350 HISTORICAL GEOLOGY 



and these constitute one of the most striking differences between 

 the geography of the present and that of pre-Glacial time. These 

 lakes are widely scattered, though in the United States they are 

 most abundant in the regions of greater relief, such as Maine, New 

 Hampshire, New York, and Minnesota, because lake basins were 

 more readily formed by drift dams across the deeper pre-Glacial 

 valleys of those regions. 



It is well known that most of the larger lakes, especially those 

 of the linear type, occupy portions of pre-Glacial stream channels. 

 All the existing lakes are due, either directly or indirectly, to 

 glacial action. Among the ways by which such bodies of water 

 may be formed are these: (1) by building dams of glacial drift 

 across old river channels; (2) by ice erosion; and (3) by accumu- 

 lation of water in the numerous depressions which were formed by 

 irregular deposition of the drift (kettle-holes, etc.). Hundreds of 

 small lakes, often not more than mere pools in size, belong to the 

 last named type, while very many of the large and small lakes are 

 due chiefly to the existence of drift dams. Certain lakes in south- 

 eastern Canada and elsewhere appear to occupy rock basins scoured 

 out by ice erosion. 



In considering the origin of glacial lakes, the so-called Finger 

 Lakes of central- western New York deserve special mention. All 

 are agreed that the lakes of this remarkable group occupy pre- 

 Glacial valleys, most of which, at least, contained north-flowing 

 streams. These lakes have dams of glacial drift across their lower 

 (north) ends, and the dams have been important factors in the 

 formation of the lakes, being in some cases perhaps the sole cause. 

 But in the cases of the two largest lakes — Seneca and Cayuga — 

 there is strong evidence, from the hanging valley character of the 

 tributaries, that the pre-Glacial valleys were notably deepened 

 by ice erosion. 



The presence of Lake Champlain is due principally to a com- 

 bination of factors, including late elevation of the land, with greater 

 uplift on the north; heavy glacial accumulations toward the north; 

 and possibly some deepening as a result of ice erosion. 



In the basin of Lake George there was a pre-Glacial divide 

 where the " Narrows" are now located, and this divide appears 

 to have been considerably lowered by ice erosion when part of 

 the Champlain ice lobe ploughed its way through the deep, narrow 

 valley. The waters are now held in by drift dams at each end. 



