Pilot Knob

Pilot Knob. . , Pilot Knob, the only example of an exposed submarine volcano in Texas, appears today as a prominent hill one mile northwest. It was formed some 80 million years ago on the bottom of a warm shallow sea which covered much of the continent during the Cretaceous period. The molten rock which it spewed forth rose to the surface from deep within the earth's crust. Following a channel opened by a belt of fractures known as the Balcones Fault zone., Reef organisms were attracted to the irregularity on the sea floor. Seeking food and protection on its slopes, they multiplied rapidly, not unlike coral reefs form now in the south Pacific Ocean. As corals, starfish, oysters and other marine life died, their shells and skeletons were fossilized around the volcano. Outcroppings of the small reefs which they formed can be seen today along the banks of Onion Creek, immediately north of Pilot Knob., As the Cretaceous sea receded, the volcano was exposed in its entirety. The long weathering to which it was subjected eroded much of the mass, leaving only the central portion or vent. Today it rises 710 feet above sea level, 180 feet above the surrounding valley. Diameter of its rim is 1.5 miles., The Pilot Knob area, 29 square miles around the volcano, is probably the best known igneous (volcanic) rock locality in the entire Balcones zone. Principal component of both the central knob and the several smaller knolls on its south and east is basalt, a dark gray to black volcanic rock. Weathered volcanic ash, similar in composition to basalt, underlies the valley around Pilot Knob and makes the land rich for farming.