62 



Types of Aquatic Environment 



stone barrier that retains Lake Erie. At Niagara 

 Falls it is making progress at the rate of about five feet 

 a year. Since the glacial period it has cut back from 

 the shore of Lake Ontario a distance of some seventeen 

 miles, and if the process continues it will in time 

 empty Lake Erie. 



Fig. 13. Evans' Lake, Michigan; a lake m process of being filled by encroach- 

 ment of plants. A line of swamp loose-strife (Decodon) leads the invading 

 shore vegetation. Further inwash of silt or lowering of outlet is precluded 

 by density of the surrounding heath. The plants control its fate. 



Pholo by E. McDonald. 



When the glacier lay across the St. Lawrence valley, 

 before it had retreated to the northward, all the waters 

 of the great lakes region found their way to the ocean 

 through the Mohawk Valley and the Hudson. At that 

 time a similar process of cutting an outlet through a 

 limestone barrier was going on near the site of the 

 present village of Jamesville, New York, where on the 



