GULF COASTAL PLAIN
The Gulf Coastal Plain ([Pl. 9]) is composed of [Cretaceous], [Tertiary], and [Quaternary] rocks and includes the eastern, southeastern, and southern portions of the State. The rocks of the area consist of sands, clays, shales, and limestones. The Texas Gulf Coastal Plain is bounded on the north and west by the Balcones [fault] zone, on the south and southwest by the Gulf of Mexico, and extends eastward into Arkansas and Louisiana.
The region has broad river valleys and uplands of low relief, but there is an increase in relief toward the interior of the State. The surface of the area slopes gradually toward the Gulf and successively younger formations are encountered gulfward.
The rocks of the Texas Gulf Coastal Plain are relatively unfossiliferous, but many of the Upper [Cretaceous] rocks contain fossils. In the central portion of the region some marine formations of [Tertiary] age locally contain well-preserved invertebrate fossils.
Geology
Geologic studies of the State of Texas have indicated the presence of rocks formed during every era and [period] of geologic time. These range from the [Precambrian] granites of the Llano uplift to the [Quaternary] gravels of the High Plains.
Plate 10
GENERALIZED [GEOLOGIC MAP] OF TEXAS
Modified from Geologic Map of Texas, 1933
[This map in a higher resolution]
![[This map in a higher resolution]](#7837905192149670112_map1_hr.jpg.id-5652087351530782209.wrap-0.html.html){kind=link}
One of the best ways to become acquainted with the geology of Texas is to study the [geologic map] of the State ([Pl. 10]). A geologic map shows the distribution and age of surface rocks and may also indicate what kind of geologic structures are present. The types of rocks that crop out at the surface may be shown by means of symbols, colors, or patterns, and these are explained by a legend which accompanies the map. On [Plate 10], colors are used to show the distribution and [geologic age] of the surface rocks of Texas. Reference to this map will give the collector some idea of the age of the fossils that might be found in a given area. Some special geologic maps may have the location of geologic structures and [formation] contacts indicated by means of symbols, such as dashed lines, arrows, and similar special markings. However, the map included in this publication does not show any of these special markings.