BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 VOL. 27, PP. 295-304, PL. 14 JUNE 1, 1916 



PLEISTOCENE DRAINAGE CHANGES IN WESTEEN NOETH 



DAKOTA 1 



BY A. G. LEONARD 



(Presented before the Society December 29, 1915) ' 



CONTENTS 



Page 



Introduction 295 



Age of the Missouri River Valley 295 



Pleistocene valley of Missouri and Yellowstone rivers 299 



Pleistocene valleys of the Yellowstone River 300 



Preglacial valley of the Little Missouri River 300 



Abnormal drainage features of Little Missouri tributaries 301 



Evidence of postglacial age of Lower Little Missouri Valley 302 



Introduction 



The continental ice-sheet produced important drainage changes in 

 western North Dakota. Its effects are particularly well shown in the case 

 of the Missouri, the Yellowstone, and the Little Missouri rivers, since all 

 these streams were forced to seek new channels. The region was three 

 times invaded by the ice-sheet — the later Wisconsin, earlier "Wisconsin, 

 and an earlier invasion, Avhich was probably Kansan or possibly sub- 

 Aftonian — but it was the earlier, or pre- Wisconsin, invasion which caused 

 most of the changes. The southerly course of the Missouri Eiver below 

 old Fort Stevenson has been attributed to the latest or later Wisconsin 

 ice-sheet, but evidence is here presented that the valley, at least in North 

 Dakota, is preglacial, using the term preglacial to mean older than the 

 oldest ice-invasion of this region — it may mean either pre-Kansan or pre- 

 sub-Aftonian. 



Age of the Missouri Eiver Valley 



As long ago as 1868 Gen. G. K. Warren made the statement that the 

 present course of the Missouri Eiver was determined by the edge of the 



1 Manuscript received by the Secretary of the Society December 20, 1915. 



XXII— Bull. Geol. Soc. Am., Vol. 27. 1915 (295) 



