CHAPTER IV. 



THE DRIFT BORDER OR GLACIAL BOUNDARY 



It would be misleading to treat the drift border as a unit, for it is really 

 a combination of the margins of several dinft sheets which differ widely in 

 age. A portion of the border in northwestern Pennsylvania seems to mark 

 the limits of a drift sheet as old as the Kansan and possibly of pre-Kansan 

 age. Immediately adjacent to this portion of the border the Wisconsin 

 drift extends to the limits of glaciation. But upon passing to central Ohio 

 the Illinoian drift is the one to mark the glacial limits. The lowan is not 

 exposed to view outside the Wisconsin within the region under discussion, 

 though a silt apparently of lowan age extends beyond the limits of the 

 Wisconsin and the Illinoian in the western part of this region. 



SECTION I. THE BORDER OF THE OLDEST DRIFT (KAIS^SAN OR 

 PRE-KANSAIST) . 



So far as this region is concerned, the oldest sheet of drift found as an 

 outlying deposit is restricted mainly to the northwestern part of Pennsyl- 

 vania, though there may be limited exposures of it in southwestern New 

 York and in eastern Ohio. In the central and eastern parts of the region it 

 seems to fall short of the limits of the later drift sheets. 



So far as exposed, this old drift has a very meager development on the 

 uplands, though it is heavy in the valleys. The general thickness on the 

 uplands probably averages less than 5 feet, while in the valleys it reaches 

 depths of 200 to 300 feet. Because of the meager deposition on the uplands 

 it becomes difficult to determine the precise limits of glaciation. Search is 

 often necessary to discover even a bowlder or a pebble of glacial origin in 

 a trip across the uplands, while valleys on either side may contain heavy 

 deposits of drift. Until, therefore, very detailed study has been made 

 the limits of the drift can be stated only approximately. As yet it is not 

 known whether the border passes somewhat directly across the uplands and 



