532 A. G. LEONARD 



explanation for the presence of the bowlders on top of the high 

 buttes is that the ice upon encountering these obstructions was 

 buckled up as it passed over them. But it seems more probable 

 that the ice sheet which was able to push across the deep valley of 

 the Missouri River and advance 40-60 miles beyond was thick 

 enough to submerge the Blue Buttes and pass on over them. The 

 terminus of the continental glacier was not far from 15 miles south 

 of the Blue Buttes, since the ice advanced only as far as the Kill- 

 deer Mountains. Back about 15 miles from the edge the ice sheet 

 therefore doubtless had a thickness over the upland plain of con- 

 siderably over 500 feet. The ice which filled the Missouri River 

 Valley must have had locally a thickness of 1,000 feet or over. 



Age of the older drift. — That the drift west of the Missouri 

 River is much older than the Wisconsin drift is evident from the 

 great amount of erosion it has undergone. Over much of the 

 region the finer materials of the till have been swept away, leaving 

 only bowlders and gravel. The older drift also differs in color, 

 being considerably darker than the Wisconsin. This pre- Wisconsin 

 drift has commonly been regarded as Kansan and there is perhaps 

 more reason for referring -it to the invasion of the Kansan ice 

 sheet than any of the other early ice invasions. 



The drift north of the Missouri River and west of the Altamont 

 Moraine appears to be younger than the drift south and west of the 

 river. It has undergone less erosion and resembles the typical 

 Wisconsin till in color. Calhoun believes that the drift of north- 

 eastern Montana is of Wisconsin age, and on his map this drift 

 is shown as extending to within about 40 miles of the North Dakota 

 line. 1 The extra-morainal drift north of the Missouri River in 

 Williams County is undoubtedly continuous with the drift sheet 

 west of here in Montana, in which case it is probably of Wisconsin 

 age, and belongs to the Early Wisconsin stage. 



If this view is correct, there are three drift sheets in North 

 Dakota — the Late Wisconsin east of the Altamont moraine, the 

 Early Wisconsin west of the moraine and north of the Missouri 

 River, and the Kansan drift west and south of the river. 



1 Fred. H. H. Calhoun, "The Montana Lobe of the Keewatin Ice Sheet," Prof. 

 Paper No. 50, U.S. Geol. Survey, 1906, pp. 52-57. 



