16 CHICAGO ACADEMY OF SCIENCES. 



Prof. Worthen early recognized this soil horizon at the base 

 of the loess and made numerous references to it in the Geology of 

 Illinois. Being situated near the surface, it is much more fre- 

 quently exposed to view than the soils at lower horizons. Prob- 

 ably, also, it has been subjected to less disturbance and removal 

 than soils which were overridden by the ice sheet. 



lOWAN DRIFT SHEET AND MAIN LOESS DEPOSIT. 



This sheet of drift was brought to notice by Mr. W J 

 McGee in his early papers on the drift of Northeastern Iowa, where 

 it is commonly referred to as the "upper till." It was found by Mr. 

 McGee to be of the same age as a deposit of loess which covers 

 Southeastern Iowa, but which Avas excluded from much of North- 

 eastern Iowa by the ice sheet (i). 



This drift sheet Mr. McGee found to be markedly dififerent in 

 its stony ingredients from the sheet that underlies it. It is sep- 

 arated from that sheet by a prolonged interval. This interval, it 

 now appears, comprises not only the third or preloessial interval 

 of AVestern Illinois, but the time covered by the Illinoian stage of 

 glaciation and the deglaciation interval which preceded it. Com- 

 prising thus the time of two stages of deglaciation and one of gla- 

 ciation, the interval shown in Northeastern Iowa is necessarily 

 very 'marked. 



The lowan sheet is thought to be represented in Northern 

 Illinois by a sheet of drift found mainly east of Rock River, 

 though extending in places west of that stream. This sheet is con- 

 cealed within a few miles east of Rock River b}^ a later drift sheet 

 (the earlv Wisconsin). 



Both in Iowa and Illinois the sheet referred to the lowan 

 stage is perceptibly fresher than the sheet of the Illinoian stage 

 of glaciation. The depth of leaching is, on an average, one to two 

 feet less than on the Illinoian drift sheet. The erosion of streams 

 is also less, apparently not more than half as great as in the Illi- 

 noian drift sheet. 



FOUETH OR POST-IiOESSIAXi INTERVAL OF RECESSION OR DEGLACIATION. 



This interval is shown to be one of considerable length by 

 the amount of valley excavation which occurred between the de- 

 position of the loess and the succeeding stage of glaciation. At 

 the close of the loess deposition the main valleys, such as the 

 Missouri, Mississippi, and Illinois, were filled througihout most 



[U See Eleventh annual report U. S. Geological Survey for a full discussion of 

 t ills drift sheet. 



