486 j. e. todd the channel of the missouri eiver 



Evidence from Montana 

 general statemext 



The Missouri River in its course throughout that State has been related 

 to the ice-sheet on the north and the driftless regions on the south, sim- 

 ilarly to that which we have noted in North Dakota and South Dakota. 

 Moreover, the history of the ice-sheet seems to be very similar to that in 

 Xorth Dakota. There is found in both cases till-covered areas outside 

 of the Altamont moraine. This till may be of Kansan age or of early 

 Wisconsin. The most recent stage of the ice was that of the Wisconsin 

 at the time of the formation of the Altamont moraine. This general 

 relation is similar to that which we have recognized in South Dakota and 

 of course argues that similar might be expected in Xorth Dakota, the 

 State between. 



That the valley was Tertiary is indicated by the geological section at 

 Glass Bluff,^ 4 miles southeast of Buford. "Glacial drift, 25 feet," lies 

 more than 160 feet above the Missouri,^ or 2,100 feet above tide. This 

 agrees well with the 'TBenny Pierre-Hay Draw" outlet, 2,200 feet above 

 tide. Afterward the Yellowstone became a component of the ^Missouri 

 and cut do^^Ti its present channel. 



POSTGLACIAL EROSIOX 



Dr. Leonard has concluded that the erosion of the Missouri Eiver along 

 its present course is too gTeat to have been accomplished by that stream 

 since the beginning of the formation of the Altamont moraine. The 

 erosion at its greatest is not more than 500 feet in depth and a mile or 

 two in width. From Montana we find that the erosion from the Missouri 

 channel from Mussel Shell River to Milk River is from 600 to 1,000 feet. 

 In the former case the material eroded is mostly clay and soft sand. In 

 the latter it is consolidated sandstone. Below Great Falls the Missouri 

 has been flowing over sandstone, so that it is readily seen the postglacial 

 erosion is 150 to 250 feet in depth. Another point is that the preglacial 

 valleys in a number of cases, especially toward the west, are cut very 

 much deeper than those of postglacial origin. This is prominent in the 

 case of an old channel of the Missouri near Great Falls, which by boring 

 near Sand Coulee was found to be 270 feet deep.^^^ 



DIFFICULTY OF REMOViyC DRIFT FROM CHAXXEL.^ 



From the map on page 38 we see how the erosion of a drift-filled chan- 

 nel seems very little, if any, affected by harder rocks for the walls of the 



s Third Biennial Kept., N. Dak. Geol. Surv., p. 19. 



^Bull Geol. Soc. Am., vol. 27, p. 300. 



1** U. S. Geol. Surv., Professional Papers, No. 50, p. 3fi. 



