174 THE ICE AGE IN NORTH AMERICA. 



ing the vicinity of Oahe, a few miles above Pierre, at the 

 mouth of Bad River. From this point to the mouth of the 

 Cheyenne glacial deposits do not encroach upon the plateau 

 to the west. But, above the mouth of the Cheyenne, the 

 line strikes off farther west, and crosses the Moreau River 

 about forty-five miles back from the Missouri, and the Grand 

 and Cannonball Rivers at about the same distance. The 

 Northern Pacific Railroad passes from the glaciated to the 

 unglaciated region at Sims Station, about thirty miles west 

 of Bismarck. 



In the chapter upon " Terminal Moraines " we will speak 

 more fully of the portion of Dakota lying east of the Mis- 

 souri River ; but west of the Missouri the deposits belong to 

 what we have denominated the fringe, or what President 

 Chamberlin perhaps more appropriately calls " the attenuated 

 border." This portion of the boundary I had the privilege of 

 studying in the summer of 1888, driving some hundreds of 

 miles through the Indian reservation, extending from Fort 

 Yates south westward to the Moreau, and thence southeast- 

 ward to the vicinity of Pierre. Here I found the border, 

 although somewhat attenuated, to be pretty sharply defined. 

 The glacial marks, however, consisted almost wholly of 

 bowlders and rather coarse gravel, and was pretty evenly dis- 

 tributed over the surface of the plateau. The formation of 

 the region is cretaceous, so that it is easy to recognize the 

 Laurentian bowlders. In size these sometimes attain a diame- 

 ter of four or ^yq feet, and frequently almost cover the 

 ground. The elevation attained by them runs up to about 

 six hundred feet above the river. We found, however, no 

 scratches upon these bowlders, nor were there any exposures 

 of till or unstratified deposit, so characteristic of the terminal 

 moraine and of the central portion of the glaciated area. 

 But from my own experience I have no hesitation in classify- 

 ing these deposits with those produced by direct action of the 

 glacier. They are what would naturally occur on the attenu- 

 ated margin of the ice- sheet. 



I found evidence, also, of a temporary line of marginal 



