492 J. E. TODD THE CHANNEL OF THE MISSOURI RIVER 



does it emphasize the similarity of the conditions of the different streams, 

 but the different areas are parts of the same system; so that we can not 

 conceive of one portion being cut down without the subsequent lowering 

 of all connected with it. Hence it is inconceivable that Lake Arikaree 

 could have drained until Lake Mobrara had cut down; in other words, 

 not before the Pleistocene. On the other hand, we can not believe that 

 the valley of the Missouri in North Dakota could have been excavated or 

 formed during the Tertiary without excluding the Yellowstone and the 

 Little Missouri on the one hand or the Cannon Ball on the other. 



We conceive that the erosion of the valley of the Missouri had cut 

 down within 400 or 500 feet of its present level by the advance of the ice 

 to form the Altamont moraine.- This would correspond to the boulder- 

 topped terraces spoken of in connection with the Little Missouri and 

 elsewhere. 



In the maximum extent of the ice, we find the time when the Pleisto- 

 cene channels were first occupied — for example, the outlets of various 

 lakes — and the lower boulder terraces mark the latest direct effect of the 

 Altamont moraine upon the stream and its deposits. 



HEAD EROSION PROMINENT 



AYe have already alluded to the prominence of this function of streams, 

 but we find it especially prominent in the history of this region. It has 

 been especially the cause of the various streams approaching the Altamont 

 moraine. While there may be traces of portions of the Tertiary courses 

 of streams still occupied and later of Pleistocene courses, the portions of 

 streams next the moraine which together form the present course of the 

 Missouri have been formed by this action of streams, namely, head 

 erosion. To illustrate, we have the Tertiary course of the Little Missouri 

 marked from its eastward bend to the mouth of Tobacco Garden Creek. 

 The course from the bend eastward would be of Pleistocene origin, prob- 

 ably during the maximum extent of the ice, and probably the earlier part 

 of it was a lake which soon became drained. The course of the Missouri 

 from the mouth of Tobacco Garden Creek to the present mouth of the 

 Little Missouri would be an example which was formed by head erosion 

 and was probably not continuous and prominent until the latter portion 

 of the Wisconsin. 



THE BIG BERTHOLD BEND 



What seems a unique but unmistakable example of this is illustrated 

 in the Great Bend in Fort Berthold Eeservation. It has already been 

 suggested by several observers of that region that the curve of the great 

 U-shaped bend marks the position of a moraine formed around the south- 



