262 Chaniberlin and Leverett — Studies of the 



average fall is less than two-thirds of this. In the upper' por- 

 tion, 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 has here, however, cut down 145 feet below the old 

 floor. But this lessens rapidly up stream, and at 20 miles 

 above the mouth, it is reduced to about 60 feet.* 



Fig. 2. 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 serves to show the increase in the rate of fall of Red- 

 bank river in its lower 20 miles, a feature due to the deeper trenching of that 

 portion. The profile also serves to show the extreme narrowness of the col 

 which separates the Redbank and Susquehanna systems, the tunnel beneath the 

 col being but 1950 feet in length. 



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, only made itself seriously felt upon the gradient of the 

 Redbank in its lower 20 miles. Such an abnormal deepening 

 is accounted for by the sudden enlargement of the drainage 

 area to several times its previous size in consequence of the 

 diversions of drainage previously discussed. We do not think 

 a simple change in the altitude, or in the general slope of the 

 region, would produce a result of precisely this nature. The 

 main stream, of course, usually leads in rejuvenated excava- 

 tion, but not in such a disproportionate degree as this nor in 

 precisely this method. 



An objection to the northwestward outlet may perhaps 

 seem to be presented by the deposits of gravel which occur 

 along the Allegheny valley between the mouth of French 

 creek and the mouth of the Clarion. In several places, not- 

 ably at the bends of the river at Brandon, at a point two miles 

 below Brandon, at Kennerdell, at Black's (Winter Hill station), 

 and at Emlenton, there are deposits on the face of the gorge 

 extending from near the river's edge up to heights of 200-300 



* Compare statement of Prof. I. C. White respecting the relative altitude of 

 water deposits on the upper and lower courses of the Conemaugh, Youghiogheny 

 and Cheat rivers, this Journal, November, 1%%*!, p. 378. 



