POST-TERTIARY DEPOSITS. 



61 



ozoic rocks and the great Cretaceous area, and finally deposit, in East Texas, a 

 sediment composed of materials from these regions in the form of a highly cal- 

 careous red sediment. The Trinity River rises in the Paleozoic rocks of 

 Northern Texas, but far east of the "Staked Plains," and passing down 

 through the Cretaceous area, becomes charged with calcareous matter. Hence 

 its sediments, though often calcareous, do not have the red color of the Red, 

 Brazos, and Colorado rivers. The Sabine rises still east of the Trinity, while 

 the smaller rivers, such as the Neches and Angelina, rise in the timber re- 

 gion, and the character of the sediments of them all varies with the region 

 they rise in and flow through. 



Colorado River Silt. — The alluvium now existing in the immediate bluff 

 of this river, and rising ten to thirty feet above the water, is composed of deep 

 red stratified sandy and clayey silt, containing many land shells and frequent 

 beds of leaves, as well as the branches and trunks of trees. This is fre- 

 quently underlaid by gravel beds such as have been described in the high 

 lands. Occasionally there is found overlying, unconformably, the red bluff 

 a deposit of deep chocolate-colored stratified clay. It occurs in lenticular 

 patches on the surface of the red bluff, and contains many beds of leaves and 

 vegetable detritus. These deposits doubtless represent old filled-up channels 

 or river bottom lakes. When a river shifts its course the old channel is 

 often converted into a lake. This gradually becomes filled up by the wash- 

 ing of detritus by rain, by the drifting of sand by the winds, and by vegetable 

 matter; and, after passing through the stage of a bog, eventually becomes 

 dry land. The soft strata of the Tertiary are especially well adapted to the 

 occurrence of such phenomena, while in hard rock they would not so readily 

 happen. Some of these chocolate clay deposits may also represent old lakes 

 in the course of the main river, filled up with sediment by the slackening of 

 the waters as they enter the wide part of its channel. Occasionally both the 

 red and chocolate bluff contain small white calcareous nodules. The allu- 

 vium now being laid down along the Colorado in the Tertiary area of Texas 

 is a gray or buff-colored sand with occasional pebble beds. 



Figures 3 and 4 show the relation of the red, the chocolate, and the gray 

 alluvium. 



Fig. 3. — Section showing relation of red and chocolate silt, a, red silt; b, chocolate silt. 



