How does the geologic environment shape rockfalls in the Texas Hill Country?

Hill Country

AUSTIN (KXAN) — Visitors won’t be able to take a splash inside Hamilton Pool during Memorial Day weekend. Travis County Parks closed down the beautiful watering hole for the foreseeable future due to falling rocks.

A geologist hired by Travis County Parks to observe the area says the rock falls aren’t going away anytime soon.

“I would not want to sunbathe or swim underneath the lip of Hamilton Pool,” said Charles Woodruff with the Bureau of Economic Geology.

The February deep freeze is part of what Woodruff says led to the fallen rocks underneath the Hamilton Pool overhang, but there are many other factors also playing a role.

Geologist Charles Woodruff and Mariel Nelson are studying rock falls in the Hill Country (KXAN Photo/Kaitlyn Karmout)
Geologist Charles Woodruff and Mariel Nelson are studying rock falls in the Hill Country (KXAN Photo/Kaitlyn Karmout)

Beneath the receding ledge of Hamilton Pool is a sedimentary rock called shale. Shale is soft, weak and can split very easily when water seeps in.

“This little piece right here is a little harder, this one is actually a sandy limestone,” said Brian Hunt with the Bureau of Economic Geology, grabbing a small pebble.

Geologists like Hunt and Woodruff can’t control the rockfalls, but they can research the environment that creates them.

“We’re drilling over here that’s going to be an observation well to see how water declines during dry periods or rises during wet periods,” Woodruff said. “Ultimately, they’re going to be looking at a total water budget and how it plays a role in the security or failure of the bluffs.”

Research well at Reimer's Ranch (KXAN Photo/Kaitlyn Karmout)
Research well at Reimer’s Ranch (KXAN Photo/Kaitlyn Karmout)

That well is at nearby Reimer’s Ranch, another Travis County Park that shares the same landscape as Hamilton Pool.

“After that water infiltrates, we can measure the level in the well that is being drilled. We can also measure the level of water in the river,” said Mariel Nelson, a University of Texas graduate student.

Nelson is part of the research team, but her focus is a popular rock-climbing wall in Reimer’s Ranch that collapsed in 2019 after a significant amount of rainfall.

“We’re interested in the larger study because we want to know how water moves through the limestone landscape,” Nelson said.

Nelson specifically studies the role of tree roots and their ability to shape the limestone cliffs.

  • A collapsed rock wall at Reimer's Ranch (KXAN Photo/Kaitlyn Karmout)
  • A collapsed rock wall at Reimer's Ranch (KXAN Photo/Kaitlyn Karmout)

It’s another piece of the bigger picture — and by better understanding the geology of the groundwater systems, it allows researchers to better understand the risks.

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