The Rocks of Devil's Slide

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With the opening of the Lantos Tunnel this week at the notorious Devil's Slide on state Route 1, the work is not finished. The winding, vertiginous old road and the 70 acres of land it sits on will be handed over to San Mateo County, which has pledged to give it a makeover and splice it into the California Coastal Trail, adding a new attraction to the Devil's Slide Coast. The facilities will include parking lots at each end, water, bathrooms, trash bins, rails and pedestrian crossings.

Hikers and bicyclists will be thrilled with the high views up and down the coast. They may feel a tickle as they look at the surf hundreds of feet below. And some of them will feel curiosity as they look inland at the sides of the road, about 1.3 miles of it, that they could never stop and see before.

This stretch of road passes through two very different sets of rocks. The south end is the granite of Montara Mountain; I showed you some of this at Quarry Park in El Granada, a bit farther south, in 2011. It holds up the rugged headlands around the tunnel's mouth. It dates from Cretaceous time, just like the granite in the Sierra Nevada—and in fact it's the same stuff, ripped out of the range and pulled northward here by the San Andreas fault. The same granite is found as far north as Bodega Head and south to San Luis Obispo County (including Pinnacles and Fremont Peak, part of a sliver of tectonic plate that geologists know as the Salinian block.

Caltrans photo

The north end is a thick pile of mudstone that dates from Paleocene time, slightly younger than the granite. This is the material that gave Devil's Slide its name. It's been falling into the sea for untold centuries, including the last century in which we have been vainly putting roads across the cliffs.


The San Mateo County geologic map describes it as marine shale and sandstone with beds of conglomerate. The conglomerate is basically sandstone that includes "angular boulders of granitic rock as long as 2 meters and smaller boulders, cobbles, and rounded pebbles of hornblende gneiss, muscovite gneiss and schist, Franciscan chert, quartzite, limestone, sandstone, and shale." That variety of material represents a long-vanished countryside rather like today's, with vigorous rivers that carried this coarse sedimentary material straight out to sea.


The contact between the granite and the mudstone is an inactive fault. Both units are in the Salinian block. And now it's time to show them on the geologic map.

Kgr, granitic rocks of Montara Mountain; Tss, Paleocene sandstone and shale; fs and fg, Franciscan sedimentary rocks; blue, Franciscan limestone
Major units: Kgr, granitic rocks of Montara Mountain; Tss, Paleocene sandstone and shale; fs and fg, Franciscan sedimentary rocks; blue, Franciscan limestone. The "V" signs mark the tunnel entrances.

That fault contact will be an attraction for many geologists and students. Local researcher Kathleen Burnham describes what she looks forward to seeing here: "Among the notable features now hazardous and difficult to access are bedding-plane trace fossils, vertical burrows filled with white carbonate, some isoclinal bedding folds, and the contact between the strata of Point San Pedro and the underlying Montara Granodiorite. The Devil's Slide project as a whole will greatly enhance accessibility of these spectacular outcrops when the project is finished, traffic diverted, and vehicles no longer whizzing by within inches."

The geologists at Caltrans have been good friends of the geological community. Now that the old roadbed is in the County's hands, let's hope they don't decide to hide these rocks. Who knows how long we have to enjoy them.