During the Cretaceous Period through the early Paleogene Period, much of Texas was covered by a shallow subtropical sea. The calcium carbonate sediments deposited during this period lithified into the limestone rock underneath the park's soil and was exposed by erosion around the creek bed. Aquatic reptiles swam in the sea as evidenced by a complete skeleton of a mosasaur found in the rocks of Onion Creek not far from the park. In the park, shells of extinct sea animals such as species of "Inoceramus" and "Exogyra" are preserved in the limestone.