Pilot Knob in southeast Austin is the eroded core of an extinct volcano formed during the Late Cretaceous, when Central Texas lay beneath a shallow sea. Instead of one peak, it consists of four rounded hills rising above a circular lowland, preserving ash deposits, mudflows, lava-hardened trap rock, and a shoreline rim left as volcanic activity declined. Dated to about 79.5 million years ago, it is one of about 75 extinct volcanoes in Central Texas. Its rocks have helped geologists date the eruptions and understand how magma altered the surrounding Austin Chalk, while its resistant beach rock also helps form Upper and Lower McKinney Falls.