EVIDENCE FROM MONTANA 



487 



valley." How much less, therefore, when both walls and filling are un- 

 consolidated, as in most of the Dakotas. 



THE YELLOWSTONE AND LITTLE MISSOURI DIVERTED FROM THEIR 



TERTIARY CHANNELS 



Mr. Calhoun, from a study of the Keewatin deposits in Montana, traces 

 the preglacial valley of the Missouri to Glasgow, on Milk Eiver, and from 

 quotation from others shows that it continues on to the northeast corner 

 of Montana, and the Little Missouri likewise shows similar points in its 

 history. 



"In the late Tertiary," according to C. M. Bauer,^^ "it occupied a well- 

 marked channel extending northward from the first prominent eastward 

 bend of the present stream to the head of Tobacco Garden Creek and 

 thence along this valley to the present Missouri," he adds, "to the Ter- 

 tiary Yellowstone near Williston." More likely the Tertiary Little Mis- 

 souri crossed the Missouri near the mouth of Tobacco Garden Creek and 

 ran north several miles farther before reaching the Yellowstone. 



During the maximum of the ice all three streams joined in a lake and 

 for a time flowed eastward over the mid-course of the smallest stream. 

 Eventually the Yellowstone and Missouri ceased to rise to Benny Pierre- 

 Hay Draw, outlet, but found the course of the present Missouri Eiver, 

 the Little Missouri determined by an outlet of the lake. The Missouri 

 and Yellowstone, on the withdrawal of the ice-sheet, found themselves in 

 well-defined valleys from Poplar and Glendive respectively to Williston. 

 The most plausible explanation of the new channels is that they were 

 eroded by subglacial and preglacial streams during the advance, the 

 culmination, and earlier recession of the early Wisconsin. Head erosion 

 we deem to have been particularly efficient during the recession of the ice. 

 It was a time of rapid deposition of stratified drift. 



Summary of Evidence 



All of the arguments presented by Dr. Leonard for the origin of the 

 channel of the Missouri as being preglacial or Tertiary are found to be 

 negative or indecisive. In other words, the case as he presents it is not 

 proven. 



The first positive argument for the glacial and Pleistocene origin of 

 the Missouri channel is the evident relations of the latter to glacial dis- 

 tribution and deposits. This argument is especially strong if no other 

 theory is capable of explaining the facts. Dr. Leonard gives us no reason 



" U. S. Geol. Siu-v., Professional Papers, No. 50, p. 31. 

 ^2 Journal of Geology, vol. 23, p. 55. 



