CHAPTER VII 



PHYSIOGRAPHIC FACTORS 



SOIL 



Land masses of the earth are covered by an unconsolidated sur- 

 face mantle of mineral particles derived from parent rock by proc- 

 esses collectively called weathering. The depth of the mantle is 

 variable depending upon disturbances and time, while its physical 

 and chemical properties depend upon the nature of the parent 

 rock and the weathering agencies that may have affected it. This 

 inorganic material may be termed soil but is usually not so con- 

 sidered until organic materials have accumulated from organisms 

 that have lived in or upon it. 



Soil Formation.— Weathering may result in purely physical 

 change, as when rock masses are broken into smaller and smaller 

 sizes, or may be of a chemical nature, producing changes in com- 

 position of the material. The two processes function together nor- 

 mally. Disintegration is largely accomplished by physical agents, 

 such as water, wind, ice, and gravity, and by expansion and con- 

 traction resulting from temperature changes. The first four agents 

 are functional through the erosive action of the load of cutting 

 material they transport and are, therefore, effective in proportion 

 to speed of movement or to force and pressure. The effects of 

 temperature are the most widespread although not always con- 

 spicuous. Differential expansion and contraction of rock materials 

 result in cracking, which is especially marked when temperature 

 changes are abrupt. The widest temperature fluctuations occur in 

 arid regions and at high altitudes where their effectiveness is indi- 

 cated by consistently coarse and angular soil particles. To a lesser 

 extent the process goes on everywhere. Prying action of plant 

 roots and excavating or burrowing by animals may contribute to 

 disintegration, but these activities are certainly of greater impor- 

 tance in their facilitating of chemical processes. Openings in the 

 soil increase aeration and the percolation of water. Shifting the 



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