While the units discussed above are the major divisions of geologic time, the geologist usually works with smaller units of rocks called formations. A geologic [formation] is identified and established on the basis of definite physical and chemical characteristics of the rocks. Formations are usually given geographic names which are combined with the type of [rock] that makes up the bulk of the formation. For example, the Beaumont clay was named from clay deposits that are found in and around Beaumont, Texas.
THE GEOLOGY OF TEXAS
The geologic history of Texas, like the geologic history of the rest of the earth, is recorded primarily in marine sedimentary rocks. These rocks provide some knowledge of the early geography and the first inhabitants of what is now the State of Texas. Most of these rocks were formed from sediments deposited in shallow seas which covered parts of the State at various times in earth history.
By studying these rocks and their relations to each other, geologists have established a geologic column for Texas.
Physiography
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]).
TRANS-PECOS REGION
The Trans-Pecos region, located in the westernmost part of the State, is an area of mountains and plateaus with broad basins between the major mountain ranges. Many different types of rocks are exposed in Trans-Pecos Texas and these include marine, fresh-water, and terrestrial deposits. In many areas igneous rocks flowed out on the surface and now overlie sedimentary rocks. There are also many places where igneous rocks have been injected into the surrounding rocks, and these igneous rocks have been exposed by later erosion.