372 C. R. KEYES OROGRAPHIC ORIGIN OF LAKE BONNEVILLE 



Aviiids and wind-driven sands. Situations of two kinds appear to have 

 escaped — those on the lee side of the main body of water and those on 

 the lee side of islands or monntains. The maximnm abrading action of 

 winds in dry countries is near the level of the general plains surface. 

 The terraces on the east shore were protected from this action by the 

 Avaters of the lake itself. On the northern and eastern flanks of the 

 mountain isles, like the Oquiri'h Txange^ tlie lake terraces retain their 

 freshness longer than elseA\herc because of the fact that the prevailing 

 winds could ]iot reach them. 



On the west shore of the old lake^ Avhere the Avaters Avere shalloAV^ Avhat- 

 ever terraces there were AA^ere necessarily quickly destroyed either by being 

 removed by Avind-action or by being overAvhelmed by drifting sands from 

 the adjoining deserts. 



PLANORASION BY WINDS 



In descriptions of Bonneville basin the smooth intermont plains are 

 repeatedly accounted for by regarding them as old lake bottoms. The 

 evidence is at first glance so seemingly obvious that the idea has been 

 extended to cover all of the flat valleys of the Great basin as Avell as 

 other parts of the arid regions. In desert tracts the notably plains-form- 

 ing tendency of Avind-action a]3pears to be its most characteristic result. 

 For a large proportion of the arid intermont plains the lacustrine hy- 

 pothesis has noAv to be given up. It is highly probable, also, that even on 

 the plains over Avhich once stood the Avaters of the lake the surface lias 

 by the late action of Avinds been made smoother than AAdien the Avaters 

 originally covered the areas. 



DERIVATION OF LAKE SEDIMENTS 



It is commoiily assumed that the lake sediments AAdiich are displayed 

 in considerable thickness throughout the Bonneville basin are mainly de- 

 rived from the fine materials held in suspension by the Avaters of the 

 tributaries entering the lake. Eecently closer examination indicates that 

 these lake l^eds are not composed of clay, but of materials of much coarser 

 texture — particles like those of loess or adobe. This being the case, the 

 inference is that the materials making up the beds are chiefly merely the 

 adobe soils blown into the lake from the neighl)oring deserts. This vieAV 

 is also supported by other classes of evidence. 



ROCK-FLOORS OF DESERT PLAINS 



Tiie Bonneville basin is one of the fcAv desert plains AAdiich is probably 

 more or less deeply filled Avitli soil materials. Although most intermont 



