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.