It’s a hallmark of any westering journey beyond the Pecos in Texas. As woodlands give way to plains and desert, the land itself seems to stretch, the horizons to widen.
It reflects a geological reality. The American West is slowly being torn apart by tectonic forces. One element of this “extension” is the Rio Grande Rift, a growing rip in the continent, traced by the river itself, from Colorado to Big Bend.
Dr. Jesse Kelsch, of Alpine’s Sul Ross State University, has studied this tectonic phenomenon in West Texas. It’s a story revealed in subtle traces, and soaring mountains.
Kelsch’s research involves lab analysis. But the heart of it is fieldwork in the Big Bend’s desert mountains.
“We at Sul Ross love our area because of the exposure of rock geologically,” Kelsch said. “We get a lot of information from that. We're not traipsing through trees, although we'd really like to be. I did spend a good solid, very hot, summer in 2021 collecting the bulk of my data.”
It’s counterintuitive – that the stretching of the continent would raise mountains. But as the continental crust is thinned, it’s also buoyed up by heat from the Earth’s depths. And ultimately, the stretching crust breaks along faults. Here, one body of rock drops, while an adjacent rock body is sent skyward.
Movements occur from earthquake to earthquake. But the cumulative effects can be stunning.
Big Bend National Park is a showcase for those effects. The “Sunken Block,” an immense, down-dropped basin, dominates the center of the park. On the west, it’s bounded by the Terlingua Fault, which has raised the iconic cliffs of the Mesa de Anguila, at the mouth of Santa Elena Canyon. Faults on the east have produced the Dead Horse Mountains and the high, striated escarpment of the Sierra del Carmen, rising 7,000 feet above the Rio Grande.
Kelsch explored these places with a technique called fault kinematics. Where masses of rock have moved passed one another, they’ve left polished surfaces and, occasionally, linear grooves called “slickenlines.”
“Fault kinematics looks at slickenlines on fault surfaces,” Kelsch said, “these little scratches that leave a record of the direction of fault movement. You can imagine the rock on one side of that fault scratching the other side of that fault.”
By looking at these scratches and grooves, Kelsch identified the directions in which the West Texas earth has been stretched. And some slickenlines are covered with calcite minerals, which can be dated. That allowed Kelsch to determine the timing of Big Bend faulting.
The stretching and breaking, she learned, began some 30 million years ago, and it occurred on a northeast-southwest orientation. Then, about 14 million years ago, the direction of stretching shifted, with this land being pulled to the northwest. That shift corresponds to the expanding influence of a distant development – the San Andreas Fault.
With a group of colleagues, Kelsch published her findings in 2025. She plans to expand her research, to continue filling in the history of our region’s rugged terrain.
“I want to move outside the Sunken Block next,” Kelsch said, “because I think me and my co-authors have told a pretty good story about the state of tensile stress as recorded in the Sunken Block. But the Sunken Block is just one little piece of the southern Rio Grande Rift.”
The Rio Grande Rift hasn’t finished its work, as significant earthquakes here in the past century make clear. And more earthquakes are sure to come. But while such events can be catastrophic, the forces that drive them have produced a landscape of singular drama.
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