790 THE ILLINOIS GLACIAL LOBE. 



The texture of our various soils being different, they are enabled to maintain a 

 variety of conditions of moisture, and they partake somewhat of these artificial con- 

 ditions in a greenhouse, the conditions in each of the soils being best suited to the 

 needs of certain classes of plants. 



The amount of moisture which a sod can maintain for a crop, under given cli- 

 matic conditions, will depend mainly, (1) upon the amount of space in the soil in 

 which water can enter; (2) upon the extent of subdivision of this space — that is, 

 upon the number of grains of sand and clay there is in a given volume of soil; (3) 

 upon the arrangement of these grains, for, as already remarked, if the grains are 

 symmetrically arranged, so that the spaces shall all be of uniform size, water will 

 move through the soil much slower than if the spaces are of very unequal sizes; 

 (4) upon the amount and condition of the organic matter in the soils. The grains of 

 clay are so exceedingly small that their number vastly exceeds the number of the 

 grains of sand and silt, so that the percentage of clay practically determines the 

 extent of subdivision of the space, and it is thus the most important ingredient of 

 the soil. 



In Illinois and adjacent districts the following- classes of soil are repre- 

 sented: (1) residuary soils, or soils formed from the underlying rock; (2) 

 stony, or glacial clays; (3) gravelly soils; (4) sandy soils; (5) loess, or 

 silt rapidly pervious to water; (6) silt slowly pervious to water; (7) fine 

 silts nearly impervious to water; (8) peaty or organic material. A tabular 

 statement is here presented which shows the origin or mode of deposition 

 and the areal distribution in Illinois of the several classes of soil : 1 



Table of soils of Illinois. 



Variety. ' Origin or mode of deposition. Areal distribution. 



Residuary Decay of the underly- Driftless portion of the State wherever the loess 



ing rocks. as well as the glacial drift is ahsent. 



Glacial clay . 



Glacial Mainly in the northeastern quarter of the State, 



where loess and silts are generally absent. The 

 Shelbyville moraine forms the southern bound- 

 ary and chiefly the western boundary, but in 

 northern Illinois glacial clays form the soil on 

 the Iowan drift area between the Shelbyville 

 moraine and the loess of the Mississippi Valley. 



Gravelly Glacial over wash, j With the glacial clay in the northeastern part of 



streams, lakes. the State, and along streams leading away from 



the Shelbyville andlater moraines. This variety 

 of soil includes gravel knolls and ridges, over- 

 wash gravel plains, terraces, and beaches. 



1 This table was published by the writer in the Report of Illinois Board of World's Fair Com- 

 missioners. 1895. 



