352 



HISTORICAL GEOLOGY 



700 miles long and several hundred miles wide, extended over the 

 whole valley of the Red River of the North in North Dakota and 

 Minnesota, and northward over much of Manitoba. It covered 

 a larger area than the combined Great Lakes. Its water was 

 held up by the united fronts of the Kewatin and Labradorean 

 ice sheets as they retreated northward. Its outlet was southward 

 through the Minnesota and Mississippi rivers until the ice 



Riehfie/cl Springs^ 



Fig. 221 

 Sketch map of central New York, showing the relation of the pre-Glacial 

 drainage to that of the present. Pre-Glacial drainage shown by dotted 

 lines only where essentially different from existing streams. (By W. J. M., 

 based on work by A. P. Brigham.) 



melted back (northward) far enough to open the outlet by way of 

 Nelson River to Hudson Bay, when the great body of water was 

 rapidly lowered, leaving only the present-day remnants, principally 

 Lake Winnipeg. The soil of this smooth old lake bed is wonder- 

 fully rich. 



Dkainage Changes Due to Glaciation 



In addition to its lakes, the glaciated area is also characterized 

 by numerous gorges and waterfalls, which are largely due to glacia- 

 tion. As a result of the very long time of pre-Glacial erosion, it 



