CHAPTER V 



SOIL FORMATIONS AND CLASSIFICATIONS 



Residual soils. Residual soils are those that are formed in place 

 from the disintegration of rocks. They consist of the least soluble 

 decomposition products, which often constitute but a small pro- 

 portion of the original rock. Thus a limestone containing 80 per 

 cent of calcium carbonate and 20 per cent of impurities (as poly- 

 silicates, etc.) may weather to a soil composed entirely of the im- 

 purities from which the calcium carbonate has been completely 

 removed by leaching, and the polysilicates may have partially 

 broken down into acid silicates, zeolites, clay, oxids of silicon and 

 iron, etc. 



Transported soils. These are also formed from disintegrated and 

 partially decomposed rock, but instead of remaining in the place 

 previously occupied by the rock, they have been transported, and 

 often retransported, by various agencies (materials from many 

 sources sometimes being mixed together), and finally deposited in 

 the places which they now occupy. Wind, water, and glaciers 

 are the chief carrying agencies. 



Glacial material (bowlder clay) is characterized by the presence 

 of worn or rounded stones, varying in size from sand grains to 

 bowlders, embedded in silty clay. While glacial drift covers exten- 

 sive areas in northern United States, sometimes to a depth of 100 

 feet or more, the glacial material is covered in many areas by a 

 deposit of loess, 1 varying in depth from a few inches to several 

 feet. 



Loess is characterized in part by the absence of pebbles. It 

 consists largely of silt, with some very fine sand and but little clay. 

 It has been transported by wind, as a rule, and in places is found in 

 high elevations and even overlying residual soils, but in deep loess 

 deposits, as in the bluffs along the Mississippi and other large 

 streams, evidences are found of some transportation by water. 



1 This word is taken directly from the German (like sauerkraut) and pronounced 

 like less, with the lips protruded as in whistling. Similarly, the English word beef- 

 steak has been adopted into the German language. 



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