80 PROCEEDINGS OP THE "WASHINGTON MEETING 



ing terraces do nut suggest such long pauses as were required for the building 

 of the Iroquois beach. So far no time measurement is of any value., The most 

 important problem is the date when the waters fell below the Iroquois beach, 

 as this would most nearly coincide with the last clays of the Glacial period. 

 But these new data show the newness of both the Saint Lawrence and the 

 modern shores of Lake Ontario. 



Presented in abstract extemporaneously. 



PLEISTOCENE DRAINAGE CHANGES IN WESTERN NORTH DAKOTA 

 BY ARTHUR G. LEONARD 



{Abstract) 



The continental ice-sheet produced important drainage changes in western 

 North Dakota. Its effects are particularly well shown in the case of the Mis- 

 souri, Yellowstone, and Little Missouri rivers, since all these streams were 

 forced to seek new channels. 



The southerly course of the Missouri River below old Fort Stevenson has 

 been attributed to the later or Wisconsin ice-sheet, but evidence is presented 

 that the valley is preglacial. This evidence is based on the presence of glacial 

 boulders and drift on the valley bottom and at many points on a terrace repre- 

 senting a former floodplain of the Missouri. The river must have flowed in its 

 broad, terraced valley at the time of the earlier ice-invasion, and on the floor 

 of this valley the glacier deposited boulders and till. When the ice-sheet in- 

 vaded the region, it blocked the valleys of both the Missouri and Yellowstone 

 rivers, and also the preglacial valley of the Little Missouri, forcing these 

 streams to seek new channels. Lakes were formed in the valleys of the Little 

 Missouri and Yellowstone, rivers, the waters rising until they overflowed the 

 divide to the Knife River. The combined waters of the three rivers flowed 

 east across Dunn County and southeast across Morton County to the mouth 

 of the Cannon Ball River. The length of this Pleistocene valley of the Yellow- 

 stone and Missouri rivers, which extends from the head of the Knife to the 

 mouth of the Cannon Ball and crosses the divides between the Knife and Heart 

 rivers and that between the Heart and Cannon Ball, is 155 miles. 



The lower 50 miles of the Yellowstone Valley was blocked with ice during 

 the Glacial period, and the waters flowed east to the valley of the Little 

 Missouri, forming at least two broad, flat-bottomed valleys connecting these 

 streams. One of these valleys, 28 miles long, is now occupied in part by Bennic 

 Pierre Creek, a tributary of the Yellowstone. 



The Little Missouri was also forced out of its preglacial valley, which is 

 now occupied by Cherry and Tobacco Garden creeks. It probably flowed for 

 a time through the Pleistocene valley of the Missouri and Yellowstone rivers, 

 but later took an easterly course and formed its present postglacial valley, 

 which extends from the mouth of Bowling Creek to the Missouri River — a dis- 

 tance of 100 miles. Evidence is given that this lower valley of the Little Mis- 

 souri is much younger than the portion above the mouth of Bowling Creek. 



Read by title in the absence of the author. 



