The Tower History Carved in Stone

In the shallow sea that's now Texas, the limestone began to form in less than 10 feet of water. If the Edwards Aquifer and Edwards Plateau sound familiar, then you're already acquainted with the limestone's formation area. Although we mostly hear about Edwards around Austin, it extends west for hundreds of miles.

"From Austin west to Fort Stockton, that's all basically Edwards limestone or its stratigraphic equivalent," said Charles Kerans, professor emeritus at the Jackson School of Geosciences.

And although the Edwards Formation is specific to Texas, comparable conditions across the world during this period led to similar rock foundations forming in parts of Britain, North Africa and the Middle East.

As far as geologic timelines go, limestones grow up fast.

"That's the cool thing about limestones in particular - they can turn into rock within 5,000 years," said Kerans. "Geologically, there's at least 5 million years of Earth history recorded in the Edwards [Formation], but those sediments turning into rocks, that's happening in a few thousand years."

Cordova shell isn't the only limestone from Texas you'll see around the Tower. There's also its less-fossilized counterpart -- Cordova cream limestone. As the name suggests, this limestone is, well, creamier than the shell. It doesn't feature any prominent rudists and instead is used as an accent to complement other stones on the steps and sides of the building.