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The next step, or platform, the Cretaceous, slopes down gradually or dies out on the step beneath it; then comes the Jurassic, which ends in white sandstone cliffs several hundred feet high; then the Triassic, which ends in the famous vermilion cliffs thousands of feet high, most striking in color and in form; then the Permian tread, which also ends in striking cliffs, with their own style of color and architecture; and, lastly, the great Carboniferous platform in which the canon itself is carved. Now, all these various strata above the canon, making at one time a thickness of over a mile, were worn away in Pliocene times, before the cutting of the Grand Canon began. Had they remained, and been cut through, we should have had a chasm two miles deep instead of one mile.

The cutting power of a large, rapid volume of water, like the Colorado, charged with sand and gravel, is very great. According to Major Dutton, in the hydraulic mines of California, the escaping water has been known to cut a chasm from twelve to twenty feet deep in hard basaltic rock, in a single year. This is, of course, exceptional, but there have, no doubt, been times when the Colorado cut downward very rapidly. The enormous weathering of its side walls is to me the more wonderful, probably because the forces that have achieved this task are silent and invisible, and, so far as our experience goes, so infinitely slow in their action. The river is a tremendous machine for grinding and sawing and transporting, but the rains and the frost and the air and the sunbeams smite the rocks as with weapons of down, and one is naturally incredulous as to their destructive effects.

Some of the smaller rivers in the plateau region flow in very deep but very narrow canons. The rocks being harder and more homogeneous, the weathering has been slight. The meteoric forces have not taken a hand in the game. Thus the Parunuweap Canon is only twenty to thirty feet wide, but from six hundred to fifteen hundred feet deep.

I suppose the slow, inappreciable erosion to which the old guide alluded would have cut the canon since Middle Tertiary times. The river, eating downward at the rate of one sixteenth of an inch a year, would do it in about one million years. At half that rate it would do it in double that time. In the earlier part of its history, when the rainfall was doubtless greater, and the river fuller, the erosion must have been much more rapid than it is at present. The widening of the canon was doubtless a slower process than the downward cutting. But, as I have said, the downward cutting would tend to check itself from age to age, while the widening process would go steadily forward. Hence, when we look into the great abyss, we have only to remember the enormous length of time that the aerial and subaerial forces have been at work to account for it.

Two forces, or kinds of forces, have worked together in excavating the canon: the river, which is the primary factor, and the meteoric forces, which may be called the secondary, as they follow in the wake of the former. The river starts the gash downward, then the aerial forces begin to eat into the sides. Acting alone, the river would cut a trench its own width, and were the rocks through which it saws one homogeneous mass, or of uniform texture and hardness, the width of the trench would probably have been very uniform and much less than it is now. The condition that has contributed to its great width is the heterogeneity of the different formations–some hard and some soft. The softer bands, of course, introduce the element of weakness. They decay and crumble the more rapidly, and thus undermine the harder bands overlying them, which, by reason of their vertical fractures, break off and fall to the bottom, where they are exposed to the action of floods and are sooner or later ground up in the river’s powerful maw. Hence the recession of the banks of the canon has gone steadily on with the downward cutting of the river. Where the rock is homogeneous, as it is in the inner chasm of the dark gneiss, the widening process seems to have gone on much more slowly. Geologists account for the great width of the main chasm when compared with the depth, on the theory that the forces that work laterally have been more continuously active than has the force that cuts downward. There is convincing evidence that the whole region has been many times lifted up since the cutting began, so that the river has had its active and passive stages. As its channel approached the sea level, its current would be much less rapid, and the downward cutting would practically cease, till the section was elevated again. But all the time the forces working laterally would be at work without interruption, and would thus gain on their checked brethren of the river bottom.