MANSFIELD: POST-PLEISTOCENE DRAINAGE. 81 
X would be degraded while that above X would be aggraded. In 
that part of the valley side included in the angle GXA, any terrace 
plain, IX, would have a steeper slope than the interfluve surface GX, 
while above the point X the former surface XC would be cloaked with 
new gravels and no terraces would be distinguishable. In the region 
under discussion there was no tendency toward aggradation. On 
the contrary the streams at all the places noted, even well up toward 
their sources, were uniformly degrading. If aggradation was taking 
place anywhere, it was in lower courses of the streams remote from the 
locality studied. 
If tilting in the reverse direction had occurred (Figure 7, IV), under 
the uniform conditions here postulated, the portions of the stream 
courses above X would be degraded while those below would be 
aggraded and any terrace plain XI in the degraded portion CXB 
would have less inclination than the interfluve surface AG. "Here 
again the conclusion is contrary to the observed fact. 
If the region were warped the base level would be raised or lowered 
according to whether the general effect on the region in question was 
one of depression or elevation. In the former case, other conditions 
remaining the same, the streams would aggrade their valleys; in the 
latter, the base level AE (Figure 7, V) would be lowered, as in GH. If 
the warping were broad enough to permit all parts of the surface in 
the region under discussion to be raised above its former grade, now 
represented by GD, as in AD, the effect would be similar to that 
already outlined for the case of simple uplift and would agree with 
the observed facts. Degradation would occur all along the line until 
the grade GD was attained and terrace slopes formed in the down 
cutting would be steeper than the interfluve surface. If, however, 
the warping were more local, so that part of the region were sunk 
below its former grade while other parts were raised above it, as in 
AC, the portion AX so raised would be degraded while the other por- 
tion XC would be aggraded. The effect would be similar to that of 
tilting as above noted and would not agree with the observed facts. 
From this discussion it seems clear that climatic oscillation, the 
occurrence of which is not denied, is not alone sufficient to account 
for the phenomena noted; but that simple uplift, or broad upward 
warping, was an important cause of the down cutting. 
DRAINAGE CHANGES. As a result of this incision various changes 
in drainage have been inaugurated in the region and may be seen in 
active progress. The upturned members of the Permian to Cretaceous 
strata are but poorly protected by the overlying gravels. Subsequent 
streams are accordingly being developed along the strike of the weaker 
