288 Geologic Story of the Colorado River. — Hill. 
of interior North America. The story of the sediments upon 
each side of this axis—east and west—although a harmonious 
history, records events of entirely different characteristics. 
This paper deals only with the eastern or Atlantic slope leav¬ 
ing for a future time the interior basin. While this section does 
not record the sediments of the Permian, Triassic, Jurassic 
and enclosed Laramie of the interior United States, it furnishes 
equally interesting data for a study of other formations. 
Topographically, the area is very diversified by erosion, but 
primarily consisted of a great eastwardly sloping plain separ¬ 
ated into two steppes by a gentle monoclinal scarp, and extend¬ 
ing north-east across the state to the ancient mountain axis that 
runs east and west across southern Indian Territory and south¬ 
western Arkansas. The uniformity of these open plains has 
been broken by a few igneous disturbances and degraded by the 
extensive erosion of the drainage systems, which, cross their 
strike at right angles and cut valleys from 300 to 750 feet in 
depth. Of all these streams the Colorado river has cut the 
deepest channel and in it can be read the whole sequence of 
geologic events. 
Within this short distance the river has worn through the 
crust of Cretaceous sediments that formed the floor of the 
plains and now traverses nearly every terrane from the late 
Quaternary to the earliest Cambrian. Perhaps no where else 
in the world can be seen a more comprehensive geologic sec¬ 
tion, a better illustration of sedimentary and igneous rocks, 
and their relation to topographic form and economic conditions 
or other geologic features dependent upon structure than 
in that portion of the Colorado which traverses the counties 
of Burnet and Travis, as illustrated upon the accom¬ 
panying sketch map. Here the erosion of the river basin has ex¬ 
posed nearly 10,000 feet of structure that would otherwise not be 
exposed, and every bend and curve seems to reveal some inter¬ 
esting topographic or geologic fact. Since these are so inti¬ 
mately dependent upon structure it is necessary that a brief 
enumeration of the formations of the section should first be 
made. This section is illustrated upon the accompanying 
figures. 
THE SECTION. 
Along this portion of the Colorado as shown in figures the 
following formations are revealed : 
