128 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



DESCRIPTION OF THE VALLEY. 



At the soin'ce of the Allegheny the dividing- ridges between the Alle- 

 gheny, Genesee, and Susquehanna rivers have an altitude of 2,500 to 2,6u0 

 feet. Down the Allegheny the highest points on bordering uplands fall to 

 about 2,500 feet near Port Allegany, to 2,425 feet near Olean, to 2,375 

 feet near Salamanca, and to jjerhaps 2,200 feet in the vicinity of Steain- 

 burg, where the present channel departs from the old one. There is a 

 slight rise southward from that point to the old divide near Kinzua, Pa., 

 the uplands reaching an altitude of 2,150 feet on the immediate borders of 

 the valley west of Kinzua. From this point there is a general descent 

 toward the mouth of the present river, the altitude near Warren being 

 about 1,950 feet, near Tidioute 1,750 feet, near the mouth of the Clarion 

 River 1,550 feet, and near Pittsburg 1,200 to 1,300 feet. A comparison 

 of these altitudes with the altitudes of the stream shows that the stream 

 flows at a level 500 to 800 feet or more below the highest parts of the 

 bordering uplands, while the rock floor in the deeply filled portions in 

 northern Pennsylvania and southwestern New York was cut 1,000 to 1,100 

 feet below the bordering uplands. The course of the stream, as may be 

 seen by reference to the glacial map (PI. II), lies within a few miles of the 

 glacial boundary, a large part being just outside the limits of the drift and 

 none of the valley far inside the drift border. 



The Allegheny Valley increases gradually in width in me northwest 

 portion from its source to the point where the present stream departs from 

 the old course near Steamburg, N. Y., the width being one-fourth to one- 

 half mile from Coudersport to Port Allegany, and one-half to three-fourths 

 mile from Port Allegany to the bend just above Olean, N. Y., below which, 

 as far as Cold Spring Creek, the width is a mile or more. 



Southward from the mouth of Cold Spring Creek the valley maintains 

 a width of a mile only as far as the vicinity of the State line. From 

 Corydon to Kinzua, Pa., it has a width of scarcely three-fourths of a mile. 

 At the old divide below Kinzua it naiTOws abruptly to a width of but 

 one-fourth mile, and remains narrow nearly to the mouth of Conewango 

 River at Warren. It there expands to a width of about a mile, and main- 

 tains this width from Warren to the bend near Irvineton, a distance of 8 

 miles. Upon passing south from Irvineton it soon decreases to less than 

 one-half mile, being much narrower than the valley of Brokenstraw Creek, 



