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to Captain Dutton, the average thickness of strata removed from an area of 13,000 to 15,000 square miles cannot have been under 10,000 feet. This may seem a startling conclusion, but it is based on evidence which cannot be gainsaid. Throughout the major portion of the plateau-country horizontal Carboniferous strata occupy the surface. As these are followed northward they gradually dip in that direction under younger strata (Permian, Mesozoic, and Cainozoic rocks), until they are buried at last to a depth of 10,000 feet and more. Now Captain Dutton has shown that this vast thickness of overlying strata formerly extended throughout the whole Grand Cafion district. This is proved by the fact that many outliers or relics of the rocks in question still remain, scattered at intervals over the broad surface of the Carboniferous strata. They form conspicuous table-shaped and pyramidal hills, rising more or less abruptly above the great Carboniferous platform. The accompanying diagram shows the general relations of those isolated "buttes" and "mesas," as they are termed, to the underlying Carboniferous rocks and the strata at T, of which they are detached outliers. The dotted line (a-b) indicates the level originally attained by the plateau. All the rock that formerly existed between a-b and the surface of the Carboniferous strata (C) has been denuded away.
How has this enormous erosion been effected, and what are the more prominent features of the denuded area? A low-lying plain of accumulation, such as a delta, cannot experience much erosion; the surface is approximately level, or has only a very gentle inclination, and any water flowing over it must be sluggish and ineffective. But conceive such a plain upheaved for several hundred feet, and it is obvious that the fall of the river to the sea will then be increased and its erosive action greatly augmented. It will therefore proceed to dig a deeper and wider course for itself. Now let us suppose that an elevated plain is traversed not by one main river only, but by numerous affluents, each with its quota of tributary streams. The running waters will continue to deepen their channels until the gradient by the process is gradually reduced to a minimum and vertical erosion ceases. The main river will be the first to attain this base-level—a level not much above that of the sea. The plain-track will gradually extend from the sea inland until the same low gradient is attained throughout the whole course of the river. In time all the affluents with their tributaries will arrive at the same stage.
But rivers do not only cut vertically; they also undermine their banks and cliffs, and thus erode horizontally; hence it follows that the valleys will be widened as well as deepened. The widening process may be greatly aided by the action of wind, rain, springs, and frost. Not infrequently, indeed, these agents play as important a part as the streams themselves. Under the conditions now described an elevated plain will in course of time be cut up into more or less numerous segments, the upper surfaces of which will represent the original level of the land; where the interval between two valleys is wide we shall have a broad, flat-topped segment; where the interval is short the segment will be correspondingly restricted in size. In a word, the segments will vary in extent according to the multiplicity and intricacy of the valley-system.
A word now as to the form of the slopes and cliffs bounding the valleys. We are dealing, it will be remembered, with an elevated plain of accumulation. The horizontal strata, we shall suppose, are more or less indurated beds of conglomerate, sandstone, shale, and limestone. All rocks, as we have seen, are traversed by natural division-planes ox joints, and these in the case of stratified rocks consist of two sets intersecting each other and the planes of bedding at approximately right angles. Horizontal strata are in this way divided up into rudely cuboidal, quadrangular, or rectangular blocks. Joints are, of course, lines of weakness along which, when rocks are undermined, they tend to give way. Thus when horizontal strata are cut into by rivers and undermined they break off at the joints, and vertical cliffs result. It does not often happen, however, that in a considerable series of strata all the beds are of quite the same character. Frequently some are relatively harder and unyielding, while others are softer and more readily reduced. Let us suppose that the uppermost bed cut into by the river is somewhat hard and difficult to grind through. In time the water saws its way down into the succeeding stratum, which we shall take to be a soft or easily eroded shale. In the overlying hard rock the river has been able to cut merely a narrow steep-sided trench. The shale, however, offers much less resistance to the vertical and lateral action of the water, and is thus rapidly intersected and washed away from underneath the superincumbent harder stratum. The latter, losing its support, then yields along its joint-planes, and a larger or smaller slice is detached from the wall of the cliff and falls in ruins. In this way the cliffs gradually retire as they are undermined—in a word, the ravine is not only deepened but widened.
Much of the rock ddbris dislodged from the cliffs falls into the river, and is gradually broken up and carried away; but some comes to rest at the base, forming a talus, and thus retards the denudation of the shale. To the action of the river we must add that of other epigene agents, such as wind, rain, springs, and frost, under the influence of which the shale weathers away more rapidly than the overlying rock, and eventually forms a sloping stage upon which the dSris derived from the receding cliffs continues to accumulate. Meanwhile, however, the river digs down through the shale and encounters, we shall suppose, another thick stratum of hard rock. Lateral erosion by the running water is now reduced to a minimum; slowly the current saws its way down
F1g. 1o. D1agrammat1c Sect1on Show1ng Stages Of Eros1on By A R1ver Cutt1ng Through Hor1zontal Strata. (After Captain Dutton.)
A, relatively hard rocks; j, relatively soft strata; r r, river at successive stages as valley is deepened and widened.
vertically, just as it did in the uppermost unyielding bed, until it again reaches a second layer of shale. The undermining action is now repeated, and a second line of rock-wall begins to retreat in the same manner as the first. And so the process goes on with all the succeeding strata through which the river cuts, until it finally attains a minimum gradient and ceases to erode. But note that, while the deepening of the ravine proceeds, the cliffs never cease to retire. Each