begin to deposit their load of gravel and silt. They thus block their own channels and are forced to flow in new courses. Thus during the lapse of ages they flow now here and now there until finally they build almost level plains covering hundreds of thousands of square miles. Other plains, such as a large section of the central United States, were once part of the sea floor, and hence for millions of years received vast deposits of fine clay and silt brought by rivers from the lands. Then the movements of the earth's crust finally brought them almost un changed to a level above that of the oceans.
Plateaus and Basin Regions as Combinations of Mountains and Plains.—Vast portions of the earth's surface, such as the plateaus of Tibet, Peru and Arizona, and the basin regions of Persia and Utah, combine the features of mountains and plains. In the plateaus a plain or region of low relief has been uplifted, and streams have cut valleys in it. Thus the valleys and their slopes have the character of mountains while the uplands have some of the characteristics of plains. On the whole, however, most plateaus are so cut up that they are more like mountains than plains, as is clearly evident in the Allegheny Plateau. In the basin regions, on the other hand, a moun tainous country has been converted partly into plains, as may be seen by the way in which the peaks of half buried mountains often stick up through great plains of gravel in parts of Nevada. Often plateaus and basins are combined as in Mexico, where Mexico City is located on a high plateau, but also in a basin which is floored with a plain of soil brought down from the mountains.
It would be highly profitable to study the various kinds of moun tains, plains, and plateaus in order to see how each exerts its own special influence on man. We should find that even under similar condi tions of climate the mountains vary greatly in the degree to which they hamper transportation and agriculture, retard education and progress, or favor the sightseer and hunter. We should find that although most plains have relatively deep scil and dense population, and are comparatively easy to traverse, they differ greatly in these respects. Unfortunately, the limits of space oblige us to confine our study to the contrast between the life of typical mountains and typical plains. We shall talk chiefly about the mountains, however, because this is the only chapter where their influence is fully dis cussed. Plains are so important that they form the chief theme in the chapters on Soil and Agriculture.