Rockfall
Living Life on the Edge
Prototype Wall rockfall at Reimer’s Ranch Park
Tom Jones
I love exploring the Texas Hill Country and often find myself at the base of a cliff. I always remember the early advice I received from a geology professor about not climbing road-cut cliffs because of the hazard created by falling rocks. On one of my early field trips as a geology student, I was eager to get to the road cut to start my assignment to measure and identify the various rock layers. I was not thinking about the danger, and a small boulder fell and narrowly missed a student standing next to me. This experience taught me to stand away from the cliff and perform a visual survey to identify potential dangers before approaching.
I am always a little nervous when standing or sitting under a cliff. I know the chance of being injured or killed by a collapsing cliff is exceedingly small, but it does happen. Unstable rock can suddenly collapse as a rockfall, or strata may move downslope as a unit or chaotically as a landslide. Virtually all parts of Texas are subject to these gravity-driven geohazards. “I would not want to sunbathe or swim underneath the lip of Hamilton Pool,” said Charles Woodruff with the Bureau of Economic Geology (KXAN News). He says the danger of rockfalls isn’t going away anytime soon.
Cypress Creek Falls at Eagle Rock in Wimberley—note the swimmer under the limestone ledge on the left and the rockfall to the right of the low-water dam. The rockfall resulted in a huge boulder collapsing into Cypress Creek.
Rockfall is the one of the most common types of mass movement and can be observed throughout the Texas Hill Country. They may be triggered by freeze/thaw action, rainfall, changes in groundwater conditions, weathering, root growth, and erosion of the surrounding material.
Beneath the receding ledge of Hamilton Pool is a sedimentary rock called shale. Shale is soft and weak and can split very easily when water seeps in. Geologists can’t control the rockfalls, but they can research the environment that creates them. At Reimer’s Ranch Park an observation well was installed to see how water declines during dry periods or rises during wet periods. The research is to determine how groundwater plays a role in the security or failure of the bluffs. Although we can’t control rockfalls, research is underway to determine the environment that creates them.
Rockfall under the Hamilton Pool limestone ledge: photo by Tom Hausler
One of the most widely reported rockfalls in the Texas Hill Country occurred in January, 2019, at Reimer’s Ranch Park, a Travis County Park that is adjacent to and shares the same landscape as Hamilton’s Pool. Click here to view/download a map of the park.
The rockfall extended along a 200-foot-long section of a limestone cliff, which broke off and crashed to the ground. Michael Brewster with Travis County Parks said it's the largest rock fall in at least 13 years. He calls it a rare event. "It just breaks off in big chunks. It doesn't crumble down like soft rock," said Professor Charles Kerans with University of Texas Jackson School of Geosciences. The geology professor explained that steady, heavy rain can cause more fracturing. "When we increase water-flow through those fracture systems, it's going to decrease the friction between the rock layers, and that will make it easier for those rocks to break away."
Rockfall from collapse of Prototype Wall, January, 2019, Reimer’s Ranch Park—Barton Springs Edwards Aquifer Conservation District
While wet weather contributes to rockfalls, apparently the sport of rock climbing does not, according to park officials. A feature designated Prototype Wall in Reimer’s Ranch Park is a good example of nature at work. In fact, Dr. Kerans says that "the combination of soft shale below and hard limestone above is an ideal scenario for this type of failure...It would be extremely difficult to predict the timing."
"It could happen anywhere at any time. It could happen right now," said Lauren Gimotty, a climber that day. And another, Sam Henry Farmer, far up at the top of the cliffs: "We're living life on the edge, literally!"