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ILLINOIS ACADEMY OF SCIENCE 



flood plain areas, would have their velocity checked and so 

 be compelled to drop a part of their load on the bordering 

 banks, which then as now, were doubtless forest covered 

 and so furnished lodgment for a part of the silts de- 

 posited upon them. A large part of the material would al- 

 ways be carried beyond the bordering hills and finally be 

 dropped upon the interstream areas. 



Even the details of thick loess distribution bordering the 

 streams are in harmony with wind deposition, for the thickest 

 loess is not where the winds blow straight across the valleys, 

 but is on the bluffs bordering those portions of the valleys 

 which trend oblique to the prevailing wind currents, so that 

 the winds followed along the valley for some distance before 

 their movements were obstructed by a bend in the channel. 

 This is illustrated by the deposits along the Sangamon river 

 in the Springfield Quadrangle. The deposits of the thick loess 

 around the margin of the Iowan drift sheet and extending 

 with decreasing thickness for many miles to the south and east 

 are also consistently explained only by the agency of the 

 wind The most striking characteristics of the Iowan drift 

 sheet are, 1, the thinness of the drift; 2, the lack of the usual 

 amount of fine material and the presence of great numbers of 

 bowlders upon its surface, and, 3, the moraine like hills of loess 

 bordering the area. All of these peculiarities would be de- 

 veloped as a result of winds blowing outward over the surface 

 of the glacier, or over the dried mud flats of the drift sheet, 

 after the glacier had retreated to the northward, but before a 

 cover of vegetation had become established upon the drift sur- 

 face. Winds sweeping unchecked over the bare drift surface 

 during dry periods would pick up large quantities of the finer 

 materials. A part of their load would be dropped when the 

 forest that fringed the border of the Iowan drift area was 

 encountered and a part would be carried farther forward. If 

 such conditions attended the withdrawal of the Iowan ice sheet 

 as resulted in the surface of the drift remaining bare of vegeta- 

 tion for a long time, the bowlders and coarser debris of the 

 till would become concentrated at the surface by the removal 

 by the winds of a large part of the finer constituents, reduc- 

 ing by this great amount the thickness of the Iowan drift. 



A part of the material gathered by the winds from the river 

 flood plains or from the Iowan drift surface would be dropped 

 at the immediate border, a part would be carried for a longer 

 or shorter distance before finding permanent lodgment in the 

 vegetation that covered the uplands. Another part dropped in 

 places not well protected with vegetation, was doubtless again 



