At Thornton Beach, the ground is made of a semi-consolidate sedimentary rock extending to Mussel Rock that does not hold together well, making the coastline very unstable. Constant surf along the Daly City coast undercuts the cliffs, rain scours and penetrates the soil and loosens the rock structure, and gravity pulls the weakened slopes down in slumps and slides. The San Andreas Fault runs 1/4 mile from this spot, and earthquake shaking also causes landslides. Geologists believe the shelf below, between this elevation and the shore, is a mighty rotational slump triggered by the 1906 earthquake. This Daly City beach was once accessible by road, but in 1982 heavy rains and unusually large tidal action undercut the cliffs from Thornton Beach south to Mussel Rock, causing a massive landslide, and safety concerns keep the 56-acre property and beach inaccessible.