120 MAN AND THE GLACIAL PERIOD. 



fact that there was a vast amount of deposition by water 

 over that area during the earlier stages of the ice-retreat. 

 This has been due partly to the gentler slope which would 

 naturally characterise the borders of an area of elevation, 

 and partly to an extensive subsidence which seems to have 

 begun soon after the ice had reached its farthest extent of 

 motion 



It should be borne in mind that at all times a glacier 

 is accompanied by the issue of vast streams of water from 

 its front, and that these of course increase in volume when 

 the climax has been reached and the ameliorating influ- 

 ences begin to melt away the accumulated mass of ice 

 and to add the volume of its water to that produced by 

 ordinary agencies. As these subglacial streams of water 

 poured out upon the more gentle slopes of the area in 

 front of the ice, they would distribute a vast amount of 

 fine material, which would settle into the hollow places 

 and tend to obscure the irregularities of the previous di- 

 rect glacial deposit. 



Such an instance came clearly under my own observa- 

 tion in the vicinity of Yankton, in South Dakota, where, 

 upon visiting a locality some miles from any river, and to 

 which workmen were resorting for sand, I found that the 

 deposit occupied a kettle-hole, filling it to its brim, and 

 had evidently been superimposed by a temporary stream of 

 water flowing over the region while the ice was still in 

 partial occupation of it. Thus, no doubt, in many cases, 

 the original irregularities of the direct glacial deposits 

 have been obliterated, even where there has been no gen- 

 eral subsidence. 



But, in the area under consideration, the loess, or 

 loam, is so extensive that it is perhaps necessary to sup- 

 pose that the central portions of the Mississippi Valley 

 were subjected to a subsidence amounting to about five 

 hundred feet ; so that the glacial streams from the retreat- 

 ing ice-front met the waters of the ocean in southern 



