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individual layer of softer rock continues to waste away more rapidly than the harder bed above it. Thus eventually a river-valley appears bounded, not by vertical cliffs, but rather by a succession of horizontal tiers of precipitous faces, corresponding to the outcrops of the several strata of harder rock— separated the one from the other by the longer or shorter slopes yielded by the shales.
Finally, we may further note that the recession of the cliffs will be much influenced by the rate at which their basal portions are undermined. Each slice removed from a steep rock-face narrows the width and increases the inclination of the sloping stage above. Hence, as Captain Dutton has clearly shown in his admirable description of the Colorado Canons, the descent of ddbris from each stage is facilitated, while the weathering of the soft rocks and the undermining of the overlying harder beds are accelerated. Thus, curiously enough, as the same author remarks, the state of affairs at the bottom influences the rate of recession at the summit.
When a river has reached its base-level and ceases to erode, the valley-slopes and cliffs, nevertheless, under the influence of weathering, continue to retire. The ddbris showered down from above now tends to accumulate below, and thus affords protection to the rocks against which it is banked. And the talus thus formed continues to rise higher and higher. The exposed strata above, however, having no such protection, weather as before, each rock-tier retreating, but at a gradually diminishing rate. What form the ground will ultimately assume will largely depend upon climatic conditions. If the climate be moist and frost be active in winter, the sharp edges of the rocktiers will be bevelled off, and the sloping surfaces will become heavily laden with ddbris and disintegrated rock-material, the further degradation and removal of which will be retarded by the growth of vegetation. Thus, in time, the sharp angles will tend to disappear, and a somewhat undulating slope will replace the more strongly marked features which the same rocks would have yielded under arid conditions.
Let us now recall what was said as to the cutting up of our elevated plain into a multiplicity of flattopped segments, and we shall see reason to conclude that these segments must be bounded by steep faces, the aspect of which will vary according to the nature of the strata and the character of the climate. If the climate be arid, and the strata consist of alternate hard and soft beds of variable thickness, the bounding walls of the segments may in some places be approximately vertical, or they may show a succession of short cliffs with intermediate sloping stages. If, on the other hand, the climate be moist, those features will be more or less softened and modified. In the former case step-like profiles will abound ; in the latter the ground will likewise ascend in stages, but these will be less accentuated, and may even be in large part replaced by continuous slopes. Again, each flattopped segment of the denuded area, eaten into on all sides, will continually contract, the bounding cliffs and slopes retiring step by step until they eventually meet atop. The flat summit now disappears, and is replaced by a sharp crest, ridge, peak, or rounded top, as the case may be. Each diminishing segment, in short, ultimately acquires a more or less strongly pronounced pyramidal form. This, however, is not the final stage. Denudation continues—pyramidal hills, dome-shaped heights, and crested ridges gradually crumble down, until at last all abrupt and prominent irregularities of surface disappear, and the once elevated plain returns to its former state, that of a gently undulating or approximately flat stretch of low-lying land. The cycle of erosion is completed.
Thus in the erosion of a plateau of horizontal strata we recognise the following stages :—(1) The excavation of deep trenches by streams and rivers; (2) the gradual sapping and undermining of cliffs, etc., the widening of valleys, and the consequent cutting up of the plateau into a multitude of flat-topped blocks or segments; (3) the progressive contraction of the segments, and their conversion into pyramidal or roundtopped hills and crested ridges; and (4) the continued reduction and lowering of the hills and final resolution of the plateau into a plain.
This plain, in the hypothetical case we have been considering, is supposed to be at a level very little above that of the sea. But the minimum level to which a region tends to be reduced need not be at such a low elevation. The streams and rivers discharging into a great lake or inland sea cannot erode their valleys below the level of the quiet water which is the receptacle of their sediment. That surface becomes for them a base-level of erosion, and all their energies are employed in the task of reducing to that level the land over which they flow. Soon or late, however, the outlet of the lake will be deepened, the surface of the latter will fall, and the base-level will, of course, be lowered at the same time. But should a slow movement of elevation affect the lower end of the great lake, and thus, by counterbalancing the work of river erosion at its outlet, maintain the surface at approximately the same level for a prolonged period of time, then denudation may eventually succeed in reducing to that base-level all the lands that drain into the lake. The lake might be entirely silted up, but so long as the movement of elevation persisted, and the river (at the former outlet of the lake) continued to saw its way down as rapidly as the ground was upheaved, the old base-level of erosion would be maintained.
We may now return to the Grand Canon district and the question of its erosion. During the progress of the great denudation the interior spaces of the district, according to Captain Dutton, "occupied for a time the relation of an approximate base-level of erosion." The whole region has been greatly elevated, but this upheaval was not effected all at one time. On the contrary, in place of one single continuous movement a succession of uplifts has taken place, each separated from the other by a period of repose. It was during one of these prolonged pauses that enormous sheets of strata, averaging some 10,000 feet in thickness, were gradually broken up and removed from the surface of the Carboniferous rocks, while the latter themselves were planed down to a flat expanse. This Carboniferous platform served for a long time as a base-level of erosion. The horizontal masses under which it lay buried were first deeply incised by the Colorado River and its affluents and their countless tributaries. The strata thus became broken up into innumerable separate blocks or segments, which, little by little, were reduced in size and most of them eventually demolished. But before the last remaining "buttes" and "mesas" could be removed a great change supervened. A general upheaval of the entire area for several thousand feet took place, and the base-level to which the district had been so largely reduced was destroyed. The gradients of all the rivers now increased, and the velocity of the currents was correspondingly augmented, with the result that the erosion of ravines and cafions recommenced.
It is beyond the purpose of these pages to trace further the history of the Grand Canon district. But those who wish to have an adequate conception of what is meant by river erosion would do well to consult Captain Dutton■s work. From it they will learn how the Colorado River has, within a very recent geological period, dug out a valley " more than 200