GEOLOGY OP THE NORTHERN ADIRONDACK REGION 293 



great Labrador ice sheet for a long time. From analogy with 

 other regions it may be inferred that there was more than one 

 advance and retreat of ice sheets over the district, but how 

 many and how extensive are purely conjectural matters for the 

 immediate region. The last ice advance destroyed, for the most 

 part, the traces of the presence of its predecessors. 



Coming down on the region from the north and northeast, the 

 Labrador ice sheet had its advance opposed by the elevated mass 

 of the Adirondacks and was forced aside by it into two great 

 ice streams which worked their way around the region. The one 

 advanced up the St Lawrence valley and turned south along the 

 west side of the Adirondacks, and thus reached and entered the 

 Mohawk valley from the west; the other moved south through 

 the Champlain valley, reaching the Mohawk at the east end. As 

 the ice increased in thickness, it encroached more and more on 

 the flanks of the region till finally it overswept the whole, and 

 persisted in this condition for a long time. While the basal cur- 

 rents of the ice continued to be controlled by the topography, 

 the main mass swept over the region in a general southwesterly 

 direction. Ultimately changing conditions brought about reces- 

 sion. The thickness was least over the highlands, and the ice 

 would first disappear there, leaving the two great currents sweep- 

 ing round the region, as they did during the advance. These 

 slowly dwindled and disappeared northerly. 



The final disappearance of the ice left the topography modified 

 both by glacial wear and glacial deposit, but with its larger 

 features little changed. Ridge slopes were smoothed, summits 

 rounded, valleys clogged with deposit, lakes produced either by 

 inequality of deposit or by local excessive downward erosion, 

 stream courses more or less modified, a host of minor changes 

 much altering the appearance of the region. 



Postglacial history 



During the continuance of glaciation changes in altitude took 



place, and at the time of final melting away of the ice from the 



St Lawrence valley the elevation was much less than at the 



beginning of glaciation, was in fact sufficiently low to enable the 



