| OCR Text |
Show GBASS VALLEY. 249 in part eroded. The eastern wall of the valley is the uplifted side of the third plateau range, comprising the Fish Lake table at the north, the Awapa in the middle, and the Aquarius at the south. This wall is everywhere due to displacement. The western side of the valley is a wall of erosion formed by the river sinking its channel and the subsequent decay of the mesas by secular waste. The origin of the valley apparently antedates the last general uplifting of the plateaus by a very long period, and its course and general arrangement were probably determined by the configuration of the country which was made at the close of the trachytic epoch of eruptions. The valley then lay between two long lines of volcanic vents, one in the Sevier Plateau, the other in the Awapa, with a broad lava field between them. The vertical movements which subsequently upheaved those tables did not displace the course of the drainage, which only established itself the more immutably in its original position. • The lowest point of the valley is not at either end, but a little south of its mid-length, opposite the head of East Fork Canon. To this point two streams flow, one from the north, the other from the south, and their waters, here uniting, pass through the canon to join the Sevier. It was evidently so from a remote epoch. The great canon itself was at first a mere depression between the central and southern trachytic vents of the Sevier Plateau, but as that mass was upraised, the fork persisted in holding its thoroughfare and cut the rising platform in twain. At one epoch the rate of elevation was sufficiently rapid to dam the fork and create a lake in the valley, which may have been 15 or 20 miles in length. Remnants of old lake beaches are still visible on the southern and eastern sides of the valley, and these possess considerable interest. They are best displayed where Mesa Creek merges from its gorge in the northwestern angle of the Aquarius. They consist of beds which are composed of a mixture of the ordinary detritus which comes from the waste of sedimentary sandstones and that which is derived from the decay of volcanic rocks. Where the former greatly preponderates, the resulting strata have the usual aspect of the lacustrine Tertiary deposits; and where the latter is in great excess the beds have the same appearance and characters as the stratified tufas else- |
