160 THE ICE AGE IN NORTH AMERICA. 



strata so highly jointed that they collapse under a slight im- 

 pact, and undermine the overlying and more resistant strata. 

 The result was an aggradation of older systems to the level of the 

 older cols and the present alternations of the old aggraded val- 

 leys with low angles of slope and gorges varying from a steep 

 V-shape to one with nearly vertical walls extending far below 

 the present water level and attesting to the torrential flows 

 which degraded them." 



In estimating the significance of these facts, however, it 

 is important to bear in mind the extensive changes of land 

 levels which took place before, during, and after the Glacial 

 period. (1) It will be shown in a later chapter that there 

 was extensive elevation of the glaciated region in Tertiary 

 times increasing in extent towards the north. In all prob- 

 ability this amounted to more than 2,000 feet in the central 

 part of the glaciated area. This, in itself, may have had 

 something to do in reversing the drainage of many northerly 

 flowing streams. 



(2) But at the close of the Glacial period the northerly 

 areas of the glaciated region were depressed so that they stood 

 at Montreal 600 feet lower than the present level, and farther 

 north still lower. It is impossible to tell how far south this 

 glacial depression extended in the northern part of the United 

 States. 



(3) Since the retreat of the ice there has been a differ- 

 ential re-eleyation of the glaciated area increasing towards 

 the north, and amounting to 600 feet at Montreal and 1,000 

 feet farther north. The axis about which this differential 

 re-elevation revolves seems to run east and west through 

 Central New York and the basin of Lake Erie. In estimating, 

 therefore, the height of the col, in the lower Allegheny or 

 middle Ohio Valley, which determined the level of Lake 

 Allegheny we must not forget that this differential northerly 

 depression during glacial times may make the water levels 

 in the upper part of the lake appear to be higher than they 



