808 R. S. Tarr — Topographic Features of Texas. 



then more elevated than the Carboniferous and the rise in the 

 land was 'from east to west. 



The extensive sedimentation of the Cretaceous time com- 

 pletely obliterated all pre-existing topography. The deep sea 

 conditions and accompanying thick deposits, so plainly proved 

 by Prof. E. T. Hill, covered all land with a thick mantle. 

 When this deposit was finally raised above the sea and a new 

 era of erosion began, the large streams which first established 

 themselves commenced to flow without regard to the topo- 

 graphic and geologic features buried beneath the Cretaceous. 

 The Tertiary beds which occupy a large portion of east Texas 

 mark the extension of the oceanic waters many miles farther 

 than at present. It is probable that the Tertiary ocean ex- 

 tended much farther inland than at present indicated and that 

 the beds have been in part removed by later erosion. It is 

 plain from this that the waters of the new drainage systems 

 emptied into an ocean not far removed from the buried Paleo- 

 zoic area. What were the conditions west of the Paleozoic 

 has not been ascertained. One stream, the ancestor of the 

 present Colorado, established itself in the Southern Carbonif- 

 erous area, and another, the Brazos, crosses this system farther 

 north. These streams with their tributaries have eroded the 

 Cretaceous from much of the Carboniferous, and the Colorado 

 has in addition removed the Cretaceous from a great area of 

 Silurian. 



The history of these operations is not thoroughly plain in 

 every point. The extensive denudation in this Paleozoic area 

 must mean that this was early a region of strong erosion, and 

 that this condition has been continued. It may also mean 

 that this was the highest region of the new land. Be this as 

 it may the fact remains that in this area, by means of two large 

 rivers and several well established tributaries, a great thick- 

 ness of Cretaceous has been eroded away and some of the 

 Paleozoic removed. This process has established a series of 

 interesting topographic features, some of which throw light 

 upon the later history of the drainage systems. 



The Colorado, which rises in the Staked Plains, while well 

 established in its lower and middle course is rapidly extending 

 its drainage area by headwater erosion in the plains. Deep 

 canons and arroyas mark its new work in this region. Over the 

 Paleozoic it flows with rapid fall and a wonderfully serpentine 

 course without regard to structural features. That it is a 

 superimposed stream is proved by the fact that it is now busy 

 eroding barriers of Silurian and other hard rocks which it has 

 unexpectedly reached after passing through the Cretaceous 

 strata. The larger tributaries flow equally without regard to 

 structural features ; but the smaller tributaries of more recent 



