LINDGHEN.] 



RESUME OF GEOLOGICAL HISTORY. 



637 



westerly trend, while subsequent events have more and more forced 

 it in a northwesterly direction. PI. LXXXVIII illustrates well the 

 present condition at the mouth of the canyon the basalt flow cut in 

 two and the two Pleistocene flood plains in the widening valley of the 

 river. Pliocene gravel older than the flood plains underlies the basalt. 

 In the right background the Miocene sandstones of Table Mountain 

 are shown, while the background at the extreme right shows the first 

 granitic hills of the Boise Ridge. 



Having thus traced the cycle of events which have taken place in 

 this region since the beginning of the Neocene, it may be well to 

 rapidly review the main points in the history. 



RESUME OF GEOLOGICAL EVENTS IN THE LOWER SNAKE RIVER 



BASIN. 



Pre-Neocene . 



Pre-Neocene or early Neo 

 cene (Miocene) . 



Early Neocene ( Miocene ) , 

 possibly extending over 

 into late Neocene. 



Late Neocene (Pliocene) . . 



Pleistocene ... 



/ The depression of the Snake River Valley outlined by 

 I orographic movements. 



f Long-continued erosion, dissecting the Boise Moun- 

 tains. 

 f Boise Canyon eroded to its present depth at its mouth. 



Large fresh-water lake occupying Snake River Val- 

 ley. 



Deposition of thePayette formation at least 1,200 feet 

 thick. 



Highest level reached, 4,200 feet above present sea 

 level. 



Eruptions of rhyolite and andesitic basalts, contem- 

 poraneous with the sedimentation. 



Orographic disturbances of the Payette formation. 



Partial drainage of the lake and epoch of erosion 

 excavating the valleys of the Snake, Boise, and 

 Payette from the Payette lake-beds. 



Basalt flows, filling the Snake River plains and the 

 Boise Canyon. 



Deposition of lacustrine sediments between the flows. 

 (Idaho formation.) 



Complete draining of lake. 



f Erosion of the Snake River basalt canyon above the 

 confluence with the Boise; Boise River cuts down 

 through the basalt to its present depth. 



1^ Terraces, up to 100 feet above the river, and present 

 flood plains along the Lower Snake, Boise, and 

 Payette rivers. 



