310 E. S. Tarr — Topographic Features of Texas. 



three times as wide. A terrace twenty-five or thirty feet 

 above the river bed, sometimes on one side sometimes on both, 

 has been cut by the high water. The river sometimes rises 

 forty or even fifty feet, as is strikingly shown by the presence of 

 rafted logs near the top of Pecan trees. Great as are the 

 floods at such time they are incapable of carrying off all the 

 sediment and the flood plains are rapidly growing, at least in 

 the area affected by the dams in the older Paleozoic, region. 

 Upright tree trunks, comparatively fresh, are sometimes ex- 

 posed for a distance of five feet above the roots, and bones of 

 the mastodon are found both in the alluvial deposits of the 

 Colorado and its tributaries. 



That there has been a period of very rapid erosion just pre- 

 vious to the present condition is attested to by numerous points 

 of evidence. The Colorado is flowing in a somewhat degraded 

 canon above the Silurian ; and where the rocks are hard 

 enough to resist rapid erosion this canon character is well pre- 

 served. The butte type of erosion so typical of the bordering 

 Cretaceous is also, I believe, an evidence of previous rapid 

 erosion. One example, the Santa Anna Mts (so called), re- 

 mains in the center of the Carboniferous. There are two 

 buttes, side by side and divided by headwater erosion, in Cole- 

 man county, fully fifteen miles from the nearest notch of Cre- 

 taceous. The capping stratum is a compact magnesian lime- 

 stone, the lowest of the Caprina division of Hill. The lower 

 beds are Trinity sands. The length along the top is a little 

 more than a mile, the height 250 feet. Five distinct large 

 creeks head in the region near these buttes, which are the last 

 remnant of a Cretaceous divide. The hard stratum on the top 

 which permits the retention of the butte outline is one of the 

 several strata which in the bordering Cretaceous gives the 

 topography its benched mesa and butte outline, a topography 

 so characteristic that a person can with safety predict Cretace- 

 ous as far as he can see the outline. 



The degraded canon character of the Colorado valley and 

 the sharp benched and butte outline of the Cretaceous seem 

 to indicate a period of hurried erosion of recent date ; and the 

 early Quaternary uplift which placed the Tertiary above the 

 sea may very well be correlated with this period. 



From this it would seem that the middle Colorado originally 

 starting on the new Cretaceous land is now superimposed Upon 

 the older Paleozoic, with which it is engaged in a struggle 

 brought about more quickly by its rejuvenation by elevation. 

 Owing to the vicissitudes then imposed, it is tending to reach a 

 temporary old age in the portion above the barriers and this 

 tendency is being increased by the peculiarities of rainfall. 



