Devil’s Slide is on the western flank of San Pedro and Montara Mountains in the San Pedro Headlands, about 3 miles (5 km) north of Montara and 1 mile (1.6 km) southwest of the Linda Mar District of Pacifica, California. It spans 2.5 miles (4 km) between San Pedro Creek to the north and Martini Creek to the south. The area is characterized by steep slopes with gradients ranging from 30% to 50%. It is notorious for frequent slope failures, including rockfalls and deep-seated landslides. Due to ongoing instability, a 1.4-mile (2.3-km) section of the Pacific Coast Highway was abandoned in 2013. The bedrock of Montara Mountain belongs to the Salinian terrane, composed of granitic plutonic rocks. These rocks are part of a batholith formed during the Cretaceous period as a result of subduction-related magma intrusion. The batholith was later displaced northward by right-lateral movement along the San Andreas Fault system. The plutonic basement rocks at Devil’s Slide range from quartz diorite to granite and are collectively known as the Montara Granodiorite. Adjacent to this are the marine sedimentary rocks of San Pedro Mountain, formed during the Paleocene. These rocks are exposed as layers of highly weathered sandstone, shale or mudstone, and conglomerate, with an estimated total thickness of 3,800 feet (1,160 m). Several types of slope failure occur at Devil’s Slide. Rockfalls, the most common, take place within the Paleocene sedimentary units. Debris flows occur in the weathered sections of both the Montara Granodiorite and the Paleocene sedimentary units. The most catastrophic events are deep-seated failures in the Paleocene sedimentary units, which have displaced the former highway grade through creeping and episodic slip events.
The Indigenous Peoples of the San Francisco Peninsula were the Ramaytush, part of the Ohlone people, with a population of about 1,400. The Ramaytush were divided into ten independent bands, including the Aramai, who likely numbered no more than 50 across two villages. One village, Timigtac, was near present-day Rockaway Beach, and the other, Pruristac, was near present-day Pacifica. In 1769, Pruristac was among the small villages visited by the Portolá expedition, the first Spanish explorers in the area. The explorers were greeted by 25 Aramai after crossing Montara Mountain from the south. Upon reaching the valley floor, they received food from the locals. Two weeks later, as the Spaniards returned south through San Pedro Valley, they noted the Aramai had disappeared. In 1786, San Pedro y San Pablo Asistencia, a ‘sub-mission’ of Mission San Francisco de Asís, was established at Pruristac, and the Aramai were among the first Native people absorbed by the church. In 1834, after the Mexican War of Independence, the mission lands were secularized. In 1839, Governor Juan Alvarado granted Rancho San Pedro, comprising 8,926 acres (3,612 ha) and all the buildings of the Asistencia, to Francisco Sanchez, the Commandante of the San Francisco Presidio and eighth alcalde of San Francisco. Sanchez retained ownership after California was ceded to the United States in 1848. During World War II, the Devil’s Slide area of San Pedro Mountain was used for a military triangulation station and observation site as part of San Francisco’s harbor defense. Before radar, a ship’s location was determined by triangulation. Military personnel used binoculars and compasses to locate ships and relay their coordinates to a command post. There were six military structures at Devil’s Slide, including three steel-reinforced concrete observation posts, two concrete and earth bunkers, and a reinforced steel observation tower.
San Pedro Mountain has long impeded transportation. From 1848 to 1937, five road alignments were attempted, each plagued by landslides. The earliest efforts were mere improvements to primitive trails used by horses and later by wagons. In 1905, construction began on the Ocean Shore Railroad, intended to connect San Francisco and Santa Cruz along the Pacific coastline, but it was never completed. A major tunnel was built at Devil’s Slide, along with numerous bridges and trestles. In 1906, the San Francisco earthquake caused significant damage, and the railroad never recovered from its losses. Mainline operations ceased in 1920. California State Route 1, part of the Pacific Coast Highway, was built between 1936 and 1937. From the beginning, highway engineers recognized slope instability issues at Devil’s Slide. The first major landslide destroyed much of the road in 1940. Within its first decade of operation, the highway was closed at Devil’s Slide for more than 200 days. Another significant landslide in 1995 forced the road’s closure for almost two years. These closures affected the growth of nearby towns and prompted a realignment of the highway. In 2013, the old road was closed and replaced by the Tom Lantos Tunnel, a double-bore tunnel routing the highway through San Pedro Mountain. In 2014, the 1.3-mile (2.1 km) Devil’s Slide Trail was opened to the public, converting the old section of the roadway into a pedestrian and bicycle route. Read more here and here. Explore more of Devil’s Slide and Pedro Point here: