LOWER ALLEGHENY DRAINAGE SYSTEM. 



147 



Evidence that additions have been made to the upper drainage basin 

 of the Allegheny, is furnished by the features of the tributar}^ valleys. It 

 is found that the trenching of the gradation plains on the tributaries is 

 conspicuous only in their lower courses. On the Redbank River, which 

 enters 22 miles below the Clarion, accurate data are obtainable, since the 

 railroad follows its valley for 70 miles and has a grade neai-ly coincident 

 with the stream and but a few feet (20 to 40) above it. The jiroiile of 

 this railroad (see fig. 7) brings out the significant fact that the stream has 

 a much more rapid fall in the lower 20 miles of its course than for some 

 distance above that point, which is the reverse of the normal law of 

 mature streams. The average fall for this 20 miles is nearly 12 feet 



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Fig. 7.— Profile along a portion of the Low-Grade Division of the Allegheny Valley Railway (so named because of the 

 low altitude at which it crosses the Allegheny Mountains). It shows the increase in the rate of fall of Redbank 

 River in its lower 20 miles, a feature due to the deeper trenching of that portion. The profile also shows the extreme 

 narrowness of the col which separates the Redbank and Susquehanna systems, the tunnel beneath the col being 

 but 1,950 feet in length. 



per mile, while for the next 20, or even 50, miles above, the average fall 

 is less than two-thirds of this. In the upper portion the present floor of 

 the stream nearly corresponds with an old floor. In the lower portion, this 

 old floor continues on to the mouth with a rate of descent a little less than 

 that of the upper portion, following the normal law. The later stream 

 here, however, enters the Allegheny about 150 feet below the old floor; 

 but this lessens rapidly upstream, and at 20 miles above the mouth it is 

 reduced to about 60 feet.' 



It appears quite evident from these facts that there has been an 

 abnormal deepening of the Allegheny since the formation of the old floor, 

 and that this has been so recent that it has, as yet, made itself seriously felt 



1 Compare statement of I. C. White respecting the relative altitude of water deposits on the upper 

 and lower courses of the Conemaugh, Youghiogheny, and Cheat rivers: Am. Jour. Soi., 3d series, 

 Vol. XXXIV, 1887, p. 378. 



