402 Foshay — Preglacial Drainage of Western Pennsylvania. 



lower Beaver valleys) at a higher elevation than any of the 

 Pleistocene terraces. It is most marked in the Beaver valley 

 on account of its being formed for the most part from the 

 more resisting rocks of the Conglomerate Series, but it is 

 easily made out in the Ohio valley from Pittsburg to Beaver 

 and also far up the Allegheny and Monongahela valleys. 

 South of Beaver, on the Ohio, it has not been observed. Ascend- 

 ing the Beaver the plain falls from 915 feet A. T. at Beaver 

 Falls to 890 feet A. T. at the mouth of the Connoquenessing — 

 a distance of ten miles. There can be no doubt that this was 

 once the bed of an ancient river at a time long anterior to the 

 First Glacial epoch, and, from its northward fall, the stream 

 must have flowed in that direction. The plain thus indicates 

 a long epoch, when the preglacial drainage had not yet cut the 

 deep canons which marked the topography of the later Ter- 

 tiary period. The clayey deposit over the plain belongs to a 

 Pleistocene epoch antedating that during which the terminal 

 moraine was formed as it seems to pass beneath the moraine 

 with its kames at the point of contact. It seems to record an 

 episode when the continent was lower than now. Possibly it 

 may be contemporaneous with McGee's Columbia formation (?) 



Following the period of the base-level plain came long ages 

 of high continental elevation — higher than the present — dur- 

 ing which all the streams of Western Pennsylvania cut chan- 

 nels far below their present beds. This epoch (Pliocene ?) was 

 either one of slight precipitation or of comparatively short 

 duration as none of the tributary streams reached a base-level 

 of erosion but were flowing through V-shaped canons of rather 

 rapid fall when the period of the terminal moraine with its 

 subsidence filled all these old channels with drift to nearly the 

 level of the old base-level plain. During all the foregoing 

 time Spencer River had drained the region in question, its 

 waters delivering into the Erie basin. After the deposit of the 

 drift in the valleys of Pennsylvania a divide was formed 

 across the old channel of Spencer River at Orwell, O., and the 

 drainage of the region became for the most part reversed — 

 the waters now finding their way into the lower Ohio and 

 thence to the Mississippi. 



The northern elevation of the continent so thoroughly 

 worked out by Gilbert and Spencer in New York and Ontario 

 occurred at this same time and so confirmed the region in its 

 drainage to the south. The modern rivers now began eroding 

 their beds of drift and are still at work. That this process has 

 gone on uninterruptedly for a long period of time is shown by 

 the fact that many of the tributaries of the Beaver and Ohio 

 have flat flood plains, underlaid by the buried channels of the 

 former drainage level, extending two miles or more back from 



