<3lacfal anfc ipreglacial Xafcee. 219 



ice have become anchored, while the ice behind 

 it has carried the drift until it is covered over 

 and piled up at the sides. When these ice 

 mountains melted away depressions were left 

 which in some cases have resulted in lakes, and 

 in others simply dry kettle holes. This 

 process has been hinted at in a former chapter, 

 but we give it here as one of the kinds of lakes 

 formed during the glacial period. They are 

 found everywhere that glacial action has pre- 

 vailed. They are found in great abundance 

 in some parts of New England on the margin 

 of the terminal moraine. These lakes, how- 

 ever, are comparatively insignificant as com- 

 pared with the great inland seas like Lake 

 Superior and Lake Michigan, that undoubt- 

 edly owe their origin largely to the ice age. 



There are other factors, however, that enter 

 into the formation of the great chain of lakes 

 on the northern boundary of the United 

 States besides those mentioned, that have 

 brought into existence the smaller inland 

 lakes. 



Glacial lakes may be divided into three 

 classes. Those found in the "kettle holes" 

 of the terminal or medial moraines, and those 

 that are formed by the deposition of the gla- 

 cial drift, as, for instance, Devil's Lake, and 

 those that are caused by ice forming dams 

 across the valley of a river that lasted only 

 during the ice age. In some lakes of the 



