376 



successive capture of parts of it by different branches of the invading 

 Muddy Fork. Fairly distinct sags in the ridges between the small sepa- 

 rate systems strengthen this suggestion. If such a drainage adjustment 

 ever toolv place, it has been so long ago that only these slight evidences of 

 it remain. Such an adjustment would be possible, and if it did actually 

 take place, it is probable the only case of its kind described. 



The first stream which still retains direct evidence of having once 

 drained into the Blue River system is Dry Fork Branch. This stream now 

 empties into Muddy Fork of Silver Creek, about a mile below Borden. It is 

 the first of the series of barbed tributaries. In succession the tributaries 

 of the old northwest drainage line were annexed to the Silver Creek sys- 

 tem. A number of these, especially those coming in from the north, are 

 decidedly barbed. The latest ones to be taken in were those in sections 

 29 and 32, between Pekin and Borden. Evidence of this successive capture 

 of the tributaries of the northwest extending stream is not found alone 

 in tlie barbed drainage pattern. The gradation plain formed by the north- 

 westerly flowing stream has not been entirely destroyed. To the northwest 

 of Borden, just above the town, is preserved the oldest recognized portion 

 of the old gradation plain. Quite a large remnant is preserved here, and it 

 still retains the silts and gravels of the old stream bed. This remnant is 

 shown beautifully on the topographic map. Fig. 1 shows its even line as 

 quite a distinct feature where it has been cut into by the reversed drainage. 

 The elevation of this ancient valley remnant is about 755 feet, whereas the 

 present reversed valley floor is 575 feet in elevation. This means that, the 

 drainage change permitted the old gradation plain to be trenched at this 

 place something like ISO feet. At the mouth of Dry Fork Branch the en- 



Fig. 1. — View of the even surface (sky ihie) of a remnant of the old gradation 

 plain of the former northwestwardly flowhig stream just west of Borden. View taken 

 from the south side of the present reversed valley. The present reversed stream is 

 entrenched at this place 180 feet below the old gradation plain. The gradation 

 plain remnant here has preserved upon it old gravels and silts similar to those shown 

 in Fig. 5. 



