The Oligocene–Miocene Catahoula Formation occurs along the length of the Gulf of Mexico coastal plain from the Rio Grande to the state of Mississippi. In Starr County, in South Texas, the formation is typified by the 20 m-thick Catahoula Volcanic Ash, interpreted to represent a single caldera eruption. The massive ash fall is the main source of the uranium that has been mined in South Texas since the late 1950s. It is also the origin of massive amounts of petrified wood, which was used in prehistory for making projectile points and continues to be used for landscaping. Diagenetic alteration of the volcanic ash produced the distinctive El Sauz Chert, which was used by native Americans for 10 000 years to make projectile points. More recently, the Catahoula Volcanic Ash was used as pozzolanic aggregate with Portland cement for the construction of Falcon Dam. The outcrop of the Catahoula Volcanic Ash in Starr County is the centrepiece of a geoheritage initiative, titled ‘Ancient Landscapes of South Texas at the Nexus of Natural and Cultural History’, whose goal is to connect the earth and social sciences to educate residents, students and visitors in how the region's geology has played a significant role in its social history.