388 



CI.OLOGY. 



of the ice blocked up many northward trending valleys and caused their si reams 

 to find new courses along the ice border. The present Ohio and Allegheny rivers 

 seem to have been formed by the union of several streams thai previously flowed 

 into the Erie basin. The Missouri river seems to have been formed by a similar 

 combination of many streams that previously flowed northerly and easterly 

 but some part of this readjustment of the drainage seems to have been later 



Fig. 513. — The unconformity between the Natchez above and the Lafayette below. 

 The line of contact is indicated by the dotted line. 



than this stage. Including these later changes, the Ohio and Missouri rivers 

 may be pictured as two great drainage arms embracing the border of the ancient 

 ice-sheet and carrying away its waters. Rather low gradients and a low ele- 

 vation in the lower Mississippi seem thus to be indicated. 



III. The Kansan glacial stage. — As defined by Calvin, Bain, and 

 others who have specially studied it, 1 the Kansan stage is represented 

 by a typical sheet of till occupying a large surface area in Kansas, 

 Missouri, Iowa, and Nebraska (Fig. 470), and theoretically extending 



1 Reports of the Iowa Geol. Survey. 



