184 THE ILLINOIS GLACIAL LOBE. 



east borders of the valleys is the result of seolian agencies, for there seems 

 no reason to suppose that the glacial waters, while overspreading the uplands 

 on the borders of the valleys, would have deposited an excessive amount 

 of sediment on the east side. Whether the seolian action is to be measured 

 simply by the excess of the accumulation on the east bluff or should be 

 made to include a portion on both bluffs of the stream is not as yet mani- 

 fest. The west winds may have carried material from one valley across to 

 its neighbor where valleys approach closely, as in the case of the Illinois 

 and Mississippi, in Calhoun County, Illinois, where geolian loess appears to 

 have covered the entire interval. The calcareousness of the seolian loess is 

 significant and has an important bearing upon the determination of the 

 source of supply and the date of deposition. The supply can hardly have 

 been derived from the great western . plains, where the material available 

 for wind transportation is largely leached of its calcareous ingredients. It 

 can not be referred to a much later date than the close of the lowan stage 

 of glaciation, since leaching would have rendered the surface portion of the 

 silts noncalcareous. It can not be referred to a much earlier date, for it is 

 separated from the Illinoian drift sheet by the Sang'amon soil and weathered 

 zone. As urged by Chamberlm, the seolian deposition appears closely 

 connected with glacial and fluvial deposition. 



