GLACIAL LAKES 335 



the three upper lakes, then joined as Lake Algonquin, discharged 

 their combined waters into Lake Iroquois at first through a great 

 channel now strongly marked across Ontario in the course of the 

 Trent River and Lake Simcoe, the so-called " Trent outlet." 

 At this time a smaller Lake Erie probably occupied the basin of 

 that lake, and later the Trent outlet was abandoned for the Port 

 Huron outlet (Fig. 364). 



The Nipissing Great Lakes. We have now followed the ice 

 front step by step in its retreat across the valley of the St. Law- 

 rence system. The successive unblocking of outlets offers but 

 one further possibility the opening of the French River-Nip- 



FIG. 365. Outline map of the Nipissing Great Lakes with their outlet past North 

 Bay into the Champlain Sea. 



issing Lake-Ottawa River, or " North Bay outlet." Though not 

 so to-day, the bed of this ancient channel was then much lower 

 than that of the " Mohawk outlet," and so soon as the glacier 

 had in its retreat uncovered this northern channel, the waters of 

 the upper lakes discharged through it past the site of Ottawa 

 and into an arm of the sea which then occupied the lower St. 

 Lawrence valley and has been called the Champlain Gulf or Sea 



