THE SOILS OF CALIFORNIA 35 



tion in soil work since 1913, in a station publication * 

 from which the following statements, leading to a 

 correct understanding of the origin and nature of 

 California soils, are selected: 



"In the humid regions of the world, and especially 

 in the humid region of the United States, practically 

 nine-tenths of the soils are either of residual or of 

 glacial origin. The glacial soils have been trans- 

 ported and deposited by ice, and while the glacial 

 deposits may be very deep, the true soil is not deep. 

 The subsoil is usually heavier than the surface, often 

 clayey, and the practical feeding depth of roots is 

 usually less than four feet. The residual soils are 

 much more extensive than the glacial soils. They 

 are formed by the destruction of rock masses, the 

 disintegrated and decomposed fragments accumulat- 

 ing on the surface of the hard rock to form the soil 

 mass. Eesidual soils usually have a surface soil six 

 or eight inches deep, resting on heavier material that 

 grades to a clay at two or three feet in depth. At 

 greater depths rock fragments are found in the clay 

 and these grow more numerous until the mass is 

 largely broken or 'rotten rock/ and finally the solid 

 rock mass is reached. The total depth of the soil 

 mass above the rock varies greatly, but usually is less 

 than four feet. 



"In the humid regions, the transported soils, other 

 than those formed by glacial action, are of little 

 extent. The flood plains and bench lands along the 

 rivers and creeks are exceedingly productive, but 



1 Circ. 210, Calif. Exp. Sta., Berkeley, March, 1919. 



