Texas Limestone Hides Fossils of Sea Creatures in the Hill Country

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Tony Maples Photography


Take a close look at a piece of Texas limestone and you will see what appear to be tiny seashells. How did these sea creatures get into the rock from the Texas Hill Country? The answer might surprise you, and you may also find it interesting how easy it is to find these fossils.

What is Texas Limestone?

Limestone along a road cut in West Texas

Photo: Facebook/Robert Briggs

Frequently used for counters and buildings, Texas limestone comes from the calcium carbonate in sea organisms from millions of years ago. Though many types of limestone exist, the type that contains visible fossils is called fossiliferous limestone. The majority of limestone comes from shallow seas, which covered central Texas during the Mesozoic era when dinosaurs ruled. Bodies of sea creatures would sink to the floor when they died, and over time, only their calcium-rich skeletons or shells remained. These formed the calcium carbonate that accounts for at least 50 percent of all limestone.

Glen Rose Formation

Dinosaur Tracks in Glen Rose in the Glen Rose Formation

Photo: Flickr/mcdlttx

Though some of the older limestones formed during the Jurassic era when the sea first swept into central Texas, the most familiar to most is that of the Glen Rose Formation. Famous for its dinosaur tracks, this formation was laid down during the Cretaceous era, after the Jurassic. But sand and dinosaur tracks only lie along the northern end of this formation, near the Glen Rose area, but further south, in central Texas, you will find mostly limestone. This formation houses many of the Texas Hill Country caves, which formed from slightly acidified water flowing through limestone and carving out the caverns.

Where to See Texas Limestone in the Hill Country

Photo: Wikimedia Commons

Limestone occurs in numerous places both below the surface and above ground in the Texas Hill Country. Visit any of the caves in the area to see drip formations caused by the reaction of water on the rock over time. Look closely at road cuts for a band of white rock. That’s limestone exposed by the cut. You will easily be able to also make out other layers of rock and possible faults in the road cuts, too. Additionally, take a look at many of the Texas historic markers and many of the buildings in the Hill Country, both of which made use of the abundant limestone in the area.

Fossils Found in Texas Limestone

Fossiliferous Texas limestone on the federal building in Galveston

Photo: Wikimedia Commons

When you see fossiliferous limestone around the Hill Country, look carefully for fossils of sea life. Bivalves, such as clams, left behind imprints rather than their actual shells, which broke down over time. Another frequently seen fossil comes from gastropods, better known as snails. Like clams, their shells demineralized, though their impression remains in the rock. Less commonly seen are other sea animals such as crabs, corals, and sea urchins.