 Surface runoff later eroded the saprolite that once surrounded the corestones and left, what are now locally called "elephant rocks" as boulders perched on the ground surface.The reddish or pink granite has been quarried in this area since 1869, and two abandoned granite quarries are within the park.These and others nearby have provided red architectural granite for buildings in states from Massachusetts to California, but most particularly in St. Louis, including stone for St. Louis City Hall and the piers of the Eads Bridge.