370 ('. K. KEYES OKOGRAPHIC ORIGIN OF LAKE BOJNNEV^ILLE 



BoiuicviJle (;liiiiate is not at all strcngtliciiiiig-. An entirely new exami- 

 nation of the data wonld doubtless lead to wholly different conclusions. 



GLACIAL HYPOTHESIS OF LAKE'S ORIGIN 



At the time when the climatic explanation of lake genesis was ad- 

 vanced, no other hypothesis seemed available. Indeed, the possible duality 

 of the Glacial epoch was then receiving wide attention. The fact that in 

 arid regions an entirely different set of erosional agencies come into play 

 that are fundamentally distiiu^t from those which are most active under 

 conditions of moist climate had not been suspected. In the attempt to 

 make a great lake in a desert land a necessary consequence of the near-by 

 presence of a continental ice-advancement, a strong bias existed and an 

 erroneous interpretation of facts resulted. Nowhere in all the elaborate 

 discussion that took place on the subject was there any direct or critical 

 evidence adduced to show that the time of the greatest expansion of Lake 

 Bonneville coincided with that of the farthest southward advance of the 

 northern ice-sheet. In light of the recent observations demonstrating 

 that this could not be, all the arguments formerly presented in support 

 of this contention prove notably weak. 



So much stress has been put on tlie direct association of glaciation 

 with the Great Basin lakes that the competency of the common cause 

 now demands definite evaluation. Most considerations are qualitative 

 only. Since the recent careful ma})ping of the glaciers of the Uinta and 

 AYasatcJi Mountains by Prof. W. W. Atwood^'^ quantitative calculations 

 are now i)ossible. The most conspicuous feature thus brought out is 

 their insignificance. So inconsequential must have been their influence 

 on lake conditions that one almost wonders Avhy the two were ever genet- 

 ically associated at all. Singularly, too, the very points of contact, the 

 meeting of lake sliore and moraine, where the critical testimony would 

 be expected to be adduced, are the most inconclusive of all. 



An important climatic feature which in mountainous arid lands seems 

 to be quite generally overlooked is the notable change in humidity that 

 takes place with elevatioji. The higher parts of the loftier mountains 

 enjoy moist, temperate conditions in the midst of the dry, torrid condi- 

 tions of the surrounding desert. Often when the latter has an annual 

 rainfall of only 5 to 10 inches the precipitation on the mountain tops 

 rises to as high as 30 inches and more, or about the same that prevails in 

 tlie upper Mississippi Valley. Under conditions such as these glaciation 

 might well be initiated on the high points without appreciably affecting 

 the moisture content over the lowland ]:)lains. When, further, it is re- 



1^ U. S. Geol. Survey, I*rottv^sioual i'aper Nu. 01, 1909, 



