1914] Buwalda: Pleistocene Beds in the Mohave Desert 453 



overlying finer fanglomerates on the north flanks dipping in the 

 same direction and disappearing under the nearly horizontal lake- 

 beds. The absence of remnants of the lake-beds on the upper 

 back slopes of the upthrown block may indicate that some of the 

 displacement occurred in pre-lacustral time, and that much of 

 the upthrown block was not covered by the lake waters. The 

 areal relations, dip, and folding of the lacustral beds at the west 

 end of the scarp plainly indicate post-lacustral movement (pi. 25, 

 fig. 1). 



Whether the fault is of the normal or of the reversed type 

 cannot be stated with certainty. The gentle contortion of the 

 lake-beds and younger fanglomerates, where they dip into the 

 fault at the west end of the scarp, suggests compression and 

 faulting of the reversed type. The contact plane referred to 

 above dips seventy-five degrees northward under the older fan- 

 glomerates, but whether its attitude is evidence as to the type 

 of faulting is uncertain. 



The degradation of the scarp and the wide breach cut across 

 it by Mohave River indicate the lapse of considerable time since 

 the faulting. While it is not possible to estimate accurately the 

 length of time, the faulting perhaps occurred not long after the 

 extinction of the lake. It may be that the faulting and the pre- 

 lacustral folding represent a single deformative period beginning 

 in pre-lacustral, and extending into post-lacustral, time. 



The flat surface of the lake-beds about Manix is believed to 

 represent nearly the top of the lacustral deposits except where 

 slightly modified by one or two washes and by wind-blown sand. 

 That this is not a flood plain developed by Mohave River below 

 the upper surface of the lake beds is indicated by the absence of 

 well-rounded fluviatile material on the present surface, and the 

 absence of terrace remnants of a higher surface about the sides 

 of the basin. Railroad levels indicate that this plane surface 

 has been tilted ten or twelve feet per mile to the east in the 

 vicinity of Manix ; in a north-south direction the surface is 

 practically level. A thin accumulation of quartz and feldspar 

 grains, left behind when the upper few inches of the soft con- 

 taining lake-beds were blown or washed away, invariably covers 

 this undissected surface. 



