4 SOILS OF THE EASTERN UNITED STATES. 



The Trinity clay could only be confused with the Wabash clay 

 among the alluvial bottom-land types, as these two are the most im- 

 portant dark-colored alluvial clays. The Trinity clay, however, is 

 invariably associated with deposition of sediments derived from the 

 black Cretaceous prairies, while the Wabash clay is confined to the 

 drainage system of the Misisssippi River, and is derived from the 

 brown or black loessial upland prairie soils. The Trinity clay also 

 possesses the distinguishing characteristic of containing lime con- 

 cretions and nodules in the subsoil. Chemical analyses of the subsoil 

 have shown from 3 to 35 per cent of calcium carbonate in the numer- 

 ous samples which have been examined. 



SURFACE FEATURES AND DRAINAGE. 



The Trinity clay resembles the majority of other alluvial soils in 

 that its surface configuration is almost absolutely level and relieved 

 only by those minor changes in elevation due to the existence of 

 low ridges and bars lying parallel with the course of the principal 

 stream along which the type occurs and of intervening depressions, 

 sloughs, or bayous which mark old stream channels. 



The altitude at which the Trinity clay is found varies considerably 

 \vith the elevation of the streams which it adjoins. In Mississippi 

 and in Alabama practically all of the type is found at an elevation 

 ranging from 150 to 400 feet above tide level, while in Texas the 

 altitude ranges from 5 or 10 feet to 500 feet above sea level. In all 

 cases the surface of the type is only slightly elevated above the normal 

 water level of the stream which it adjoins. This elevation ranges 

 from 5 or 6 feet in the minimum instances to an elevation not exceed- 

 ing 15 to 20 feet in extreme cases. Almost invariably the Trinity 

 clay occupies the first bottoms, although in some areas in northern 

 Texas it is found developed upon older second bottoms at somewhat 

 higher altitudes. As a result of its close association with the stream 

 drainages and of its practically level surface, the natural drainage 

 conditions over the Trinity clay are almost universally poor. It is 

 only upon the higher ridges and upon limited areas of the second bot- 

 toms that natural drainage conditions are adequate. In practically all 

 other areas artificial drainage must be installed, either through the 

 opening of more direct stream channels or through the drainage of 

 portions of the type into the deeper sloughs and bayous by means of 

 open ditches. Considerable areas of the Trinity clay have thus been 

 reclaimed and brought under cultivation. 



In addition to defective drainage the greater part of the type in 

 all areas where it occurs is annually subject to overflows at the time 

 of the spring freshets. A few areas of considerable agricultural 

 importance escape this overflow. Very few areas thus far encoun- 



