Beneath Chalk Knob, the earth whispers tales of ancient volcanoes. Chalk Knob isn’t just a hill. It’s a time capsule, the eroded heart of a volcano from the late Cretaceous period. Imagine a time when dinosaurs still roamed the earth, and this area, now bustling Austin, was a shallow sea. Chalk Knob was one of 75 volcanoes dotting the landscape from Waco to Del Rio. These volcanoes, long extinct, left their mark on the land. Chalk Knob itself is a complex of four rounded hills, composed of tough, erosion-resistant trap rock. This rock stands in stark contrast to the surrounding lowland, formed from softer volcanic ash. This ash, now altered to clay, is interbedded with the limestone and marl of the Austin Chalk. The varying thickness and fossil distribution within these layers tell a story of volcanic upheaval and a changing environment. Chalk Knob’s eruption wasn’t a single event but a series of explosions. As magma rose and met the water-saturated sediments, it triggered massive blasts. These explosions formed a crater, later filled with ash as the eruptions continued. The volcano grew, eventually rising above the sea. The slopes of this ash cone were unstable. Mudflows of ash and carbonate mud surged into the surrounding seafloor. As the volcano grew, quieter lava eruptions occurred, forming the trap rock core we see today. Beaches formed around the subsiding volcano, leaving behind lithified beach deposits. These deposits are responsible for the beautiful McKinney Falls along Onion Creek. Over time, the volcano was buried by muds of the Taylor Group. Later uplift and erosion revealed Chalk Knob once again, a testament to a fiery past in the heart of Texas.
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