62 



GULF TERTIARY OF TEXAS. 



b 



Fig. 4. — Section showing relation of red and chocolate silt and recent river sand, a, red silt; 

 b, chocolate silt; c, recent river sand. 



Above the river bottom are found three or more successive terraces, finally 

 reaching the upland gravel, as explained on page 60. 



The red and chocolate alluvium underlies extensive river bottom areas and 

 gives rise to rich clay or loamy soils, varying somewhat in character with the 

 local changes in the alluvium, but generally of a black or brown color, and 

 very productive. 



Brazos River Silt. — The alluvium now found along the banks of the Brazos 

 rises ten to thirty feet above the water, and is composed of stratified red 

 and chocolate clay, often with beds of yellow, black, or light watery-green 

 colored clay. The whole deposit is highly calcareous, and often contains 

 many small white calcareous nodules. The yellow, black, or watery-green 

 clay generally occupies a position below the red and chocolate, though in 

 some cases it is underlaid by thin beds of the latter. The two deposits gen- 

 erally merge into one another, and in only very few cases was a sharp line of 

 separation seen. These clays are underlaid by gravel beds one to six feet 

 thick. 



The alluvium now forming is a gray or buff sand, and in low water thin 

 seams of clay are interstratified with it. 



Above the silt deposits are a series of gravel terraces, often poorly defined, 

 and gradually reaching the upland gravel.* 



The Rio Grande Silt. — The alluvium now forming the immediate bluffs 

 of the river varies in different parts of its course. It rises from ten to thirty 

 feet above the water, is all characterized by being highly calcareous, and con- 

 taining many land shells (Bulumulus). From Eagle Pass to Laredo it consists 

 of a fine gray silt, rarely showing any signs of stratification, in which respect it 

 differs very markedly from the alluvium of the Brazos and Colorado. For 

 some distance below Laredo it continues the same, though indistinct signs of 

 stratification begin to appear; and from Roma on to Point Isabel, at the 

 mouth of the river, it is markedly stratified, and often colored a light brown 

 by iron. It frequently contains small white calcareous nodules, as on the 

 Brazos. In the upper part of the river this alluvium is underlaid by pebble 



*These terraces are much more difficult to define in the Tertiary than in the Cretaceous 

 area, as in the former case the red-colored river deposits and the underlying older strata are 

 of the same general color, and often of the same sandy consistency, while in the Cretaceous 

 country the underlying chalks and clays form a radical contrast. 



