SOILS OF ILLINOIS. 795 



which borders the Illinois, a belt several miles in width. Aside from this 

 main district, there is considerable silt of this class between the Rock and 

 Mississippi rivers, in northern Illinois, capping' the earlier or Ulinoian drift 

 sheet. 



On the newer (Wisconsin) drift, as stated above, silts slowly pervious 

 to water cover large districts in central and eastern Illinois to a depth of 

 several feet. In northeastern Illinois such a silt capping is not a common 

 feature. 



Wherever- silts of this class occur the vegetation is usually prairie 

 grass, and there is a blackening of the soil by humus to a depth of several 

 inches, often 2 feet or more. This class of silts affords a highly productive 

 soil, one which will yield fair returns even under most careless methods of 

 farmino-. Corn and grass are the staple products, but other crops have a 

 fair yield. 



FINE SILTS NEARLY IMPERVIOUS TO WATER. 



The fine silts nearly impervious to water are of two classes — white 

 clay and "gumbo." The first class covers the uplands of much of southern 

 Illinois. The second is common in portions of modern river valleys, remote 

 from the current and subject to overflow only in periods of extreme high 

 water, and has great extent along the Illinois and Mississippi river bottoms. • 

 A less compact silt, found in river bottoms, is known as potato land. 



The white clay is a pale-colored deposit, scarcely at all blackened by 

 humus. It covers the greater part of Illinois south from a line running- 

 eastward from Litchfield, Illinois, to the Wabash Valley, near Terre Haute, 

 Indiana. It also covers much of Clay, Vigo, and Sullivan counties, Indiana. 

 It is so poorly drained that much of the water stands on the surface until 

 removed by evaporation, while in seasons of drought scarcely enough water 

 rises from below to supply the loss from evaporation. In the southeastern 

 portion of Illinois and southwestern part of Indiana there is, however, a 

 looser soil, less easily influenced either by excess or deficiency of rainfall. 

 In those districts the surface is hilly and the white clay is much more eroded 

 than in flat tracts. The drift is so thin, also, that the rock, in many places, 

 comes sufficiently near the surface to have become uncovered by erosion 

 and thus to give character to the soil. 



There are extensive districts in south-central Illinois with very flat sur- 

 face where the white clay soil is underlain at a depth of a few inches by a 



