105 
THE GEOLOGIC EVOLUTION OF THE NON- 
MOUNTAINOUS TOPOGRAPHY OF THE TEXAS 
REGION. AN INTRODUCTION TO THE 
STUDY OF THE GREAT PLAINS.* 
By RoBT. T. Hill, Austin, Texas. 
East of the Pecos river the topography of Texas is a vast 
series of plains and drainage valleys eroded into plains. It is 
characterized by a sub-horizontal structvire of diverse terranes, 
the most extensive of which are of Neozoic, post Paleozoic age. 
Approached from the northwest the "Staked plains" are a con- 
tinuation of the Great Plains of central United States, which 
separate the Rocky mountain front from the Mississippi valley, 
and are composed of Miocene strata. In the eastern border of the 
state the plains are a continuation of the great coastal plain of 
the Atlantic and gulf region. Each has its own topography, 
flora, fauna, and cultural conditions and, in general, a substruc- 
ture of horizontal strata void of mountain folds and corrugations. 
In the south of the state the great plains of Colorado meet and 
show their relation to the coastal plain of the Atlantic. 
Across the centre of Texas from the Rocky mountains at Las 
Vegas, New Mexico, to the coast at Galveston, a profile and sec- 
tion can be constructed which will reveal in detail the history of 
deposition and degradation of the region, including two great 
periods of eruptive activity. 
Portions of this section show for long distances a monotonous 
uniformity, but in the fift}' miles on either side of the capital is 
revealed a comprehensive view of all the formations from the 
alleged Archsean and Algonkian to the present time. 
The only formations immediately bearing upon the origin of 
the plains — both coastal and great — are of Neozoic origin, which, 
as a rule, may be considered the product of alternate subsidence 
and elevation accompanied by the invasion and recedence of the 
gulf of Mexico. 
During these time-intervals the processes of land degradation 
and coastal sedimentation together with changes of climate varied 
so as to produce the specific difl'erences now existing in com- 
position of formations, residual soils, consequent drainage growth 
and resultant topography. Before giving the details of these 
processes of Neozoic time it may be proper to review our limited 
knowledge of the topographic features which existed at the close 
*This article is a continuation of my chapters on the Geography and 
Topography of the Texan Region, published in this magazine. 
