Texas landscape

Famed mountain range stretches into Central Texas

The Hill Country is known for its distinctive geological features: pocked limestone outcroppings that often jut out of the ground in great slabs, clear springs, and caves tucked into canyons and near rivers.

The karst-like terrain, carved out over thousands of years by water running along the limestone beds, points to a geological secret, however. If you were to drill 1,500 feet below the surface some twenty miles west of Georgetown, through all those accumulated layers of 125-million-year-old limestone, you would run into the Appalachian Mountains.

“The Balcones Fault Zone that extends from Dallas, to Waco, to Austin, to Del Rio [is] the surface expression of [these] underlying mountains,” explains geologist Don Beaumont, who before retiring served for more than forty years as a geologist with Texaco and Knowledge Systems. “The displacement on these faults drops our limestones from fifty feet to several hundred feet down the east side of the Appalachian Mountains, while the limestones over the mountains remain largely undisturbed. Movement along the Balcones Fault [has given] rise to caverns, springs, and some of our subsurface drinking water, [plus] undetermined natural gas potential.”

Don Beaumont leading a Senior University field trip looking at the Balcones Fault

The buried mountain range bears little resemblance to the one stretching through the coastal South Atlantic states, known as the Smoky Mountains in the Carolinas and as the Blue Ridge Mountains in Georgia. Near the Mississippi River, these mountains slip underground, hidden under long-accumulated river deposits. They continue to run underground until Arkansas, where they emerge as the Ouachita Mountains. Don explains that the discovery of a link between these ranges is fairly recent. “Only in the last fifty years,” Don says, “have we suspected that they were connected.”

The technological developments that made it possible to trace the connection between the Appalachian and the Ouachita ranges also revealed that the massive interconnected chain does not end in Arkansas. Instead, the mountains return underground and then snake through Central Texas, concealed under multiple layers of limestone. “With the development of reflection seismic subsurface imaging in the middle of the twentieth century,” Don explains, “[we] can record the configuration of rocks to a depth of 25,000 feet and deeper. We now know that [all of these] mountains are connected.”

Although springs, caves, and limestone outcroppings bear little evidence of these buried mountains today, Don explains that the chain once towered over the landscape. It formed nearly 300 million years ago, he said, when Africa and South America collided to create the supercontinent Pangaea. At the time, they would have rivaled today’s Himalayans, with peaks of up to 30,000 feet. “Subsequent erosion reduced them to ‘rolling hills,’” says Don, who regularly teaches and speaks about geology at Senior University and at local clubs. “Then [the hills] were buried by the limestones we know about here in Williamson County.”

The discovery of these hidden mountains suggests new possibilities for hiking the famed Appalachian Trail. Texas hikers can get started right away—and quite near home!

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