154 Hausman — Glacial Modification of Drainage 



peneplaned, then reuplifted and dissected by the reju- 

 venated streams to a stage of early maturity, when the 

 country was covered by the first advance of the ice of 

 the Pleistocene glaciation. The main preglacial drain- 

 age lines of the area seem to have been toward the north, 

 and the advancing ice impounded this north-flowing 

 water in proglacial lakes which rose in level until they 

 overflowed at the lowest points in the east-west divide 



The 



between north-flowing 



and south-flowing streams. 

 Fig. 1. 



Fig. 1. Portion of a sketch map of New York State to show location of 

 the area under discussion (in black). 



ice continuing in its advance, moved with the maximum 

 freedom, in the fastest and deepest currents, and hence 

 with the greatest erosive effectiveness, in the north-south 

 valleys, and, overriding the lower divides between them 

 and the south-flowing drainage, considerably reduced the 

 elevations of these divides or cut them down entirely. 

 Later, during the periods of recession of the ice fronts 

 in the various valleys, moraines, together with the vast 

 deposits of glacial streams, filled in the troughs of the 

 glacially eroded valleys, often to great depths, and 

 fashioned the present relief of their bottoms and sides. 

 Many of these north-south valleys were, moreover, so 



