LATE TERTIARY HISTORY OF UPPER MISSOURI RIVER 57 



caused a relative rise of this portion of the continent, together with 

 some folding. The streams were now set for the task of erosion. 

 During this time the Yellowstone succeeded in maintaining its 

 course across the Glendive anticHne, a long, narrow arch extend- 

 ing southeastward from the Yellowstone River, Montana, to the 

 northwestern corner of South Dakota, and the Upper Missouri 

 maintained its course across the Poplar dome, a low quaquaversal 

 structure at Poplar, Montana. 



Deposition again began on the Great Plains in Oligocene time.^ 

 The change in level causing deposition may have been the result of 

 warping, the mountains on the west rising relative to the plains 

 and furnishing more sediment. 



A general uplift following the Oligocene inaugurated a great 

 period of erosion and base-leveling on the northern Great Plains. 

 In places, particularly in Saskatchewan,^ the great rivers deposited 

 some gravel and coarse sediment where the gradient became low 

 and the load too great. However, during the late Tertiary, the 

 Upper Missouri and Yellowstone carried away thousands of feet 

 of strata and carved for themselves magnificent broad valleys in 

 the soft Cretaceous and early Tertiary deposits.^ In the vicinity 

 of Highwood Mountain the erosion in the valleys probably 

 amounted to about 3,000 feet. Farther east, near Sentinel Butte 

 and the Kildeer Mountains, about 1,000 feet and near Turtle 

 Mountain about 500 feet of material was excavated. Between 

 these monadnocks the country was reduced to a peneplain. Then, 

 according to Upham, followed the early Quaternary uplift and the 

 glaciation of the northern part of the continent. The outlets to 

 the north became blocked with ice and later were partly filled with 

 drift and outwash. Ponding of waters and marginal drainage 

 began. Lake Agassiz and Lake Souris were formed, as well as many 

 other smaller lakes, most of which are now extinct. Just where the 



' A. G. Leonard, geologic map of North Dakota, 1913, showing present distribu- 

 tion of White River formation. 



== R. G. McConnell, "On the Cyprus Hills, Wood Mountain and Adjacent Terri- 

 tory," Canada Geol. Survey Report, N.S., I (1885) 70-c. 



3 Warren Upham, "Tertiary and Early Quaternary Base-levehng in Minnesota, 

 Manitoba, etc.," American Geologist, XIV (1894), 235-46. 



