Evolution of the Mountainous Texas Region. — Hill. 113 
their greatest work in their distant sources and merely cutting 
deeper channels in the areas through which they extend. 
Let us return to the part played by the ancient Paleozoic floor 
in this chapter of events. 
Extending across the heart of Texas from north to south as a 
low lying barrier between the gulf and the interior post-Paleozoic 
seas, it was so base leveled and degraded that when the profound 
Cretaceous subsidence began it was buried beneath thousands of 
feet of off-shore sediments, but of a softer, and more easily de- 
graded nature than the firm granitic, hard limestone and quartzite 
beds of the Paleozoic, which especially composed its lower part 
and southern end. 
In the great series of loading down the coastal plain coastward 
of it, and the consequent oscillation of elevation and subsidence 
of the vast plains region, it was natural that this solid core of 
Paleozoic rocks had greater resistance for the accompanying 
strains and stresses, and finally under continued loading the great 
balcones fault developed approximately along the coastal margin, 
leaving sharp projecting land adjacent to the Appomattox sea 
upon which degradation was facilitated; this has partially re- 
moved its Neozoic covering, and will ultimately degrade it to base 
level. 
The topography of the Texas plains is the product of the etch- 
ing of a series of sedimentation plains of post Eocene age by a 
series of consequent autogenetic drainage s^'stems. The plains 
are the product of the oscillations of the Cordilleran continent, 
whereby the waters of the gulf and accompanying base leveling 
swept back and forth across the Texan region. These oscilla- 
tions began with the overcoming of the central Paleozoic barrier 
at the beginning of the Comanche or Lower Cretaceous epoch, 
attained their maximum in Upper Cretaceous time, as recorded in 
the Dakota base leveling, when the sea extended over much of the 
present Rocky mountain area. In Laramie- Eocene the oscilla- 
tion was almost as profound and far reaching. Following that 
epoch was decreasing intensity; the line of marine base level 
extended less and less inward during the invasions of Miocene, 
Pliocene, and Pleistonic time. 
Dependent upon conditions of deposition the rocks of these 
different periods are of ditlerent degrees of consolidation, and 
consequent resistance to denudation. 
