348 HISTORICAL GEOLOGY 



when the ice had withdrawn a little farther northward, a lower 

 outlet was formed through the Trent River by which Lake Algon- 

 quin drained into Lake Iroquois. The old Trent River channel is 

 now higher than the Detroit outlet, but some of the proofs for the 

 former Trent outlet are as follows: (1) The presence there of a 

 large, distinct river channel; (2) the convergence of the beaches 

 toward that channel; and (3) the fact that the land was then con- 

 siderably lower on the north or northeast side of Lakes Ontario 

 and Erie than on the south side. For example, in following the 

 old Iroquois beach we find that it gradually rises to higher levels 

 until it is several hundred feet higher at the east than near the 

 mouth of Niagara River. This tilting of the beach has been due 

 to warping of the land since the lake existed, and it is evident 

 therefore that during the Algonquin-Iroquois stage the Trent 

 River channel was lower than that past Detroit. During the 

 Algonquin-Iroquois stage the waters of all the Great Lakes region 

 discharged through the Mohawk-Hudson valleys, and the volume 

 of water which flowed through the Mohawk Valley must have 

 been as great, if not greater, than that which now goes over 

 Niagara Falls. During this time the St. Lawrence Valley was still 

 buried under ice. 



Still later the ice withdrew enough to allow the Algonquin- 

 Iroquois waters to discharge along the northern base of the Adiron- 

 dack and into what appears to have been ice-ponded waters in 

 the Champlain Basin, and thence into the Hudson Valley. The 

 Mohawk Valley outlet was thus abandoned. 



Finally the ice withdrew far enough to free the St. Lawrence 

 Valley when the waters of the Great Lakes region dropped to a 

 still lower level, bringing about the Nipissing Great Lakes stage 

 (see Fig. 220). The Nipissing Lakes found a low outlet through 

 the Ottawa River (then free from ice) and into the Champlain 

 arm of the sea. Post-Glacial warping of the land brought the Great 

 Lakes region into the present condition, but this, and the Cham- 

 plain subsidence, being really post-Glacial features, will be de- 

 scribed below. 



Other Existing Lakes and Their Origin 



Counting all, from the smallest to the largest, there are within 

 the glaciated area of North America tens of thousands of lakes, 



