THE SOIL SURVEY 723 



an intimate nature that they cannot be recognized by 

 inspection. However, they are correlated with the origin 

 and mode of formation of the soil, and therefore the use 

 of those factors in the classification is justified as an aid 

 to rapid and accurate field identification. 



621. Source of material — the soil group. — The soils 

 of a region may be similar in many properties because 

 they have been derived from the same kind of rock. They 

 may be similar also because they have been derived from 

 the same mixture of different rock materials. As a result 

 of the many kinds of rock and the different proportions in 

 which they may be mingled, many groups of soil series 

 may be recognized. Some of the commoner groups of 

 rocks identified with these differences are acid and basic 

 crystalline rocks, shales, sandstone, and limestone. 



622. Agency of formation — the soil province. — The 

 way in which a rock formation has been broken down and 

 the residue brought to its new resting place affects both 

 the chemical and the physical nature of the resultant soil. 

 The six groups of forces that have been predominant 

 in the formation of soils are : (1) weathering, or the 

 decay and disintegration of rocks in place, forming a 

 residual soil ; (2) biological processes, which form organic 

 matter and give rise to cumulose soils ; (3) water in 

 streams, lakes, and oceans, which reduces, transports, 

 and sorts soil-forming materials, and which imparts to its 

 deposits a distinctly stratified arrangement; (4) atmos- 

 phere, especially as regards wind, which exerts an abra- 

 sive and sorting action similar to that of water but with 

 a very much smaller range in the texture of the strata 

 formed, and with a type of stratification also distinct 

 from that formed by water ; (5) glaciation, or the action 

 of continental masses of ice, the deposits from which are 



