158 THE ILLINOIS GLACIAL LOBE. 



southeastern Iowa and western Illinois. In the vicinity of Iowa City, 

 along the borders of the Iowan drift sheet, the loess is so porous that water 

 readily penetrates it and tile draining is seldom necessary. But upon 

 passing southward across Washington, Louisa, Henry, and Des Moines 

 counties, the texture becomes gradually finer, and extensive areas in the 

 southern part of the State require tile draining. In northeastern Missouri 

 the texture is still more compact, so that a large portion of the rainfall is 

 disposed of by surface evaporation. Similarly in western Illinois the loess 

 of Henry, Rock Island, and Mercer counties is more porous than that of 

 counties immediately south. But in this region the belt of porous, loess 

 along the Mississippi and that along the Illinois occupy so much of the 

 interval between the streams that there remains only a narrow strip of 

 compact loess in the vicinity of the divide. East from the Illinois River, 

 as far as the divide between this stream and the Kaskaskia, the loess is 

 sufficiently porous to require but little tile draining. The Kaskaskia 

 drainage basin and much of southern Illinois, like northern Missouri, has a 

 surface silt too compact to absorb the rainfall. The loess in this region has 

 become so puddled (perhaps because of the process of soaking and drying 

 to which it has been subjected) that underdrainage is difficult. Through- 

 out much of the region a hard ferruginous crust immediately underlies the 

 soil at a depth of 12 to 15 inches or less, which needs to be broken before 

 underdrainage can be established. The compact silts of southern Illinois 

 present an ashy appearance near the surface, which gives them the name 

 of "white clay." It is rare to find even an inch of black soil on their sur- 

 face. In this respect they are in striking contrast to the silts west of the 

 Kaskaskia Basin, in which a black soil several inches in depth is generally 

 present. The white clay district is continued eastward into Indiana and 

 southwestern Ohio, occupying much of the interval between the glacial 

 boundary and the southern edge of the Wisconsin drift sheet. 



The mineralogical constitution of the loess and determinations of the 

 size of its grains has been given considerable attention by Salisbury, and 

 the results are presented in the Sixth Annual Report of this Survey. The 

 samples selected were chiefly from localities adjacent to the Driftless Area 

 on each side of the Mississippi. It was found that of 150,887 particles 

 measured, 146,894, or about 97£ per cent, are less than 0.005 millimeter in 

 diameter, leaving only 2| per cent of greater size. The largest particles 



