DEVELOPMENT OF THE TOPOGRAPHY. 521 



their disturbed structure in increasing quantity southward. The sediments 

 of the great Neocene lake epoch, which constitute the Llano Estacado 

 formation, and which were laid down horizontally between the mountain 

 blocks of the above area, are largely composed of Comanche debris. So 

 extensive was the denudation and erosion of this little appreciated Neocene 

 lake epoch that the western two-thirds of the Comanche series was de- 

 graded, and entered into the composition of these lake deposits. It has been 

 my pleasure during the past year to find several remnants of the Comanche 

 series west of the Llano Estacado outcropping beneath its escarpment of 

 Tertiary beds. 



That this first great denudation of the Comanche series took place since 

 the Eocene is further demonstrated, first, by the utter absence of Eocene debris 

 in the sandy littoral beds of the latter formation : the base-leveling of 

 Eocene time did not cut down to the Comanche series. Secondly, by the 

 fact that the Comanche debris again enters into the composition of the post- 

 Eocene formations of the coastal region, of probable synchronous age with 

 the Llano Estacado epoch. 



The second epoch of destruction of the Comanche series by denudation 

 thus far recognized was in late Quaternary time, when the Gulf coast coin- 

 cided with the present eastern border of the Cretaceous. By this process 

 the older strata are exposed, and the escarpments of all the terranes are 

 slowly receding eastward. 



It is impossible at present to enter into a discussion of the evolution of 

 the present drainage, across the strike and with the dip, which has produced 

 the unique and characteristic topography, further than to say that there are 

 two important stages in its history independent of the above-mentioned 

 Neocene Llano Estacado epoch, when the extension of the Comanche ter- 

 ranes westward from the lOOtfi meridian were almost entirely degraded, and 

 their debris entered into the composition of the Llano Estacado sediment : 

 The older is the system of rivers embracing the Red, Colorado, and Brazos, 

 all of which have, by headwater erosion, cut their way completely across 

 the Comanche area and deep down into the Paleozoic floor. The second 

 and later epoch of erosion belongs to a superimposed drainage system 

 composed of such streams as the Trinity, Paluxy, Lampasas, Guadalupe, 

 Nueces, Frio and Devils rivers, which are now carving the great plateaus 

 once separating the streams of the older system into remnantal buttes and 

 mesas and reducing them to base-level. 



By this double erosion and degradation by far the greater part of even 

 the post-Llano Estacado remnant of this magnificent series has been elimi- 

 nated and what is now exposed, although covering an immense area of 

 country, is only a remnant of the previous extent. 



The present topographic forms of this erosion can be readily understood. 



