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Show 220 GEOLOGY OF THE HIGH PLATEAUS. or plains, their velocity is at once checked by the diminished slope and the coarser debris comes to rest. These streams lie (within the mountains) in ravines usually profound, with steep flaring sides, and opening upon the valley bottoms or plains through magnificent gateways, and every long range or ridge has usually many such gateways opening at intervals of a very few miles along its flank. At the gateway the stream begins to surrender a part of its freight and to build up its channel. The check given to the velocity of the stream here is marked, indeed, but less incisive than might at first be supposed. The profile of the bed of the stream does not have an angle at this point, but is curved very gently, and is concave upward. Indeed, it is so throughout the entire course of the stream outside the gate and generally for a considerable distance inside the gate. Thus the velocity of the stream slows down gradually and not suddenly. As the velocity gradually diminishes so the stream gives up more and more of its load. But the stuff which it drops along any small part of its course is by no means of the same size; that is to say, there is no rigorous sifting of the material in such a manner that the stones or particles at any given place are of uniform size, while finer ones are carried on to be scrupulously selected where the slope and velocity are less. On the contrary, all sorts are deposited everywhere. Nevertheless there is a tendency to sorting. Higher up the slope there is a greater proportion of coarser deposit; lower down there is a larger proportion of finer deposit; but everywhere the coarse and the fine are commingled. Where the stream is progressively building up its bed outside of the gate, it is obvious that it cannot long occupy one position; for if it persisted in running for a very long time in one place it would begin to build an embankment. Its position soon becomes unstable, and the slightest cause will divert it to a new bed which it builds up in turn, and which in turn becomes unstable and is also abandoned. The frequent repetition of these shiftings causes the course of the stream to vibrate radially around the gate as a center, and in the lapse of ages it builds up a half-cone, the apex of which is at the gate. The vibration is not regular, but vacillating, like a needle in a magnetic storm; but in the long run, and after very |
