470 J. E. TODD PRE-WISCONSIX CHANNELS IX DAKOTA-NEBRASKA 



the Ponca, as indicated on the map. Along this course the altitude is 

 from 1,400 to l,i50 feet, and there is no swell rising more than 20 or 30 

 feet above the general slope. Doubtless the configuration resulted from 

 the work of glaciers which overrode the region. It is reasonable to be- 

 lieve that the upper part of Platte Creek from as far north as AVhite 

 Lake may have all drained through this channel in the earlier and later 

 portions of the Kansan stage of the ice. However, no direct evidences 

 of this was obtained from excavations or deposits. The profile of tliis 

 A'alley corresponds with the slope of the streams just described, allowing 

 for about 100 feet deepening during the interglacial epoch following the 

 Kansan or pre-Wisconsin stage, as found in the case of the James and 

 Vermilion. Its level corresponds fairly well with that of the Hartington- 

 Coleridge and later channels, which were occupied by the peripheral 

 waters of the Kansan stasre. 



"to" 



CON^CLUSIONS 



It is believed that the evidence presented above sustains the interpre- 

 tation of events as outlined at the beginning of this paper. There may 

 also be added with somewhat less confidence the following inferences : 



1. The edge of the pre- Wisconsin ice-sheet did not extend so far west 

 as that of the Wisconsin sheet, although it reached considerably farther 

 south. 



2. The Wisconsin ice did not reach as far south as the southern edge 

 of the earlier ice-sheet because the river valleys, particularly of the Xio- 

 brara and James, were then much deeper and therefore a much more 

 serious obstruction to extension in that direction. Possibly, also, the 

 later ice may have been thinner or of higher temperature. 



3. On the west and southwest, on the contrary, the marginal drainage 

 may have flowed just beyond the edge of the earlier ice, and by its inter- 

 glacial erosion removed obstacles, so that the later ice filling the new 

 valley advanced several miles farther Avest than the earlier ice had done. 

 As a result the Missouri, the master stream of the time, then located its 

 present valley, which is farther west than the one it occupied in the pre- 

 Wisconsin time. This was done in a way very similar to the shifting of 

 the drainage from the ancient Ponca Creek and ancient Niobrara River 

 to the present valley of the Missouri, as stated above and shown on the 

 map, plate 25. 



