But this incredible feat of engineering didn't come without serious costs and continuing hazards. Hardly a year goes by in which some portion of Highway 1 isn't shut down by mudflows and landslides. The Big Sur section of the coast has been especially hard hit in recent years. But there is one part of the highway that we Californians constantly heard about in the news, but not any longer. It just sort of disappeared from the public consciousness. It's a section between Pacifica and Half Moon Bay called the Devil's Slide.
The problem child of this geologic mess is the gray colored unit labeled Tss on the geologic map below. It is an unnamed sequence of sandstone, shale, and conglomerate of Paleocene age, from around 55-60 million years ago, just after the great extinction that wiped out the non-avian dinosaurs and many other species. These weakly consolidated sedimentary rocks have been severely deformed and sheared by nearby fault zones into a jumbled mass of unstable blocks prone to sliding. The main slide surface is about 150 feet beneath the surface, and the entire unstable block is 4,000 feet long, and as much as 900 feet high.
For the park brochure and trail map of the Devil's Slide Trail, use this link:
For a geology road trip along the coast south of San Francisco check out this link:
For engineering details on the slide, check out the following USGS Bulletin:
For a geologic map that covers the slide area, check out: