138 



PHYSIOGRAPHY 



were blocked up in some places. After the ice melted, therefore, 

 the surface waters followed former valleys in some cases, and flowed 



Fig. 145. A later stage in the development of Lakes Chicago and Maumee. 

 The ice has retreated, and the outlet of Lake Maumee is being shifted. 

 (U. S. Geol. Surv.) 



Fig. 146. The Great Lakes at the Algonquin- Iroquois stage. The outlet to 

 the sea is by way of the Mohawk Valley. (Taylor.) 



