288 GEOLOGY OF THE HIGH PLATEAUS. 



proposition that tlie 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 hori- 

 zontally. 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 hori- 

 zontal stratum of Lower Eocene beds upon which the local drainage sys- 

 tem 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 



