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ILLINOIS ACADEMY OF SCIENCE 



2. A belt of moraine-like hills, composed of loess instead 

 of till, borders the Iowan drift sheet in Iowa, as shown in 

 figure 1, and continues as an unusually thick deposit, but with 

 decreasing thickness for a considerable distance south and 

 east of the margin of this drift sheet. 



3. The loess is very thin and patchy or entirely absent, 

 over the surface of the younger (Iowan and Wisconsin) drift 

 sheets in all of this region. 



4. Over the upland areas of the older (Kansan and Illi- 

 noian) drift sheets the loess has its greatest development in 

 Iowa and northern Illinois, where its average thickness is 

 10 to 15 or more feet. The thickness diminishes notably 

 towards the south and east, being only 1 to 4 feet over the in- 

 terstream areas in southern Illinois south of the Kaskaskia 

 river, and south of the Wisconsin drift sheet in Illinois, In- 

 diana, and Ohio. In this region of thin loess the deposit is not 

 so homogeneous as it is farther north ; in many places the upper 

 12 to 18 inches being very porous, and of a white, ashy ap- 

 pearance, and often being referred to as "White Clay." Below 

 this white superficial portion the material is brown and much 

 more clayey and compact, and tends to break into small pris- 

 matic blocks when dry. This peculiarity is thought to have 

 been developed by the finest particles of the surface loess 

 having been carried downward by ground water, leaving the 

 upper part very porous, but forming a compact deposit where 

 these finer particles became lodged in the small cracks and in- 

 terstices lower down. The general distribution of the loess 

 in Illinois is shown on the accompanying map. (Plate 1). 



COMPOSITION OF THE LOESS 



The loess is composed of very small angular undecomposed 

 mineral particles, of which quartz predominates and feldspar, 

 hornblend, calcite, dolomite and other minerals are also com- 

 mon. The texture is usually quite uniform except for occa- 

 sional pebbles ranging in size to one inch in diameter, which 

 occur in the lower 2 to 5 feet of the deposit. It is thought 

 that these pebbles were not originally deposited with the loess 

 as will be shown later. Some of them are of chert or limestone 

 or sandstone of local origin and others are of crystaline rock. 

 Any kind of pebbles common in the underlying till may occa- 

 sionally occur in the lower part of the overlying loess, not only 

 on the hills and slopes, but also over the more level areas. 



The following table of mechanical analyses of loess from 

 a number of localities shows the texture of the upper part of 

 this deposit in different parts of the state. 



