GRASS 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 apparentlj^ antedates the 

 last general uplifting of the plateaus by a very long period, and its course 

 and general airangement were probably determined by the configuration 

 of the country which was made at the close of the trachytic epoch of erup- 

 tions. 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 estab- 

 lished 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 depres- 

 sion between the central and southern trachytic vents of the Sevier Pla- 

 teau, 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- 



