Texas geology is incredibly variable and full of amazing surprises. Elevation ranges from 0 to 8,749 feet, and the age of Texas rocks ranges from approximately 2-million to 2-billion years old!
Our oldest rocks are from the Precambrian Era and are most notably found in the Llano Uplift, smack dab in the center of Texas. During the Precambrian Era, the earth was violently changing. Viscous rock called magma eventually cooled and hardened to form granitic domes below Earth's surface. Once the materials at the earth's surface eroded away, some of these domes, such as the iconic Enchanted Rock, became exposed.
Here in San Marcos, most of our bedrock is limestone formed from events of the Cretaceous Period (~135 - 65.5 MYA). Texas was covered by a shallow, warm sea full of oysters, clams, fish, urchins, and much more. Upon death, the remains of these animals ended up on the seafloor and became covered by sediments. This cumulation of animal remains and sediment would eventually lithify to form vast beds of Texas limestone. If you keep an eye out, you can often find fossils of these organisms still persisting in the rock today!