682 THE ICE AGE IN NORTH AMERICA. 



buried gorge is supported by piles driven thirty feet into 

 the alluvium. Above A this ravine lies wholly in the en- 

 veloping loess. 



From this it is evident that the rock erosion in the lower 

 part of the tributary entering at A is neither post-glacial nor 

 interglacial, but is a remnant of preglacial erosion when the 

 whole region was so much elevated that the river had lowered 

 its bed a considerable depthbelow that now occupied by it. At 

 Omaha the rock bottom of the river is known to be eighty 

 feet below its present bottom. Since the advent of the con- 

 tinental ice-sheet the preglacial gorge has been aggraded to 

 a considerable extent, and meanwhile, during the Iowan stage 

 of the glacial period the loess bluffs of the Missouri Valley, 

 here so pronounced, were deposited on the ever rising flood- 

 plain of the torrents which for many centuries poured out 

 from the melting ice during the closing stages of the epoch. 

 The Lansing skeleton antedates the deposition of the entire 

 loess formation. 



Positive evidence of the aqueous character of the loess 

 overlying the skeleton, was noted by Mr. Upham ; namely , 

 a " distinct darker layer of loess mostly about two inches thick 

 but in part merely a threadlike line, traceable continuously 

 through all the seventy-two feet of the west wall of the tunnel, 

 running about three or four feet above the limestone floor, 

 and one foot or a little more above the base of the loess. " 

 This extended in a straight plane descending from, south to 

 north about one inch to ten feet. Other lines of nearly horizon- 

 tal stratification were also observed, thus clearly showing that 

 it is an aqueous deposit of the same character with that of 

 the loess of the whole valley. 



The most plausible suggestion for the later deposition has 

 been made by Professor J. E. Todd,* who thinks it possible 

 that the loess on which the Concannon house stands may be 



* "Recent Alluvial Changes in Southwestern Iowa," "Proceedings 

 of Iowa Academy of Science," 1907, pp. 257-266. 



