MANSFIELD: POST-PLEISTOCENE DRAINAGE. (8 
of Post-Oligocene age” occurred after the completion of the aggrada- 
tion stage, for the abandoned saddles and gravel patches are asso- 
ciated with the upper level of the gravel deposits. ‘These changes 
could not therefore have been the result of late Tertiary uplift or tilting, 
as suggested by Darton. They were probably initiated by the con- 
ditions that produced the second period of incision. ‘The “recent 
stream robbing” seems to the writer to be but the uniform continuation 
of the “offsetting” just discussed, for his observations show that all 
the larger captures at least, in the Boulder-Whitewood Creek region, 
took place before the later gorges were cut, and that the altitudes of 
the saddles and gravel patches are all related to the upper, more gentle 
slopes of the valley walls. The persistent offsetting of drainage, in 
such a manner as to develop northeast-flowing master streams is not 
satisfactorily explained by climatic oscillation alone and appears to 
indicate northeast tilting. It seems hardly probable that Whitewood 
Creek, for example, could retain its supremacy and trench the anti- 
cline on which it has been superposed, had not its direction coincided 
with that of tilting. That the tilting began rather slowly and was 
later accelerated seems to be indicated by the contrast in the slopes 
of the valley sides of earlier captor streams, such as the one which 
diverted Whitewood Creek from its Boulder valley course, and the 
streams of the present gorges. In the former case the valley sides are 
flaring, while in the latter incision has been so rapid that the slopes 
are generally precipitous. 
The argument here set forth lends support to the view that Post- 
Oligocene incision began as the result of a gradual increase in relative 
humidity and that the later aggradation, by which the Pleistocene 
gravels were deposited, was occasioned by a return to more arid cli- 
matic conditions. Minor oscillations within cach of these periods 
may have occurred but no evidence bearing on that side of the question 
is now at hand. In later Pleistocene times a gradual northeast tilting 
produced a second incision which was accompanied by extensive 
readjustments of drainage and was later accelerated so that steep- 
sided gorges were produced. This view does not appear to be incon- 
sistent with Johnson's conclusions with regard to the Great Plains 
region, for it is entirely possible that differential uplifts in former 
areas of disturbance like the Black Hills and Bighorn Mountains 
might die out rapidly in the region of the Great Plains, in which case 
the ensuing destructive and constructive effects would die away also. 
Moreover the lake shores of the Great Basin, to which Johnson appeals 
in support of his theory of climatic oscillation without crustal move- 
