698 IDAHO MINING DISTRICTS. 



gravels." Among them are the deposits of Gold Hill, East Hill, and 

 Barker's claim at Idaho City, as well as the Ranch Company's claims 

 and other gravels near Placerville. They rest partly on the lake 

 beds, partly on granite; and while these gravels at Idaho City may 

 have been deposited by Moore Creek as very high bench gravels, the 

 gravels at Placerville form a part of a drainage system differing from 

 the present one. 



During the period of general erosion following the retreat of the 

 Payette lake important events took place. The lower canyon of 

 Moore Creek was scoured of its accumulated gravels. The lake beds 

 were disturbed and acquired a decided dip to the west. One block 

 of them at Idaho City sunk down between fault lines, being thus pre- 

 served from the erosion which destroyed the larger part of them.- 

 The gravels laid down on the lake beds also show a tilting westward, 

 though at slighter angles than the latter. Near Placerville and 

 Granite there were extensive disturbances, which greatly changed 

 the old drainage. Along the eastern side of the Boise Ridge these 

 disturbances, it would .appear, took the form 'chiefly of faulting, for 

 along this line the old gravels are cut off; but the amount of this 

 faulting is not easy to establish. In this connection the occurrence 

 of a little gravel on the summit of the Boise Range is of great inter- 

 est, and it is difficult to explain.. It is not believed, however, that 

 the disturbance was so great as to create the whole of this ridge at 

 this time. 



Finally, at the close of the Pliocene came the eruption of the Snake 

 River basalts. The Moore Creek flow originated at some point on 

 Grimes Creek a few miles below the basin, and as it dammed the 

 stream to an elevation of 100 feet the natural result was the accumu- 

 lation of bench gravels above by the checking of erosion. As the 

 creek gradually wore through the basalt filling, the level at which 

 bench gravels were formed gradually sunk. Thus the bench gravels, 

 lining the stream up to 100 feet above their beds, are directly due to 

 the Pliocene basaltic eruption, and represent in the basin the deposit 

 of the Pleistocene times. 



