416 THE ICE AGE IN NORTH AMERICA. 



sissippi from southeastern Minnesota to its mouth ; but its 

 greatest thickness and extent are found in the basin of the 

 Missouri River from southern Dakota to its junction with the 

 Mississippi, and upon the region crossed by the Platte or 

 Nebraska River, its longest tributary from the west, which 

 takes its head-waters from a large district of the Rocky Mount- 

 ains. The continuity of this formation from the borders of 

 the ice-sheet and the glaciers of the Rocky Mountains to the 

 shores of the Gulf of Mexico, the absence from it of marine 

 shells, and the presence of land- and fresh-water shells, indicate 

 that its deposition was by slowly descending floods, uplifted 

 upon the surface of this sediment which was being accumu- 

 lated during every summer through a long epoch, in the same 

 manner that alluvium is now spread upon the bottom-lands of 

 our rivers at their times of overflow. The occurrence of the loess 

 in Guthrie, Carroll, Sac, and Buena Vista counties * in Iowa, 

 covering the region next west of the terminal moraine, with its 

 surface fifty feet above these drift-hills and one hundred above 

 the undulating area of till adjoining their east side, proves that 

 during the time of deposition of this part of the loess the ice- 

 sheet extended to this limit, and was a barrier preventing the 

 waters by which this sediment was brought from flowing over 

 the lower area of till that reaches thence east to the Des 

 Moines River. When the ice-sheet retreated beyond the water- 

 shed of the Missouri basin, the principal source of these floods 

 and their sediment was removed, and the subsequent work of 

 the rivers which cross the area of the loess has been to excavate 

 their present valleys or channels, bounded by bluffs of this 

 formation, f 



A supplementary hypothesis to account for the subsidence 

 assumed by some is, that a bodily depression of the crust of 

 the earth was produced by the weight of the glacier. If one 

 is inclined at first thought to reject this cause as inoperative 

 because of its relative insignificance, he should reflect that 



* " The Ninth Annual Report of the Geological and Natural History Survey 

 of Minnesota," p. 30'7 et seq. 

 \ Ibid., pp. 337, 338. 



