THE PLEISTOCENE OR GLACIAL PERIOD. 



381 



Another result is to be seen in the changes in the courses of the 

 streams. In many cases, pre-existing valleys were filled with drift, 

 so that when the ice melted the old channels were obstructed at many 

 points, and surface drainage was forced into courses which were partly 

 new. In other cases, the ice, by encroaching on the middle course 

 of the valley, as in the case of the Ohio, forced drainage around its front, 







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Fig. 509. — Diagram illustrating characteristic drainage in the glaciated area of 

 southeastern Wisconsin. 



and the drainage lines thus established by force, were often held after 

 the ice melted. 



There are few streams of great length in the area covered by the 

 ice, which were not turned from their old courses for greater or less 

 distances by the ice or the drift. The Mississippi, the Ohio, and the 

 Missouri, the master streams of the United States within the glaciated 

 area, and a host of their tributaries, as well as many streams tribu- 

 tary to the St. Lawrence, suffered in this way. The history of some 

 of these changes has been studied in detail, 1 but the history of the 



