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Show 278 GEOLOGY OF THE HIGH PLATEAUS. latter debouched. The Fremont River, however, still maintains its course in one of those old canons for a distance of 4 or 5 miles. It leaves the low flats of the valley to enter the rising slopes of the Awapa and flows through a rocky gorge which becomes four or five hundred feet deep. Thence it emerges into the valley plain again and pursues its way to the foot of the valley, where a salt marsh, covered with saline pools, has been built up by the accumulation of fine silt. It is interesting to pursue this subject further and to view it in relation to future instead of past time. The river leaves the valley through the great gap between the mountain and the Aquarius, and the passage has been named the Red Gate. Thence it flows off into the heart of the Plateau Country, reaching the Colorado by a profound canon. Throughout the greater part of this distance the river is a rapid stream and is slowly sinking its channel. Its rapid descent begins half a mile beyond the point where it crosses the great fault, and it is apparent that here, too, it is lowering its bed; for old terraces of river gravel and loess are seen at different levels within the Red Gate in an excellent state of preservation, and the river has cut a broad and deep channel through them. It is only a question of time how deep the channel may be cut, for where it leaves Rabbit Valley the altitude is almost exactly 7,000 feet above sea-level, and the junction of the river with the Colorado is less than 4,500 feet. Estimating the course of the stream between the two points at 100 miles, the average descent is not far from 25 feet to the mile, which is about the same fall as prevails in nearly all the tributaries of the Colorado in this part of the country. All of them are evidently corrading their beds. Here and there local flood plains are formed, occurring along stretches of the streams where the fall is slight; but such flood plains are merely temporary in the secular life of the river. The}7- are succeeded by rapids which are gradually eating their way backwards, and in a brief period the stretches of still water will become rapids in turn. In time, then, the Fremont River will cut down its channel at the outlet of Rabbit Valley unless the fault at the Red Gate increases its throw. In the absence of such increase in the fault the stream will ultimately carry back the excavating process into the valley and the extensive alluvial beds will be gradually attacked and eroded away. |
