380 



Silver Creek system. The small wash from the north now sends its water 

 into both systems. The rapid headward erosion of the new system will 

 soon cause all of it to be deflected to the southeast. Likewise, the remain- 

 ing tributaries of the old system must be taken over into the new system. 

 Muddy Fork of Blue River would normally remain at its present elevation 

 for a long period. In the meantime, the new system will invade farther and 

 farther to the northwest, and in a short time, geologically speaking. Muddy 

 Fork of Blue River itself will be taken over into the Silver Creek system. 

 The headward erosion of the invading system will be relatively rapid, since 

 it has mainly alluvium to work upon in order to capture Blue River. One 

 might go still further in anticipation of this successive piracy. The invad- 

 ing system will extend itself in the direction of the present flow of Muddy 

 Fork of Blue River and capture tributary after tributary of the present 

 stream, just as it has done in the past after capturing Dry Fork Branch of 

 the old system. By following the line of a graded stream in this manner, 

 the successive stream piracy must be relatively rapid. Such successive 

 stream piracy will continue as long as the stream gradient in the reversed 

 direction is more favorable for headward erosion than the normal present 

 direction. Whenever these stream gradients reach a balanced condition 

 the adjustment is complete and the drainage systems have arrived at the 

 beginning of the old age condition. 



Development of Special Bed-Rock, Terraces. 



Another result of the above described stream adjustments must be men- 

 tioned in this paper. The barbed tributaries of Muddy Fork of Silver 

 Creek have been adjusting themselves to a direction of flow in accord with 

 that of the main stream where they enter it. They have a tendency to 

 adjust themselves in such a manner that the junction of the main stream 

 and the tributary form an acute angle pointing in the down stream direc- 

 tion. Practically all of the barbed tributaries have been and are making 

 this adjustment. Those on the north of the main stream have much more 

 perceptibly oriented themselves in the down stream direction than those on 

 the south. This is because the dip of the rock favors a migration of the 

 main stream against the south bluff, especially in the non-graded iwrtion of 

 the valley. This has resulted in a shortening of the tributaries from the 

 mouth and the consequent nullification of their orientation in the down- 

 stream direction of the mainstream. While this direction adjustment of the 

 Itarbed tributaries has been taking plac(\ the valleys liave also been deep- 

 ened by down cutting. The combination of this direction adjustment at the 

 mouth of the tributaries and the down cutting has permitted bed-rock 

 benches or terraces to come into existence on the upstream side near the 

 mouths of the tributaries. (See Fig. r>.) Some of the tributaries have 

 more than the one set of terraces. They range in height from 10 to 25 feet 

 above the present valley flat or above one another. ,,It is probable that new 

 accession of drainage due to capture above has had something to do with 

 the development of these bed-rock terraces, since the resulting more vigor- 



