Beneath our feet lies the heart of an ancient giant. I am talking about Polecat Knob. This isn’t just any hill. Polecat Knob is the eroded core of an extinct volcano. It sits near Austin-Bergstrom International Airport and McKinney Falls State Park.
Pilot Knob’s story begins millions of years ago. About 79 to 83 million years ago, to be exact. Central Texas was then a marine shelf. Volcanoes like Pilot Knob dotted the landscape. Pilot Knob is one of about 75 late-Cretaceous volcanoes. They stretched from Waco to Austin and beyond.
Imagine magma pushing its way to the surface. It met waterlogged sediments. The water flashed into steam. A massive explosion created a crater. This explosive cycle repeated at Pilot Knob. Ash piled up. An ash cone grew above the shallow sea.
These eruptions weren’t always violent. Sometimes, magma intruded without contacting seawater. This resulted in quieter lava flows. These flows cooled and solidified. They formed the core of Pilot Knob.
Beaches developed around the volcano as activity waned. One beach deposit formed a resistant rock. It extends north for miles along Onion Creek. This rock is responsible for Upper and Lower McKinney Falls.
Later, mud from the Taylor Group covered Pilot Knob. The area uplifted during the Tertiary era. Erosion exposed the ancient volcano. Today, the terrain reflects the different rock types around the complex.
Geologists have used two methods to date Pilot Knob’s volcanism. The first method involves stratigraphy and fossils. The second uses isotopic age dating of igneous rocks. Both methods complement each other.
The volcano influenced the surrounding rock layers. Volcanism occurred during the deposition of the Dessau and Burditt formations. These layers are thinner near Pilot Knob. Ash beds are found within the Dessau Formation.
Potassium-argon dating puts Pilot Knob’s age at 79.5 million years. This agrees with the age derived from fossils.
The magma that fed Pilot Knob came from deep within the Earth. It originated in the Earth’s upper mantle. The Ouachita structural belt may have provided a pathway for magma to rise.
Two types of magma existed. One was basaltic. The other contained nepheline. Some rocks contain inclusions of spinel peridotite. These may be fragments of the upper mantle.
Only nepheline-bearing rocks have been found at Pilot Knob. No basaltic rocks have been located.
Pilot Knob can be seen south of Austin on Highway 183. Look to the right after crossing Onion Creek. A circuit can be made via Dee Gabriel Collins Road and McKinney Falls Parkway. Dee Gabriel Collins Road follows the northern rim of the volcano. The lowland is drained by Cottonmouth Creek. The central core is easily visible.
Most of Pilot Knob is on private land.
Pilot Knob even made it into pop culture. The TV show 9-1-1: Lone Star featured a scenario where Pilot Knob reactivated.
So, next time you’re near the airport, remember Polecat Knob. It is a silent witness to a fiery past. A reminder that even Texas has volcanic roots.