As volcanic activity diminished, beaches developed around the volcano. One such beach deposit, now lithified and resistant to erosion, extends several miles to the north of the volcano. It appears along Onion Creek, where it is responsible for both Upper and Lower McKinney Falls. The entire shelf continued to subside after volcanic activity ceased, and muds of the Taylor Group gradually covered the entire volcano. During the Tertiary era, the central Texas area was uplifted, exposing the volcano as younger sedimentary rocks were eroded from the Cretaceous volcanic rocks. Today the terrain at Pilot Knob reflects the relative resistance to erosion of the different rock types that appear around the volcanic complex.