RATE OF FALL OF ALLEGHENY RIVER. 125 



required to spread a deposit 40 to 100 feet in depth over the surface of the 

 gradation plain seems surprisingly great when the limited extent of glaciated 

 countr}' tributary to this drainage system is considered, But this must be 

 doubled or trebled to fill the trenches. It would be far in excess of the 

 outwash from the Wisconsin, for that has been sufficient to fill the trench to 

 an average depth but little more than 100 feet, and it had the advantage of 

 contributions from the earlier gravel deposits all along the line. 



It should not be inferred that the gradation plains of the Upper Ohio 

 region had reached only an incipient state of trenching down to the time 

 when those of the Middlp and Lower Ohio had suffered trenching to their 

 full depth. The glacial deposits of the Upper Ohio region apparently l^elong 

 to a stage of glaciation much earlier than that which furnished the drift of 

 the Lower Oliio region. The latter is of Illinoiau age, while the former, as 

 indicated below, seems to be fully as old as the Kansan drift, if not of pre- 

 Kansan age. A sufficient amount of valley trenching may have occurred 

 in the Upper Ohio region between the earliest glaciation and the Illinoian 

 stage to bring it into harmony with that of the Lower Ohio system. The 

 precise amount of trenching can scarcely be estimated in the present stage 

 of investigation. 



ALLEGHENY RIVER. 



The Allegheny River, the main headwater tril:)utary of the Ohio, di-ains 

 an area of about 11,500 square miles, 2,000 square miles being in south- 

 western New York, and 9,500 in northwestern Pennsylvania. The river 

 rises in Potter County, Pa., near the sources of the Grenesee and the Sus- 

 quehanna, and runs northwestward into southwestern New York. It there 

 turns southwestward and holds this general course to its mouth at Pitts- 

 burg. The length of the valley is about 325 miles. The length of the 

 stream is but little more, for tliroughout much of its course its meanders 

 conform to the windings of the valley. In the headwater portion, for about 

 160 miles, the ciuwes are less sharp than in the lower portion of the valley, 

 though tlie course is far from direct. In the lower portion the valley makes 

 several sharp oxbow loops, in some instances nearly severing the prominent 

 ndges inclosed by them. 



RATE OF FALL. 



The source of the river is near the crest of the Allegheny Mountains, 

 in passes which stand aVjout 2,200 feet above tide. The highest ridges in 



