Oligocene Shoreline
Geologic units of the Coastal Plain of Texas (everything south, southeast of the San Antonio, Austin, Dallas corridor) get progressively younger towards the coast (Geology of Texas, 1992). In South Texas, the youngest materials are closest to the Gulf of Mexico and materials are progressively older moving inland. These units were deposited along a regressive shoreline as inland materials were carried by rivers to the coast and deposited, building land further seaward since the beginning of the Cenozoic (65 MYA). The eastern edge of the Catahoula formation can be interpreted as the Oligocene (34 - 23 MYA) shoreline. A little further west, the Roma sandstone was deposited along the then shoreline during the Late Eocene (40 MYA). The oligocene shoreline is particularly interesting as the interaction between the coast and ash that makes up the Catahoula formation produced unique features such as the Sauz Chert and large deposits of petrified wood.
-Geologic Map of Texas image