373 
resistant Mitchell limestone with its capping of Chester sandstones. The 
hard rock below has acted as a barrier permitting the stream to reach a 
graded condition where it passes over the non-resistant strata, while in 
the hard rocks time enough has. not yet elapsed to permit a graded condi- 
tion to come into existence. Second, the condition may be the result of 
rejuvenation by uplift, as explained by Ashley. Under this explanation the 
effects of the rejuvenation have not yet been transferred to the middle 
and upper reaches of the stream, and these upper reaches still possess the 
old graded condition while the lower reaches are steepened as a result of 
rejuvenation. The third explanation offered is the same as that given 
above as an explanation of the partial or local peneplain stretching north 
from New Albany. Under this explanation rejuvenation took place on ac- 
count of a major drainage line, the present Ohio River, taking the place 
of a small pre-glacial stream. 
It is likely that the peculiar gradient of Blue River is a combination of 
the three conditions offered in explanation. The effect of rejuvenation by 
uplift would ordinarily be transferred gradually up stream. The hardness 
of the rock of the lower reaches of the stream has much delayed the trans- 
ference of the rejuvenated condition of the stream gradient, permitting 
the retention of the graded condition in the mechanically unresistant rocks 
above. The later rejuvenation caused by the replacing of the minor local 
stream by an important major stream since the beginning of the Pleistocene 
has given an additional steepness of gradient to the lowest reaches of the 
stream. The transference of this steepened gradient beyond the lowest 
part of the stream has not yet taken place, because of insufficient time since 
the last change in stream regimen. It is not thought that subterranean 
drainage of much of the tributary space should influence the gradient of 
Blue River, unless it can be shown that much water which formerly went 
into it is now diverted to another stream by an underground channel. Blue 
River probably receives as much of this sort of drainage as it loses. Should 
considerable tributary space, however, have its waters diverted much 
farther down stream than these waters formerly entered, there would be 
some change in the gradient locally. It may be mentioned further that 
the graded condition of Muddy Fork of Blue River may have been partly 
brought about by the loss of a large tributary in the vicinity of Pekin by 
surface piracy. 
A NOTABLE CASH OF SUCCESSIVE STREAM PIRACY. 
The Development of Muddy Fork of Silver Creek. 
The factors as above outlined in the topographic development of the 
Knobstone cuesta region have permitted considerable areas of the old 
uplifted Tertiary peneplain to exist near the crest of the escarpment. But 
such a topographic condition as exists in the region of the escarpment 
with the present drainage systems is rather unstable. There is such 
an unequal amount of work being done by the set of streams that flow 
from the scarp eastward and northward and the set of streams which 
