A SKETCH OF THE LATE TERTIARY HISTORY OF THE 

 UPPER MISSOURI RIVER 



CLYDE MAXWELL BAUER 

 U.S, Geological Survey, Washington, D.C. 



The story of the Missouri River forms one of the most interesting 

 chapters in the geologic history of the Great Plains. The valley 

 of the present stream is composed of many parts, some relatively 

 young and others moderately old, but as a unit it is youthful when 

 compared with the valley of the Mississippi. Notwithstanding 

 its youth, its history is so complex and so Httle known that only a 

 mere sketch can be given at this time. All that will be attempted, 

 therefore, will be to point out a few late Tertiary river channels, 

 which may indicate former courses of the Missouri River and some 

 of its larger tributaries, and sketch their bearing on the history of 

 the main stream. One of these stream courses has been traced 

 from Poplar, Montana, northeastward to the North Dakota 

 boundary. Another valley, partially abandoned, extends north- 

 ward from a point 6 miles west of the mouth of Bowlin Creek on 

 the Little Missouri River to the mouth of Tobacco Garden Creek 

 on Missouri River (see map, Fig. i). 



F. H. H. Calhoun^ has mapped a number of preglacial valleys 

 in north-central Montana, many of which are connected with the 

 history of the Upper Missouri. One of the most important of 

 these valleys extends from the mouth of Little Sandy Creek north- 

 eastward for about 35 miles to Milk River near Havre. Below 

 its junction with this old valley the valley of Milk River is 2-2 1 

 miles broader than above and clearly indicates that the head- 

 waters of a larger stream, probably the Missouri, emptied into it 

 and followed its course previous to. the ice invasion. East of the 

 mouth of Little Sandy Creek the present Missouri flows in a nar- 

 row, rocky channel for many miles and joins the old channel at the 



' F. H. H. Calhoun, "The Montana Lobe of the Keewatin Ice Sheet," U.S. 

 Geol. Survey, Prof. Paper 50. 



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