452 University of California Publications in Geology [Vol. 7 



tains. The fangiomerates are believed to be but slightly older 

 than the Manix Beds. That this deformation was pre-lacustral in 

 date is shown by the limitation of the lake-beds to the west side 

 of this anticlinal divide. Further, the presence of beds of 

 extremely coarse, unworn, undecomposed material, with little ad- 

 mixture of fine fragments, at some horizons in the fanglomerate 

 series indicates that the slopes bordering the basin were steep- 

 ened during the deposition of the fangiomerates. These coarse 

 deposits are in sharp contrast with the deposits of gentle slopes, 

 which are finer and contain only occasional large rock masses. 

 The Manix Beds near Afton are tilted gently to the west and 

 north, due apparently to continued arching along the north-south 

 anticlinal axis mentioned above. The absence of embankments 

 composed of well-rounded fluviatile material within the narrow 

 valley cut in the lake-beds by Mohave River above the point where 

 it crosses the arched rim of the basin at Afton, probably indicates 

 that no sudden uplift has occurred along this axis since the 

 Mohave commenced cutting into the lake beds. Such an uplift 

 across its path would suddenly lower the river gradient and 

 cause the stream to deposit embankment materials in its valley 

 for a certain distance above the barrier. 



A bold east-west scarp which is traceable for about two miles 

 rises along the north bank of the Mohave south of Field (pi. 24, 

 fig. 2). While the most striking part of the scarp has been cut 

 back and emboldened by the undercutting of the Mohave, its 

 general extent marks a fault of perhaps one to two hundred feet 

 displacement in the fangiomerates and Manix Beds. Near the 

 west end of the escarpment the lake-beds and the fine fangiom- 

 erates immediately underlying them may be seen dipping north- 

 ward at a low angle into the coarse older fangiomerates, which 

 have been upthrown and likewise dip to the north (fig. 1). The 

 lake-beds and underlying fangiomerates are gently contorted 

 where they dip into the fault. Three-fourths of a mile eastward 

 down the Mohave River there is exposed a nearly vertical plane 

 of contact between the coarse older fanglomerate and the finer 

 material; it is perhaps not the main fault-plane (pi. 24, fig. 1). 

 Where the Mohave crosses the escarpment still farther down 

 stream the coarse fangiomerates dip gently northward, with the 



