Geological Outings Around the Bay: San Bruno Mountain

San Bruno Mountain has been in the way for a long time. Roadbuilders and railroad companies had to cut their way around this peak, which walls off San Francisco from the San Mateo Peninsula. Ranchers found no forage on it, and urbanizers made little headway on its rugged slopes. Megalomaniac planners once proposed blowing it up and using it as fill to turn the whole Bay into "made land" à la Foster City. And I'm sure the airline pilots at SFO would rather it weren't there either.

But three sets of users benefit from the mountain today. A quarry on its north side, in Brisbane, produces crushed rock. Several large broadcasting towers serve the city and peninsula from its long ridgetop. And the rest of us can hike this distinctive place in peace and relative quiet because San Bruno Mountain is a state park that happens to be run by San Mateo County.

The mountain serves as an island of habitat for some of the Bay Area's most iconic vegetation. As usual, the park literature will give you lots of detail on the plants and little about the rocks. But here's what we know, starting with the bedrock map (excerpted from the San Mateo County geologic map).

The rock is composed of the coarse, raw sandstone known as graywacke along with some finer-grained shale. It's strongly layered, as seen in this roadcut on the Bay side along Bayshore Boulevard.

Here's a piece of it close up. The fresh rock, like you get in quarries, is gray, but at the surface it's brown because of iron oxides that form by weathering.

The sandstone has a lot of rock particles (lithics) in it, which are rich in iron and which break down readily under surface conditions. The thin brown crust is made of manganese oxides and is coating the surface of a fracture. As you turn pieces of it in the sunlight, you'll see tiny glittering flakes of white mica in it. Under the microscope it displays metamorphic minerals like pumpellyite, showing that the stone was subjected to heat and pressure after it formed.

This body of stone is classified as part of the Franciscan Complex, and correlated rocks (the San Bruno Mountain terrane) run north through western San Francisco, including Land's End. The Marin hills from Stinson Beach to southern Tomales Bay, across from Inverness, also are part of it. It has no fossils in it, but the experts say it formed in a marine setting.

A paper published in November 2012 (Dumitru et al., Geology) assigned this rock an Early Eocene date (53-49 million years ago) and concluded that its sand came in a huge flood of sediment from Idaho, where vigorous volcanism was going on at the time. This was not what anyone thought. Most Franciscan sandstones, which are older, are thought to arise from the Californian upheavals in the Sierra Nevada region. Since that time the sandstone was heated and intruded by dikes of lava around 14-12 million years ago, while it was being manhandled by the San Andreas fault system like the rest of the Coast Ranges.

But knowing all that doesn't make San Bruno Mountain any more beautiful. And even on hazy days when the amazing view around is occluded, you can still admire the mountain itself.

Wrapping around the mountain's west side is the Colma Formation, which underlies the cemeteries of Colma and surroundings. This is a package of ice age sand, about 125,000 years old, that is the remains of the seashore at a time when the polar ice caps were even smaller than they are today. The ocean was about 10 meters higher than today, and sands from this time are found in the East Bay, all over San Francisco, and up and down the Pacific coast on wave-cut platforms. This map shows the situation at the time the Colma Formation was laid down.

Much of the Colma Formation is recycled sand from the earlier Merced Formation, but surely the rain and waves here added lots of brown sand from San Bruno Mountain, too. You can see the Colma sands at the far west end of the park, in Daly City at the place on the geologic map where the third question mark sits marking the Hillside fault. This is an exposure in Hillside Park.

Like the sandstone of San Bruno Mountain, this sand has a large proportion of lithics, accounting for its dark color. The area looks and feels just like beach dunes, but it's some 400 feet above the sea today. That is a testimony to the geologically rapid disruption of the Bay Area's land by the San Andreas fault system. Nearby is a small patch of dune habitat that San Bruno Mountain Watch is keeping an eye on.

Now is a good time to start visiting San Bruno Mountain instead of driving around it. The wildflower season is just starting, heralded by this ceanothus.