PROVISIONAL HISTORY OF ORIGIN OF THE MISSOURI 493 



western tip of a local ice-lobe, and the straight valley reaching across 

 the top of the U is the bed of the preglacial course of the stream, and 

 that in some way this straight valley — which, by the way, is more than 

 100 feet higher than the present river and slopes downstream — became 

 filled by ice in the advance of the glacier which formed a bridge for the 

 onward motion of the ice. The suggestion is perhaps insufficient and 

 presents several difficulties which we need not stop to consider. Mean- 

 while the water of the stream above the U found its easiest pathway to be 

 around the outer rim of the moraine. The head erosion acted along the 

 more vigorous course of the water, namely, the outside of the moraine, 

 until they completely connected the channel below with the channel above 

 this U-shaped bend. This is a very unusual example of head erosion, 

 the general tendency being rather to cut off bends than to form them. A 

 portion of the ice may have fallen below the pressure of the ice behind 

 and remained stationary, like a boulder pavement, except the stationary 

 lower mass was ice instead of till. The rest of the glacier passes on to 

 form the moraine. One fact favorable to this view is that the cross-valley 

 is at right angles to the direction of the ice movement or the course of 

 the glacier. 



In conclusion we note that from whatever direction we have approached 

 the region we have been considering, we find that the present course of 

 the Missouri Eiver is more recent than the Tertiary and earlier than the 

 end of the Pleistocene, or more definitely during the Wisconsin Stage of 

 the Glacial period. 



Moreover, we have been reading one of the most interesting chapters 

 of glacial history, but let no one think that we have been reading the 

 most interesting. We know no reason why the stream changes were not 

 as numerous and striking in the Nebraskan or the lUinoian as we have 

 found them to be in the Wisconsin. 



