Balcones Escarpment is ‘where the West begins’
The Balcones Escarpment, a geological scar millions of years old running across Central Texas, is one of the most important geological features in the state. It has played a huge role in shaping what Texas looks like, as well as where and how the state’s population has grown.
While you’ve probably heard of the Balcones Escarpment, you might not really know where it is — or even what it is. Or what role it played in forming the Texas Hill Country.
Here’s everything you need to know about the Balcones Escarpment:
What is it exactly?
In simple terms, an escarpment is a continuous line of cliffs or steep slopes facing in one general direction. The Balcones Escarpment is a zone of faults that runs through the middle of Texas, geologically dividing the state. Its curved line carves from Del Rio to just north of Waco, with the Hill Country starting just west of the escarpment. It separates the Edwards Plateau — the southeasternmost tip of the Great Plains — from the Coastal Plains bordering the Gulf of Mexico.
The Balcones Escarpment was likely formed between 20 million and 25 million years ago, according to the Bureau of Economic Geology at the University of Texas at Austin.
“Geologically, that’s pretty recent,” said Charles Woodruff, a geologist with UT’S Bureau of Economic Geology. The oldest limestone in the Hill Country is estimated to be about 100 million years old.
In a process that took millions of years, geologists say, the Balcones Escarpment was formed as the Coastal Plains bent downward due to the weight of deposited sediment, while the central Texas interior of the Llano Uplift remained relatively stable. The east side of Austin is about 700 feet lower than the west side as a result of this geologic activity.
“When you’re in San Antonio, you can get one of those big buildings and on one side, you can see the Hill Country across the Balcones Escarpment, but the other side, you’re looking at the Gulf Coastal Plain,” Woodruff said. “And that’s the boundary between two of the great geographic provinces of North America.”
How does it shape Texas and the Hill Country?
The Balcones Escarpment forms a series of cliffs and clifflike structures on the surface,
and serves as an entry to the Texas Hill Country, both physically and visually.
Woodruff said he was struck by the Balcones Escarpment the first time he encountered it more than 50 years ago.
“I saw those stair-step hills of the limestone, the hard and soft limestone layers and everything,” he said “I had this gut feeling that this is it. This is where the West really begins.”
Woodruff says one of his colleagues contends the Hill Country is a misnomer, because it’s really the valleys, formed by the area’s many streams, that give the topography its texture.
The streams winding through the Hill Country eroded the top layer of sediment and left the area’s limestone exposed. The rock is synonymous with the region, used in constructing numerous buildings. The erosion deposits dissolved calcium carbonate into the region’s groundwater, making it “hard.”
John Cannon, a geologist and assistant professor of earth and planetary science at the University of Texas at San Antonio, said the exposed limestone, along with other rocks in the fault, is effective at funneling and filtering water into the Edwards Aquifer, a water source for millions of people in Central Texas. The uplift of the Balcones Escarpment and the streams spewing from it created a karst landscape, which helps filter groundwater.
The rugged limestone now carved by winding roads and booming development is more suited for ranching than farming, Woodruff said. The region’s famous wineries are an exception, but the Hill Country isn’t the only hilly region to recognize that potential.
“Some of the very famous locations in France, Spain and Italy are on rocky uplands, maybe similar to the Hill Country,”
Woodruff said. “The great Champagne district of France is on the same kind of rock that you see in the zoo in Brackenridge Park, or up in what the city of Austin is built on.”
Is there a downside to the escarpment?
While the rolling terrain of the Hill Country draws in people with its beauty, it also can create problems. One of those is the potential for flash floods. Large amounts of rainwater can’t pool on inclines, meaning
downpours can send waves of water roaring through the area. Despite recent record-setting drought, the area along the escarpment has been dubbed “Flash Flood Alley.” Rapid development has increased the amount of impervious cover — materials like concrete that don’t absorb water or allow it to pass through — and that contributes to flash floods.
The escarpment also presents at least some risk of earthquakes, although experts say it is very small. While the ground hasn’t moved in recorded history, according to the University of Texas, it could move again. Two small earthquakes in 1893 and 1902 are thought to have been a result of fault movement.
The escarpment likely caused earthquakes in the distant past, but it isn’t comparable to the San Andreas fault that regularly shakes California. Cannon said the belief that “tectonic deformations” such as faults are confined to the margins of tectonic plates is outdated. Low-level seismic activity occurs all across the country, and some active faults can be 1,000 miles from a plate boundary and still cause tremors.
“Over tens of millions of years, sure, it could easily be reactivated by tectonics again,” Cannon said, “but not any time in our human lifetimes.”