NIAGARA Fx^.LLS AND VICINITY 59 



As the ice front continued to melt away, retreating northeastward, 

 drainage at a lower level was permitted along the ice front to the 

 Hudson valley, and the sea. As a result, the water level sank, the 

 Chicago outlet was abandoned, and Lake Warren became much 

 contracted and in part cut up and merged into new bodies of water. 

 The largest of these was glacial Lake Algonquin, which occupied 

 the basins of the three upper Great lakes, and seems to have been 

 for a long time independent of Lake Erie, which after the division 

 of Lake Warren was for a time much smaller than it now is. (Fig. 

 II and 13) 



The critical period in the development of the lakes, with reference 

 to the birth of Niagara, was the uncovering of the divide at Rome 

 (N. Y.) and the consequent diversion of the drainage into the 

 present Mohawk valley. This brought with it a subsidence of the 

 waters north of the Niagara escarpment to the level of this outlet, 

 which was considerably below that to which the other lakes could 

 subside, owing to the rocky barriers which kept them at greater 

 altitudes. As a result Niagara river came into existence, though at 

 first it was only a connecting strait between Lake Erie and the 

 subsiding predecessor of Lake Ontario. The overflow from Lake 

 Erie occurred at the present site of Blackrock, because there hap- 

 pened to be the lowest point in the margin of the lake. It is not 

 improbable that a small preglacial stream had predetermined this 

 point, either flowing southward into the river occupying the Erie 

 basin^ or northward as an obsequent stream into the Tonawanda. 

 The course of the river below Blackrock was determined by the 

 directions of steepest descent of the land surface, which was prob- 

 ably predetermined to some extent by preglacial streams. As soon, 

 liowever, as the level of the waters of the Ontario valley sank below 

 the edge of the Niagara escarpment at Lewiston, a fall came into 

 existence, which daily increased in hight as the level of the northern 

 lake was lowered. From that time to the present, Niagara has 

 worked at its task of gorge-cutting, the present length of the gorge^ 

 from Lewiston to the falls, marking the amount of work accom- 

 phshed. 



