SOILS OF THE EASTERN UNITED STATES AND THEIR USE XX. 



THE TRINITY CLAY. 



GEOGRAPHICAL DISTRIBUTION. 



The Trinity clay is an alluvial soil occupying large portions of the 

 river bottoms of all of the principal streams which flow through and 

 out from the Cretaceous prairie areas of Alabama, Mississippi, and 

 Texas. The materials from which this alluvial soil is formed are 

 derived by erosion from' the black Cretaceous prairie uplands, and 

 are carried down to the main stream channels to be redeposited as 

 alluvial bottom lands along their courses not only within the region 

 of the prairies themselves, but also, in the case of the larger streams, 

 for considerable distances seaward. The Trinity clay is thus found 

 only in the three States mentioned, and it is doubtful if any large 

 areas will be encountered in other States. Within these limits, how- 

 ever, the type is widely distributed, having been encountered in over 

 20 different soil surveys to an aggregate amount of 570,434 acres. 



CHARACTERISTICS OF SOIL AND SUBSOIL. 



The coloration of the surface soil of the Trinity clay varies only 

 to a limited degree between dark brown or drab on the one hand, 

 and almost jet black on the other. The surface soil has a depth 

 ranging from 6 to 15 inches with an average of about 10 inches. It 

 usually grades downward through a slightly lighter colored zone into 

 a drab, brownish-yellow, or gray clay subsoil which is character- 

 istically many feet in thickness, especially along the major streams. 

 When wet the surface soil is plastic and sticky, but as it becomes 

 dry it not infrequently sun-cracks and granulates to a considerable 

 degree. The subsoil is invariably waxy and plastic. Both the surface 

 soil and the subsoil are unusually stiff and impervious clays even 

 among the many heavy clay soils encountered in the alluvial bot- 

 toms of the various rivers of the United States. Large numbers of 

 mechanical analyses of the subsoil of the Trinity clay have shown 

 a clay content ranging from 35 per cent to 69 per cent, with numerous 

 instances where more than one-half of all of the subsoil material con- 

 sisted of this very finely divided and plastic constituent. Very little 

 material coarser than silt is ever present. 



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