454 University of California Publications in Geology [Vol. 7 



No evidence of movement in Recent time was noted in the 

 region. Three river terraces cnt on the fanglomerates and ex- 

 tending along the Mohave near the fault south of Field are, to 

 the eye, parallel with the present river bed. No accumulations 

 of rounded gravels are found on these terraces. It is believed 

 that their formation was due to inequalities in the rate of low- 

 ering of the local base level, controlled by the downcutting 

 through the barrier below Afton. Terraces formed by accumu- 

 lation of rounded gravels, which might indicate slackening of 

 stream gradient through deformation, are apparently absent. 



Mode of Origin and Cause of Disappearance of Manix Lake 



Mode of Origin. — Manix Lake came into existence as the 

 result of the ponding of waters furnished principally by Mohave 

 River. Unless the climate of the region was much less arid in 

 Manix Lake time than it is today, it required a stream of consid- 

 erable size to support this lake during the entire year against 

 the drain of two hundred or more square miles of evaporating 

 surface and sufficient overflow to keep the lake water fresh. This 

 stream must have headed in an area of much greater precipitation 

 than occurs in this desert region. The attitude of the lake-beds 

 indicates that, in this immediate region, topographic conditions 

 have not changed so greatly since Manix time as to suggest the 

 possibility that some stream other than the Mohave was the source 

 of the water. Other than the Mohave there exists, moreover, no 

 notable stream in the region. 



If the absence of rounded gravel in the twenty-five-mile section 

 of fanglomerates exposed by Mohave River in the Manix Lake 

 basin indicates that this stream did not pass through the region 

 in pre-lacustral time, it is quite possible that its entrance into the 

 basin Avas effected simply by the lengthening of its lower course 

 in a direction determined by the relief. The Mohave is a stream 

 which derives its waters from the higher parts of the San Ber- 

 nardino Mountains and wastes away in the dry desert. Such 

 extension of this stream might be the result either of increased 

 water supply due to uplift of the San Bernardino Mountains, 

 or the extension may have been the result of the greater general 



