THE IOWAN DEIFT SHEET. 137 



being- immediately underlain by an older sheet of drift. On hillside expo- 

 sures also, some distance within the border, it has been found that in places 

 a thin deposit of Iowan drift occurs at the tops of hills, while the greater 

 part of the slope exposes Illinoian drift. In not a few places in the midst 

 of the Iowan drift area only the Illinoian drift is found at the tops of the 

 hills, though the hills were apparently covered by the Iowan ice sheet. 

 There is apparently a smaller amount of loess associated with this ice lobe 

 than with the Iowan of eastern Iowa. The districts immediately outside 

 its border have only a thin coating, 5 or 10 feet, while the thickness of the 

 loess within its borders is even smaller in amount. It would seem a liberal 

 estimate to allow an average of 10 feet of both till and loess as the product 

 of the Iowan invasion in Illinois. It is therefore sufficient to modify but 

 little the features of the country which it occupies. 



STRUCTURE OF THE DRIFT. 



The drift of the Iowan invasion has generally a more sandy constitu- 

 tion than that of the Illinoian which underlies it. Along the wag'on roads 

 and in other situations where it has been exposed to the action of slight 

 wash, it frequently presents the appearance of fine sand, where the Illinoian 

 drift would present the appearance of clay. Not infrequently the matrix 

 appears to be nearly free from clayey material. There are other places, 

 however, where the till has a stiff clayey matrix, but is readily distinguish- 

 able from the Illinoian by its fresher surface Associated as this till sheet 

 is with the loess, the latter being apparently a derivation from the former, 

 a sandy till is a natural product, the fine material being- carried into the 

 loess. This difference in the two tills greatly affects the character of the 

 soils. Where the Iowan drift is present a dark soil is usually developed, 

 while in the outlying districts of northwestern Illinois the soil is of a yellow 

 or brown color. 



In places the Iowan till is covered to a depth of several feet by loess, 

 but in the Pecatonica Basin and much of the country east of Rock River 

 the loess amounts to scarcely more than a skim coating 1 or 2 feet in depth, 

 a coating such as may have been deposited by wind action since the final 

 withdrawal of the ice sheet or have accompanied the melting- of the ice. The 

 portions best protected by loess present a till scarcely at all stained or 



