MANSFIELD: POST-PLEISTOCENE DRAINAGE. 79 
QUESTION OF UPLIFT. As in the case of the Black Hills, the early 
erosion of the Bighorn Mountains, the accumulation of the gravels 
and the subsequent incision of both may be explained by either of the 
hypotheses of crustal movement or climatic oscillation. As regards 
the earlier erosion history, the writer has collected no data in favor of 
either hypothesis; but he has found some evidence which seems to 
indicate uplift, in some form, as a cause of the later incision; and 
that is the relation of the angles of interfluve surface and terrace plain 
slope above mentioned. 
Climatic Oscillation. If we consider the hypothesis of climatic os- 
cillation we must assume a constant base level. Under such conditions, 
any increase in the cutting power of the stream, permitted by an in- 
crease in the relative humidity of the region, would tend to lower the 
grade and make the angle of slope of the terrace plains AC and AD 
(Figure 7, I) less than that of the interfluve surface AB. If now more 
arid climatic conditions should return and cause the streams to cease 
degrading and to begin instead a process of aggradation, the ensuing 
changes in the slopes of the terrace plains would be simply the reverse 
of those already indicated and in no case would the slope of a lower 
terrace plain be steeper than that of its neighbor above. It seems 
evident therefore that climatic oscillation cannot be regarded as the 
only cause of the incision noted. 
Crustal Movement. The hypothesis of crustal movement may be 
considered under the several heads of simple uplift, tilting, and warping. 
If the region were to undergo simple uplift as a rigid body the result 
would be to lower the base level AE to the position GH (Figure 7, II) 
without altering the angle of inclination of the interfluve surface AB. 
Assuming the process of uplift to be sufficiently slow to permit the 
grading process practically to keep pace with it, so that we need not 
consider the retreat of a waterfall in our section, we should find that 
all the terrace plains, such as GC, formed in the down cutting, would 
be steeper than the interfluve surface AB. As uplift slackened and 
ceased, if other conditions remained unchanged, the later terrace 
plains would gradually become parallel with the interfluve surface 
AB, as in GF. This supposition accounts for all the observed facts. 
If the region were tilted on an axis shown in section at F (Figure 7, 
III) in such a manner as to make the line AE, which formerly coincided 
with base level, take the position GH, the interfluve surface AB 
would take the less inclined position GC. In order to maintain the 
grade AB, which the uniformity of other conditions here assumed 
would necessitate, the portion of the stream course below the point 
