72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
On the other hand Johnson (p. 628-631) in a study of the Great 
Plains comes to the conclusion that both the construction and dissec- 
tion of the heavy gravel deposits that constitute the upper members 
of the sedimentary series in those regions may be explained as the 
result of climatic oscillations without the aid of crustal movements. 
He argues that since the streams of the Great Plains have cut through 
the covering gravels to the old topography beneath and are there 
showing a tendency toward planation, the former inclination of the 
surface was not materially different from the present; or, if at all, it 
was probably greater because there is no indication that the earlier 
streams reached grade; hence it is unlikely that deformation could 
occur and yet permit the return of substantially the original conditions. 
He suggests a possible correlation of the changes in the Great Plains 
area with the different stages of the Quaternary Lakes of the Great 
Basin region and cites as evidence of the quiescence of Pleistocene 
times the general parallelism of the former lake shores. 
The question is thus raised whether the modifications of Black Hills 
drainage and topography in Pleistocene and Post-Pleistocene times 
may not be explained as the result of climatic oscillation rather than 
of uplift. On the assumption of a constant base level changes of 
climate from less to greater relative humidity, and the reverse, would 
produce corresponding changes in the activity of streams, the former 
condition resulting in degradation, the latter in aggradation. 
If, as Darton states, the larger topographic features of the region 
were developed in Pre-Oligocene time, it is probable that the broadly 
opened ancient valley of Boulder Creek was of Pre-Oligocene origin 
and that it was revived in the early Pleistocene period of erosion and 
aggraded to a level somewhat lower than the shoulder that marked 
the level at which Post-Oligocene incision began. Figures 2 and 3 
represent the shoulders carved at this time on the slopes of Pillar Peak, 
White Rock, and other hills. This revival and subsequent aggrada- 
tion may be explained by either of two hypotheses or by a combination 
of both: (1) uplift followed by subsidence; (2) climatic oscillation. 
In the absence of positive evidence of uplift or subsidence in connec- 
tion with the revival and aggradation of Boulder valley and in the 
light of Johnson’s work on the Great Plains and the work of Gilbert 
and Russell on the Great Basin area, it seems wiser to assign these 
early changes to climatic oscillations, since it is very generally believed 
that such oscillations occurred within the period to which these changes 
are referred. : 
The “offsetting of Pre-Oligocene valleys northward through canyons 
