EXPRESSIONS OF ARID EROSIVE INFLUENCES 373 



plains of tlie arid regions appear to be areas of profound degradation 

 and to have well defined rock-floors only scantily covered by soil, the 

 valley under consideration seems to be an exception. The lake sediments 

 are of especial interest because of the fact that they are mainly neither 

 cla3^s nor sands, but materials of intermediate-sized gTain. Instead of 

 being brought in by streams, these materials were originally doubtless 

 dusts that settled on the surface of the lake waters. The persistent body 

 of water was the only tract where wind-blo\A'n soils could come to rest 

 within the limits of the arid land. The area presents a marked contrast 

 to the Mojave plains to the southward, where the upturned strata are 

 conspicuously beveled, the resulting surface being a typical rock-floor 

 which in many spots is swept clear of soil. 



Recapitulation 



Summing up the salient features of the foreging record of recent ob- 

 ser^■ations on the conditions surrounding the genesis of a great body of 

 water in a desert region, it appears that 



(1) Tlie presence of such a lake as ancient Lake Bonneville is not 

 necessarily dependent on notably moister climate than at present ; 



(2) The existence of the lake is genetically associated with the recent 

 and extensive. uprising of the Colorado dome; 



(3) The Red Rock Pass marks the location of the main tributary 

 rather than the outlet of the old lake; 



(4) The lake probably ne\er had an outlet, the disposition of its aug- 

 mented waters being entirely accounted for by the high evaporation and 

 the ready lateral expansion of its borders ; 



(5) The Bonneville Lake occupied an old but little disguised river 

 l)asin, across the lower end of \\'hich developed an orographic barrier that 

 the ancient stream was unable to corrade apace, the lake thus formed 

 representing an orographically dammed river valley; 



( 6 ) The beginning of the lake epoch long antedated the Glacial epoch, 

 even the time of greatest expansion being long prior to the commence- 

 ment of Glacial time ; 



(7) The extensive modifications of tlie ancient river's headwaters have 

 occurred immediately before, during, and subsequent to the Glacial epoch ; 



(8) The pre-lacustrine river suffered both depedalization and decapi- 

 tation ; 



({>) The lake's liistory prior to the formation of the highest shore- 

 lines was probably niucli longer than tlie lapse of time since; 



