1914] 



Buwalda: Pleistocene Beds in the Mohave Desert 



455 



precipitation, which is supposed to have characterized the climate 

 of glacial time. Regarding the latter possibility, it is of interest 

 also that the Manix Lake deposits apparently record two periods 

 of lake expansion with an intervening period of lake contraction 

 or dessication. This is presumably caused by two periods of 

 relatively abundant water supply separated by a period of rela- 

 tively slight precipitation. These conditions are a parallel to 

 the two periods of expansion and the intervening period of 

 contraction or dessication believed by Russell to have obtained 

 in Lake Lahontan ; this lake is considered to have existed during 

 the glacial period. Another possibility is that Manix Lake was 

 formed by the turning of the Mohave River into this basin as a 

 result of the deformation which the land surface underwent just 

 previous to the Manix Lake time. The possibilities above consid- 

 ered, so far as they relate to Manix Lake, cannot in our lack of 

 more detailed knowledge, be considered more than conjectures. 



If the course of the Mohave River lay across this territory in 

 pre-lacustral time, it seems probable that the initiation of lacustral 

 conditions came about through the uplift across its path of such 

 barriers as the irregular anticline east of Afton, through which 

 the river has since cut a canon. 



Disappearance of the Lake. — At the point of outflow over 

 the rim of the basin east of Afton the Mohave undoubtedly low- 

 ered its channel with comparative ease through the fanglomerates 

 arching over the older rocks. Its downcutting in the underlying, 

 more resistant rocks undoubtedly proceeded more slowly, how- 

 ever, and it is a notable fact that the lake-beds were deposited 

 approximately up to the level of the top of the older rocks where 

 these underlying formations are exposed in the section along the 

 Mohave just east of Afton. In this locality the river has cut its 

 channel downward at least seventy-five feet in these more resist- 

 ant rocks (pi. 25, fig. 2). 



During the existence of the lake its level gradually fell as 

 its outlet was lowered by downcutting. Simultaneously the level 

 of the accumulating sediments gradually rose. When the falling 

 level of the lake and the rising level of the sediments met, the 

 lake became extinct. Further downcutting has slowly lowered 

 the local base level of the Mohave and enabled it to trench the 



