CHAPTER VII. 



DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 



We will begin the consideration of this part of our 

 subject, also, with the presentation of the salient facts in 

 North America, since that field is simpler than any field 

 in the Old World. 



The natural drainage basins of North America east of 

 the Rocky Mountains are readily described. The Missis- 

 sippi River and its branches drain nearly all the region 

 lying between the Appalachian chain and the Rocky 

 Mountains and south of the Dominion of Canada and of 

 the Great Lakes. All the southern tributaries to the Great 

 Lakes are insignificant, the river partings on the south 

 being reached in a very short distance. The drainage of 

 the rather limited basin of the Great Lakes is northeast- 

 ward through the St. Lawrence River, leaving nearly all 

 of the Dominion of Canada east of the Rocky Mountains 

 to pour its surplus waters northward into Hudson Bay 

 and the Arctic Ocean. With the exception of the St. 

 Lawrence River, these are essentially permanent systems 

 of drainage. To understand the extent to which the ice 

 of the Glacial period modified these systems, we must first 

 get before our minds a picture of the country before the 

 accumulation of ice began. 



Preglacial Erosion. 



Reference has already been made to the elevated con- 

 dition of the northern and central parts of North Amer-' 



