SOILS OF THE EASTERN UNITED STATES AND THEIR USE XXVI. 



THE HOUSTON CLAY. 



GEOGRAPHICAL DISTRIBUTION. 



The Houston clay is an important and extensively developed soil 

 type of the Cretaceous prairie region of the Gulf Coastal Plain. It 

 has been encountered in 21 areas located in 4 States and mapped 

 to an aggregate extent of 763,688 acres by the Bureau of Soils. The 

 most extensive occurrences are in central and western Alabama, 

 northeastern Mississippi, and west central Texas. In all of these 

 areas it is associated with the black prairie soils. 



CHARACTERISTICS OF SOIL AND SUBSOIL. 



The surface soil of the Houston clay is prevalently a brown or 

 black loamy clay ranging in depth from 4 to 10 inches underlain by 

 a stiff, plastic, light-brown, yellow, or gray clay, which at an average 

 depth of about 20 inches passes into a light-gray or white rotten 

 limestone or chalk. The color of the surface soil is somewhat varied, 

 depending upon its topographic position, the darker colored areas 

 occurring in more level situations and in slight depressions, while 

 the light-colored brown or gray surface soils are found upon steeper 

 slopes where erosion has largely removed the surface soil material. 

 The Houston clay is a residual soil derived from the weathering of 

 the Selma chalk in central Alabama and northeastern Mississippi; 

 of the "rotten limestone" of the Jackson stage of the Eocene in 

 the south central counties of Mississippi; and from the Austin chalk 

 in west central Texas. In all cases these formations consist of 

 impure argillaceous limestones or marls of Eocene or Cretaceous 

 age exposed in broad belts along the outcrops of the formations. 

 Within each of the areas where these formations and their derivative 

 soils occur there are also areas of varying size where more recent 

 deposits have been laid down over the marls. In consequence 

 there has been more or less mingling of material from other sources 

 with that derived directly from the underlying marl, which has given 

 rise to some variation in the composition and coloration of the type, 

 particularly the surface soil. The brown or reddish-brown surface 



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