TUFACEOUS BEDS OP EAST FOBK CA^ON. 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 persist- 

 ence 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 geolog- 

 ical 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 thir- 

 teen 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 topograph} 1 - pro- 

 duced 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. 



