Thursday, 12 February, 2026 / Edition 96

If you’ve ever wanted to pedal across some of the oldest exposed rocks in Texas, Spider Mountain Bike Park in Burnet County delivers—no boots required. Tucked into the Texas Hill Country, Spider Mountain is known for its lift-served downhill trails, rocky technical features, and unforgiving line choices. What many riders don’t realize is that the rocks shaping every turn, ledge, and drop formed in warm, shallow seas nearly half a billion years ago.

Let’s roll into the geology beneath the tires.

Molly Turko

Structural Geologist, Devon Energy

Cambrian–Ordovician Carbonates: Microbial Mounds Under Your Wheels

 Microbialite structures on Spider Mountain, 2in Andy Cube and battered hand for scale. Courtesy of Molly Turko

Much of Spider Mountain is built on the Point Peak Formation, an approximately 490-million-year-old carbonate unit deposited along the southern margin of Laurentia during the Cambrian–Ordovician. At the time, central Texas sat near the equator and was submerged beneath a shallow tropical sea.

Point Peak geology overview: The Point Peak is well known for containing microbiolites—organosedimentary deposits formed by benthic microbial communities that trapped and bound sediment and/or served as sites of mineral precipitation. These structures represent some of the earliest large-scale ecosystem engineering on Earth.

Spider Mountain geology overview: At Spider Mountain, distinct internal layering is often difficult to identify within these microbial buildups.

• Rather than classic laminated stromatolites, many of the exposures are more consistent with thrombolites or leiolites, which lack obvious stratification and instead display clotted or massive fabrics.
• Whether you’re navigating a rock garden or rolling a limestone slab, you’re riding directly over microbial structures that grew quietly on an ancient seafloor long before vertebrates ever existed.
• These microbial carbonates formed in shallow marine environments encircling Laurentia, recording stable platform conditions and clear waters ideal for microbial growth.

Find it fast: These coordinates will help you locate the microbialites I mentioned quickly:  30.835753°, -98.337820° 

A message from AAPG

Martina Birnbaum/Shutterstock.com

Join AAPG for the upcoming Buffalo Bayou K-12 Educators Trip 2026! This free, three-mile walk will provide teachers with resources to guide students in science, with examples that impact learners directly. 

Experts in sedimentation will examine the impact of Hurricane Harvey and other flooding events. Specifically, we will look at:

• Hydrologic data, stream deposition, and erosion

• How a stream responds to flooding events

• Sedimentary behavior in peels. We will then discuss them as art objects.

• The human impact of storms

• Efforts to manage storm impact on our community

Don't miss this opportunity to learn how flooding events have a profound effect on constantly shifting and changing bayous!

Register here. 

Lion Mountain Sandstone: Trilobite Hash and Ancient Shores

 Lion Mountain glauconitic green sandstone and white trilobite hash. Courtesy of Molly Turko. 

Also cropping out along the trails is the Lion Mountain Sandstone, a slightly older Cambrian (roughly 500 million years old) unit that records a different slice of this ancient marine system. This sandstone is quartz-rich, commonly green and glauconitic, and locally streaked with eye-catching white bands.

Find it fast: Here are the outcrop's coordinates, so you can find it easily, should you choose to venture out: 30.839317°, -98.339782° 

Those white streaks are trilobite hash.

Definitions: In geology, hash refers to a rock composed of broken shells and fossilized invertebrate fragments.

• These accumulations form when skeletal material settles to the seafloor, fossilizes, and is later reworked by waves or currents into sedimentary structures.
• At Spider Mountain, trilobite fragments were concentrated and redistributed within the sand, leaving behind bright fossil-rich layers that stand out against the darker matrix.

Standout geology: The Lion Mountain Sandstone represents deposition along shallow marine shorelines, and together with the Point Peak Formation, it preserves some of the oldest exposed sedimentary rocks in Texas. The sediment was sourced from the eroded, unconformable crystalline basement of the North American craton, shedding quartz-rich material into the Cambrian seas.

Go deeper: Watch my LinkedIn video on the Lion Mountain Sandstone here. 

Shoutout to AAPG President-Elect John Casiano for introducing us to these exposures on a cool field trip traversed on mountain bikes!

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Plan Your Visit!

 Ruby Patterson and Molly Turko at the base of the lift Courtesy of Molly Turko

If you're ready to get out on the trails, here's some helpful information and what I'd recommend. 

Trip tips:

• Location: Spider Mountain Bike Park is located in Burnet County, Texas, about one hour northwest of Austin.
• Best time to visit: Fall through spring offer the most comfortable riding conditions. Summer temperatures can be extreme, and exposed rock surfaces retain heat.
• Trail access: Spider Mountain is a lift-served bike park. Purchase a day pass or lift ticket in advance, especially on weekends.
• Gear: Full-face helmets and pads are recommended due to sharp limestone and sandstone features.
• Nearby stops: After your ride, explore additional Cambrian–Ordovician outcrops around Lake Buchanan and Inks Lake State Park.

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