BOUXDARY OF THE GLACIATED AREA, 



159 



with the eastern highlands a trough through which the torrents 

 cut their way southward. The presence of high water sedi- 

 ment at Franklin shows that the col which imposed the height 

 to the lake was south of that place, and also shows that all 

 intervening cols were submerged and the drainage systems 



Pig. 52— Lake Allegheny. B. Baraesville. C. Clarendon. F. Franklin. 

 O. Oil City. T. Thompsons. W. Warren. 



I. Irvine. 



between them aggraded . When the most distant and restrain- 

 ing col became degraded so that the interior ones came within 

 the area of scour they were cut down gradually and readily, 

 as the porous and loosely cemented sandstones rapidly yield 

 to water carrying only its own sands, and the shales carry 



