ROCK FLOOR OF ALLEGHENY RIVER. 127 



In this connection it may be remarked that the river has a rock bed at 

 but few places in its entire length, and the rock floor lies usually 20 to 50 

 feet below the stream. In the headwater portion, as indicated below, it lies 

 at a great depth below the stream ; yet in that portion a rock ledge is 

 crossed by the stream at Limestone Falls, about 7 miles above the point 

 where the river returns from New York into Pennsylvania, and a fall of 3.84 

 feet occurs in a distance of 650 feet.' The stream there is near the left 

 bluff, and a buried channel is to be expected in the middle of the valley 

 that will extend perhaps 200 feet below river level. 



KOCK FLOOR. 



In the upper 20 miles of its course the rock floor is but little below 

 stream level, but in the next 20 miles it becomes covered to a depth of 

 about 200 feet, while at Olean, and for nearly 30 miles below that city to 

 Cold Spring Creek, it is covered to a depth of fully 300 feet. From the 

 mouth of Cold Spring Creek, near Steamburg, N. Y., the Upper Allegheny, 

 as indicated by Carll, formerly led away from its present course to enter the 

 Lake Erie Basin." 



Down the present Allegheny from Cold Spring Creek the rock floor 

 shows a rise of about 150 feet in the 25 miles to Great Bend, Pa., 8 

 miles above Warren, where the old divide pointed out by Carll has been 

 crossed. It drops about 70 feet in the 8 miles to Warren, below which, for 

 10 miles or more, it appears to hold a nearly uniform level about 1,100 feet 

 above tide. A descent then begins, which apparently continues the entire 

 1 75 miles to the mouth of the stream, at a, level 20 to 50 feet below low- 

 water level. In this lower end of the valley, therefore, it resembles the 

 rock floor of the Ohio, but in the upper part its relation to the present 

 stream is different. Although the rock floor in the lower end of the valley 

 shows a descent with the present stream, it does not follow that there has 

 been no change of drainage. Here, as on the Ohio, the old divides have 

 been cut down below the level of the stream sufficiently to give a gradient 

 in harmony with it. But in the upper part of the Allegheny the old streams 

 flowed in plains so far below the level of the present river that their rock 

 floors have not been touched by it. 



^Eoberts's Report, Senate Doc. 89, Forty-sixth Congress, second session, pp. 13 and 23. 

 2 Second Geol. Survey Pennsylvania, Kept. I^ 1880, pp. 333-355. 



