Lisa Grant Ludwig, a faculty member at the University of California, Irvine, and her colleagues examined a number of these perilously arranged boulders
near Silverwood Lake. Stacked in gravity-defying arrangements near the San Andreas Fault these granite boulders should have been toppled by earthquakes long ago but resolutely remain.
Grant Ludwig contends that the precariously balanced rocks have survived because interaction between faults has weakened earthquake ground shaking near them. One such interaction, according to the researchers, might be a rupture that began on the San Andreas Fault but then jumped over to the San Jacinto Fault bypassing certain areas. “These faults influence each other, and it looks like sometimes they have probably ruptured together in the past,” Grant Ludwig said. For hikers, keep your eyes open for these precariously balanced rocks, consider the implications, and suggest your own explanation.
One of the future plans of this website is to create a companion site focusing on the Geology of the PCT. Most of us walk the trail totally oblivious to the wealth of geological information that surrounds us.