CHAPTER XV. 

 SOILS. 



SOURCES OF SOIL MATERIAL. 



The principal sources from which the soils of the region under discus- 

 sion are derived are the glacial drift and the loess, with its associated silts 

 of glacial age. The underlying rocks are indirectly a source of much 

 material, since their decomposed surface portions were incorporated in the 

 drift, but they constitute a minor source so far as direct contribution is con- 

 cerned. Lakes and streams attending the melting of the ice sheet have 

 deposited material in considerable amount, and it is thought that the wind 

 also has been influential in distributing fine material over portions of the 

 surface of the region. The present streams are also a source for soil in the 

 districts over which they spread in their flood stages. 



The preceding discussion has shown that portions of the surface of this 

 region date from the Iowan stage of glaciation, namely, the portions cov- 

 ered by the Iowan loess and associated silts, and the portion in which the 

 Iowan till sheet forms the surface. These deposits form the surface of con- 

 siderably more than one-half the region. In the remainder of the region 

 the Wisconsin drift constitutes the main part of the surface, though small 

 areas have been occupied by lakes and streams since the withdrawal of the 

 Wisconsin ice sheet. The area covered by Lake Chicago is the most con- 

 spicuous illustration. 



CLASSES OF SOIL. 



Inasmuch as soils are largely composed of fragments of rock material, 

 they are naturally dependent to a certain degree upon the character of the 

 rock from which they are chiefly derived. This is especially true in ungla- 

 ciated regions, but in glaciated regions also the soil is found to depend upon 

 the character of the underlying deposits. There are several modes of classi- 

 fication in use, based usually upon either chemical constitution or physical 

 texture or characteristics. The classification which seems to best serve our 

 purpose is based mainly upon physical characteristics. This classification 

 also sets forth what appear to be the characteristics of chief importance. 

 The elaborate investigations made by Prof. Milton Whitney, of the United 



