PHYSIOGRAPHIC FEATURES OF THE REGION. 299 



that portion of the Atlantic coastal plain known as the Mississippi em- 

 bayment, while on the northward the whole is limited by the Ouachita 

 group of mountains, which are distinctly Appalachian in age and struct- 

 ure. In fact, the area embraced upon the accompanying map represents 

 a base, away from which the different features of Texas expand to the 

 south and southwest. 



The eastern half of this area, including southwest Arkansas, Choc- 

 taw Nation and Bowie and Red River county of Texas, is one of the 

 most difficult for exploration. The littoral terranes of different ages are 

 all of an unconsolidated ferruginous nature, and it is only by long and 

 careful study that they can be distinguished one from the other. At the 

 first glance, the Trinity sands so much resemble lithologically the Da- 

 kota, and the Eocene so closely simulates the Plateau gravel and the 

 Second bottoms that one would think them the same. The problems 

 are somewhat similar to those met in studjdng the later formation of 

 the North Atlantic coastal plain, except that they are much more com- 

 plicated by the great bottoms and flooded rivers, and absence of any 

 topographic maps, to say nothing of dense forests, excessive humidity 

 and malarial climate. Upon the other hand, the area represented by the 

 western and southern portion of the map is a beautiful prairie region, 

 with simple and conspicuous stratification, diversified by strips of tim- 

 ber — a country radically different in every respect from the eastern part. 



Physiographic Features. 



Topography. — The northern border of the region is limited by the 

 Ouachita mountain system, which, with its vertical folds and consoli- 

 dated rocks of Paleozoic age, forms a northern l)arrier to the geologic 

 features of the region to be discussed. 



South of Red river the low, rounded divide of Red and of Sulphur river 

 extends eastward, but is devoid of sharp topographic sculpture, thereby 

 differing from the Kiamitia prairies north of the river. It consists of a 

 low central ridge of rich prairie land, diminishing in area eastward, with 

 tim])ered slopes and valleys. This prairie is mostly the outcrop of the 

 up})ermost Cretaceous formations. 



The Kiamitia prairies and gravel mesas lie north of Red river and 

 south of the mountains. The former are small, flat calcareous prairies, 

 interrupted by scarps and bounded on all sides by forests. To the east- 

 ward, in Arkansas, they are supplanted by extensive Ioav mesas, capped 

 with gravel. The Kiamitia prairies are the outcrop of Lower Cretaceous 

 beds. 



The. Red river valley, the representation of which extends longitudi- 



