Pilot Knob is the only example of an exposed submarine volcano in Texas and 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 that covered much of the continent during the Cretaceous period. Molten rock rose to the surface from deep within the earth's crust through 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 and, seeking food and protection on its slopes, multiplied rapidly in a manner like coral reefs 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 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. Long weathering 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, and the 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. The 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, and weathered volcanic ash similar in composition to basalt underlies the valley around Pilot Knob and makes the land rich for farming.