CHAPTER VII 



CLASSIFICATION OF SOILS 



Need of Classification. The formation of soils by means of 

 the various agencies described has given rise to great complexity. 

 As in any other natural group of objects, the study of the relation- 

 ship existing between the different members of the group is neces- 

 sary for a complete understanding of them. This brings about com- 

 parison and classification. A very simple assumption would be that 

 all soils derived from the same kind of rocks are the same. They 

 do usually have some points of similarity, but so many modifying 

 factors have been at work that important differences are produced 

 even in these. It must be remembered that ,soils are very complex 

 bodies, due to the infinite variety of rocks from which they are 

 derived and the large number of agencies taking part in their 

 formation. 



By far the larger portion of soil material is moved from fhe 

 place of its origin for varied distances, perhaps hundreds of miles. 

 In its travels it may be deposited over and over again, and as a 

 general rule the loose surface of the earth is a mass of drifting 

 material, here to-day and a hundred or even a thousand miles from 

 here in the next geological age. 



BASIS OF CLASSIFICATION 



1. Geological. Soil is a geological formation derived from 

 rocks by geological forces. It is very natural, then, that the geo- 

 logical formation should be used as the basis of classification. A 

 number of States have made general soil maps, basing the areas 

 upon the geology. In many cases this may serve a good purpose, as 

 where the soils are closely related to the underlying geological for- 

 mation. In other places the same formation may give rise to a 

 great variety of soils, and in this case a classification on a geological 

 basis would mean nothing. In extensive glaciated regions the soil 

 usually bears little or no relation to the geological formations 

 beneath the drift. General soil divisions may be based upon the 

 geological agencies that have produced them, and in this way form 

 an important factor in classification. This gives rise to residual, 

 glacial, loessial, alluvial, and other formations. 

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