22 



NATURE AND PROPERTIES OF SOILS 



than the inner and more protected portions. This differential 

 heating tends to set np lateral stresses which may cause the 

 surface layers to peel away from the parent mass. This phe- 

 nomena is spoken of as exfoliation. The differential expansion 



SWTH DAKOTA 



MINNESOTA' 



Fig. 4. — Approximate distribution of loess in central United States. 



of tl^e rock minerals of course plays a part in this disintegra- 

 tion, although exfoliation readily occurs in rocks which are 

 more or less homogeneous. While this form of weathering 

 may go on alone, it is much accelerated by chemical action 

 and the prying of freezing water. 



