362 C. R. KEYES OROGRAPITTC ORIGIN OF LAKE BONNEVILLE 



latter has merely sunk straiglit down a sliort distance, witli no bottom 

 lands bordering and no yalley on either side^ sloping gradually down to 

 the water's edge. Only along its lower reach^ before it debouches into the 

 Columbia Eiver, does it become notably canyon-cutting. This fact is as 

 remarkable as it is unexpected. From any commanding point the pros- 

 pect which spreads out before the eye leaves the impression that the 

 Snake Eiver was somewliere toward its headwaters suddenly turned ont 

 of its original course on to the illimitahle lava plain and there picked a 

 way that was the lowest line. Surely this stream can not have worked 

 very long in its present position. AVith the volume of water which it 

 carries, with a high gradient of 8 feet to the mile, and with an elevation 

 of 4,500 feet above sealevel in southeastern Idaho, it should, if it really 

 were an old river, have its channel sunk deep in the ground in a canyon 

 that in. grandeur would be second only to the Grand Canyon of the Colo- 

 rado in Arizona. Farther north, in central AYashington, the Columbia 

 River, when it was dammed by an ice-tongne in late Glacial times, cut the 

 deep canyon of the Grand Coulee, 100 miles long, through solid basalt. 



The edges of lava flows which cross the course of the Snake River at 

 several points where they produce falls do not seem to have been suffi- 

 ciently extensive to act as dams to prevent the river's normal cutting 

 effect. These falls a23pear rather to attest the extreme recency of the 

 date at which the stream was directed over its present course. 



OROGEAniic Movi-:ment in Bonneville Area 



REGENT DTA ATROPHIC CHANGES 



Although in the Great basin the effects of mountain-making move- 

 ments are wide-spread and extensive, they mainly date to a more remote 

 ]3eriod than has been generally assumed. Except in a few places, late 

 diastrophic change nowhere appears to impart to the region its dominant 

 relief expression. Nevertheless in the Bonneville jjasin recent diastrophic 

 activity is quite appreciable. It is measurable l)y the differences in eleva- 

 tion of the ancient shoreline at different points. The plane of the old 

 water level is a warped surface, with an incremental valne between ex- 

 tremes of nearly 400 feet, or ;il)()iit the distance between tlie Bonneville 

 and ProNX) terraces. 



Over the western half of the basin the jiresent differences in elevation 

 of tlie old lake surface is barely 25 feet. In the other hali' of the basin 

 there is a pronounced eastward tilting of nearly 200 feet. Thus is shifted 

 the present (jreat Salt Lake against the ])ase of the AVasatch Range, 

 whereas otherwise the remnantal Ixxly of water would be occupying the 

 Great Salt Desert 50 miles to the southwest. 



