The Geology of Central Texas and Its Present Day Wonders

by Susan Marx with an introduction by Travis White

“You can get a good cross-section of the history of the earth by examining rocks from Llano to the Gulf Coast – starting with the granite dome of Enchanted Rock, all the way out to the present time.”
(Dr. Mike Collins)

Growing up in the Texas Hill Country I often took for granted the beauty and character that is this region of the country. It wasn't until I took a geology class at Texas State that I began considering how my part of the world was geologically formed. For extra credit, my professor offered a day trip one weekend; he took our class on an excursion through the Texas Hill Country. We stopped at various geologic marvels along the way all day; the trip certainly changed my perspective. I distinctly remember one stop on the trip, we were traveling along the winding FM 1431 highway between Marble Falls and Austin when our professor had the bus stop at a vantage point overlooking an impressive and vast view of the Hill County. "Take a look everyone, if you've ever wondered what the bottom of an ocean looks like, it's right in front of you."

They say everything is bigger in Texas and that includes its geologic history. The heart of Texas is as diverse as it is vast and multi-dimensional. The region has a massive flat-topped plateau, rolling hills, steep canyons, deep gorges, rich farmland and alluvial plains.  Pre-Cambrian rocks can be found at Valley Springs and Enchanted Rock that date back more than 1 billion years and one of its most prominent features is a distinctive geologic feature known as the Balcones Escarpment – a dramatic strata of rock that cuts a dramatic swath northeast to southwest from Waco to the Texas Hill Country.

How did it get this way? What are the forces behind the rock formations and diverse landscapes we find here? It isn’t just the landscapes that are out-sized.  The history of how Central Texas came to be all that it is today took place on a massive scale dating back billions of years. It’s a complex tale, to say the least but we can get a good grasp of it by examining some of the main events that sculpted the landscape.

Dr. Charles Woodruff, a consulting geologist and U.T. professor calls the Llano Uplift, in the northeast corner of the Edwards Plateau, the most distinguishing feature of Central Texas. “The Llano Uplift has been a positive feature in Central Texas since the end of the Pre-Cambrian period about half a billion years ago. It’s like a big granite doorstop. The ancient Ouachita Mountains bent around it and later, owing to pressures on the soft underbelly of the continent, buckled under it,” Woodruff said.

Chronological geologic timeline of Central Texas

“The Balcones zone was formed under conditions of strain during the Tertiary era, when a down warping occurred near the Gulf Coast with a moderate uplift inland. Water-bearing formations passing beneath the plateau to the plains are broken across by the Balcones fault group, and much water is forced to the surface by artesian pressure. Barton Springs, San Marcos Springs, and Comal Springs are examples of the resulting artesian wells or springs.” (excerpt from The Balcones Escarpment by Patrick Abbott and Charles Woodruff)

This weakened area in the earth’s crust generally describes the outline of the Balcones Fault Zone - a band of faults up to 20 miles wide more-or-less tracing the root fracture of the Ouachita.

The land to the south of the Balcones has a stair-stepping quality. It undulates outwards from a series of fracturing events which broke the surface rock into a complex array, mixing it with the bands of sand that extend beyond to form the coastal plain.

Owing to the uplifted landmass of the Balcones Escarpment and the shallow soils of the Edwards Plateau, extreme flooding events are common in Central Texas and that continues to sculpt the landscape of the region. Cool air descending from the north meets the warmer, wet air from the gulf at this geologic crossroads, causing a slowing and sometimes a stall of weather systems over the area.

Canyon Gorge, for example, was created almost overnight during the summer flood of 2002.  Thirty-five inches of rain fell during a one-week period and the raging waters overwhelmed the Canyon Lake Dam.  The overflowing water fell at an estimated 67,000 cubit feet per second for several hours and cut an extensive, 50 foot-gash in the stone.  It was a boon for geologists though because it exposed all the layers of the Glen Rose formation as well as some dinosaur tracks and fossils.

In fact, Texas has one of the best collections of dinosaur tracks in the entire nation owing to its special geology. The most famous are in Dinosaur Valley in North Texas but the tracks at the Heritage Museum in Canyon Lake rival them. There are also some dinosaur tracks at Bamberger Ranch in Blanco and in Austin. A few trackways have also been spotted west of San Antonio and out around Del Rio.  They are all found in the Glen Rose formation of sedimentary rock created in the Cretaceous.

Just when you were thinking that’s amazing, another dimension unfolds.  The limestone that’s so prevalent in Central Texas left behind a network of aquifers, caves and caverns. The Edwards Aquifer, for example, supplies most of the drinking water and supports the endangered species living in it. It wasn’t always this way. The geologic environment continues to evolve and never more profoundly than beneath the surface. When it rains, trace amounts of carbon dioxide given off by living organisms, enters the surface erodes the softer sediments in the rock way, leaving a porous, water-bearing rock behind. 

This porous limestone covers much of Central Texas and is known as karst. As the slightly acidic water seeps down into the subsurface it wears cracks in the stone and, in places, whole sections of rock are eaten away. Caves, caverns and sinkholes result and they’re all over the place. Meanwhile, over the eons, as the water drips into these hidden, underground spaces trace minerals are left behind, creating the phenomenon of speleothems (e.g., stalagmites, stalagtites, etc). 

The caves and caverns of Central Texas are gloriously fascinating natural wonders that inspire and inform us about the life of the aquifers. They also provide scientists with valuable information about prehistoric climates and the now extinct wildlife that once lived here.  The geologic legacy that produced the Central Texas we know and love today is deeply profound and if you know where to look, you can see many of its episodes written in the stone.

Caves and Caverns of Central Texas

Inner Space CavernLocated in Georgetown off I-35, This is a large, complex cave with lots of formations and stands out from many due to its displays of Pleistocene-era mammal bones of tigers, jaguar, mammoth, wolves and bats. Visitors enter the main cave room on cable car.

Longhorn Cavern State ParkLocated in Burnet, this natural landmark is a cave that was formed by the eroding action of an underground river that has long since receded.  It has been used by Indians and bandits and was once a speakeasy during Prohibition.  There are walking trails, camp grounds and daily tours.

Cascade Caverns Located between San Antonio and Boerne. The tour of this cavern follows a main passage as it enlarges until it ends at a large room containing a stunning, 90-foot waterfall. 

Cave Without A Name Located northeast of Boerne. The tour follows a staircase, which spirals down to a pit and then opens into a wide passage full of stalagmites and stalagtites. The cave ends at a large stream.

Natural Bridge CavernsLocated near New Braunfels off I-35. This is Texas’ largest show cave. It has a variety of large formations and a beautifully lit flowstone floor.

Bracken Bat CaveLocated off I-35 near Garden Ridge, This cave hosts the largest bat colony in the world - approximately 20 million Mexican Free-tailed bats migrate here annually to give birth to their young.

Caverns of Sonora Located 15 miles south of Sonora off the 10 freeway. George Veni (Executive Director of the National Cave and Karst Institute) calls the Caverns of Sonora “the most beautiful caves on the planet.” The caverns are characterized by an interior maze of crystalline formations, which formed around parallel fractures in the stone.

Devils Sinkhole State Natural AreaLocated on the Edwards Plateau about 6 miles northeast of Rocksprings.  This sink is the largest single cavern in Texas and it has a lake inside it. This site is also a stopover for Mexican Free-tail bats.  Advance reservations only.

Kickapoo Cavern State Park – Located in 22 miles north of Brackettville. There are 20 known caves here with 14 miles of mountain bike trails and 18 miles of walking and birding trails. The main cave is currently undeveloped but it has dramatic formations and wild tours are available.