For a list of some of the fossils that can be found, click HERE . The Pilot Knob volcanic complex is 2 miles in diameter. Beach deposits developed around the ash mound created by volcanic explosions. These deposits crop out along Onion Creek and are responsible for McKinney Falls. The rockshelters along the creek were used as campsites by ancient indians about 2,000 years ago, and ancient Indian campsites have been excavated along Onion Creek in southern Travis County and along Barton Creek south of the Colorado River.
Evidence indicates the area was very popular with the nomadic tribes.
In more modern times, legend has it that the French explorer La Salle stayed at a camp on Onion Creek in the 17th Century. In 1832 an early Mexican land grant was given to a signer of the Declaration of Independence, Jose Antonio Navarro, on Onion Creek. Stephen F. Slaughter, which Slaughter Creek is named after, was another Onion Creek landowner. Onion, Slaughter, and Williamson creeks lead to what is now McKinney State Park.