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Show 288 GEOLOGY OF THE HIGH PLATEAUS. proposition that the rivers are older than these structural features, that their courses were initially determined by the configuration of the surface when the region emerged from its lacustrine condition in Middle Eocene time, and have persisted in holding those initial positions in spite of all changes. It happens, however, that in the cases before us the flexure is much older than the rivers. The age of the Water Pocket monocline is Pre-Tertiary, at least in the northern part, and we infer that the whole monocline is of one age. This seems at first to be in contravention of the law. But the anomaly is apparent only and not real. For we have seen that in Thousand Lake Mountain the Tertiary lies nearly horizontally across the denuded edges of the Cretaceous and Upper Jurassic and rests upon the Jurassic white sandstone. The same relation is found in the Aquarius. In the eastern half of the plateau the Cretaceous is wanting and the Tertiary rests upon the Jura. A little west of the middle of the plateau upon the southern flank is seen another ancient monocline with its throw in an opposite direction to that of the Water Pocket flexure. This, too, is of Pre-Tertiary age, and upon its slopes the Cretaceous again comes in with full force, and across its beveled edges lies the Lower Eocene horizontally. Thus while this pair of flexures was forming the intervening uplifted block was undergoing erosion, and at a later epoch it was submerged to receive a blanket of Lower Eocene strata. If now we attempt to replace the beds which have been stripped off by the later erosion of Miocene and Pliocene time, we must extend the Tertiary beds eastward (and southward) indefinitely, so as to cover the Water Pocket flexure unconformably, and also to cover the Cretaceous mesas which lie beyond it. Thus, after the Middle Eocene, the locus of the flexure was covered with a sensibly horizontal stratum of Lower Eocene beds upon which the local drainage system was laid out. As the erosion went on the streams sank their channels and the upper strata were denuded. The Water Pocket fold was in time exhumed and the streams cut down into it from above. And since its exhumation it has been greatly ravaged by erosion. Directly east of us, beyond the domes of the flexure, rise the Henry Mountains. They are barely 35 miles distant, and they seem to be near neighbors. Under a clear sky every detail is distinct and no finer view of |