BOUNDARY OF THE GLACIATED AREA, 



165 



valleys is mainly preglacial, and the high level gravel terraces 

 which line the troughs through their entire extent were de- 

 posited during the various stages of this rock erosion. The 



Fig. 53. — Map of Upper Allegheny Valley, Pa. 



difficulty of accepting this hypothesis, arising from the fact 

 that it involved believing that the deposition of the high 

 level terraces went on without the filling of the lower and 



