In order to discuss the distribution and exposures of the rocks of
Texas, it is helpful to be familiar with the _physiography_ of the
State. Physiography deals with the study of the origin and description
of land forms, such as mountains, valleys, and plains. Plate 9 is a map
of Texas which shows the major physiographic provinces within the State.
The majority of the land forms in Texas have been produced by the
processes of erosion attacking the structural features of an area.
Certain other land forms may be related to the effects of igneous
activity which resulted in the accumulation of large masses of igneous
rocks. The Davis Mountains are an example of surface features produced
in this manner.
In discussing the physiography of Texas, three major physiographic
provinces will be recognized. These are (1) the Trans-Pecos region, (2)
the Texas Plains, and (3) the Gulf Coastal Plain (Pl. 9).