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Show TUFACEOUS BEDS OF EAST FOBK CANON. 243 EAST FORK CANON. East Fork Canon is a great chasm cut through the Sevier Plateau transversely at its narrowest part, dividing that uplift into two portions. It is wholly the work of erosion, and is an excellent example of the persistence of a river channel in spite of the great displacements of the country along its course. The East Fork of the Sevier .River carries the entire drainage of Grass Valley, and has evidently done so through several long geological periods. Grass Valley, as will be seen by the map, is the long narrow depression lying at the eastern base of the Sevier Plateau, and is parallel to Sevier Valley, lying west of the plateau. Between the loci of these two valleys the plateau has been, through the later periods of geological time, gradually hoisted several thousand feet. The uplift has been greatest upon the west side of the table, which is bounded by the great Sevier fault. From the western crest-line the plateau slopes eastward ; at first very gently, then with a more pronounced descent as far as the wall of the Awapa Plateau. There is no fault on the east side of the Sevier table, but in some portions there is a cliff or abrupt slope caused by long ages of erosion. Ten or twelve miles north of the canon are the central vents of the Sevier Plateau, already described as of very ancient date. Twelve or thirteen miles south are found the great andesitic and still greater trachytic centers of eruption. Far back in Pliocene time this fork flowed between these volcanic piles from east to west, joining the main stream of the Sevier River at the foot of Circle Valley. The great changes of topography produced by the elevation of the Sevier Plateau have in no manner affected the location of the fork, which has only sunk its channel as the table slowly ascended. Very grand and imposing is the valley which it has carved through this uplifted mass. It is not one of those deep, narrow chasms cut into the earth, but a terraced valley of notable width, a distance of 2 to 5 •••<- miles separating the summit walls, with only a narrow bottom below. In the natural section thus made nearly 4,000 feet of beds, composed wholly of volcanic materials, are exposed. The river near the point of maximum cutting just grazes the top of the yellow Tertiary lacustrine beds, exposing only a few acres, but enough to assure us that we have here the entire volcanic series. |