Igot} VEGETATION OF WESTERN TEXAS 115 
PHYSIOGRAPHY AND GEOLOGY, 
In general the ‘‘greater Texas region” of Hill, to borrow his 
illustration, lies as a stairway leading from the coast by a suc- 
cession of steps to the cordilleras. On the west there is a 
balustrade formed by the southern prolongation of the Rocky 
mountain cordilleras and plateau; on the north the balustrade is 
the Ouachita system, extending east and west in Oklahoma and 
the Indian Territory. The slope leads up from the Gulf level 
to 5000 feet at the base of the cordilleras at the northwest. The 
geological areas are the ‘“‘stair steps,” speaking approximately. 
Beginning with the latest, they are the coastal plain (coast 
Neocene), the Fayette prairies (marine Eocene), the Blac« 
prairie (Upper Cretaceous), the Grand prairie (Lower Creta- 
ceous) ,the denuded areas of the Granite country, the Carbonifer- 
ous sandstones and shales, the Red beds (Permian), and finally 
the Llano Estacado (non-marine Tertiary). The “balustrade” 
of the trans-Pecos mountains and plateaus (except the Stockton 
plateau, which is part of the Great plains area) is quite distinct 
from the regions just cited, The tilting of the strata and weath- 
ering and erosion have given rise to diverse physiographic fea- 
tures, which are in close harmony with the geologic structures 
and strongly reflected in the vegetation (figs. z, 2). Since the 
edaphic factors stand out so prominently in determining plant 
formations, they will be given a proportionate prominence in 
this discussion. In general, the physiographic and geologic 
provinces as defined in Hill’s Physical geography of the Texas 
region> are employed in this paper, except that the Great plains 
region is made to include all east of the front ranges of the 
trans-Pecos Texas and north of the Rio Grande plain, with 
meridian 97.5° as an arbitrary boundary on the east. This is 
done because the physiographic ecology of the vegetation is 
most significant when it is borne in mind that the present condi- 
tions are but a stage in the leveling down of a former higher 
plain which covered this entire area, and whose remnants we 
have in the Great plains proper, the Edwards plateau, and buttes 
5 Topographic atlas of the U.S. Texas folio. 
