Bay Nature magazineFall 2002

Ask the Naturalist

What, and where, are the oldest rocks in the Bay Area?

October 1, 2002

A: The oldest rocks in the Bay Area are metamorphic rocks associated with the granitic rocks at Point Reyes, Bodega Head, and Montara Mountain. They have traveled a long way in space and time to get here. They all occur west of the San Andreas Fault on the Salinian Block, which is attached to the Pacific plate and is moving northward past the North American continental plate. The Salinian Block has come north from Southern California or perhaps Mexico, carrying the metamorphics with it.

Metamorphic rocks are those that have been altered by heat and/or pressure. The Bay Area’s metamorphic rocks were originally layered sedimentary rocks—sandstone, shale, and limestone. They were altered some 80 to 90 million years ago when the the Farallon plate (predecessor of the Pacific plate) was colliding with the North American plate all along the west coast. The Farallon was subducted beneath the North American and began to melt far underground, forming magma. As the magma rose in the crust, the existing crustal rock—referred to as country rock—was altered by pressure and the heat of the magma. In the process the sandstone, shale, and limestone were changed into quartzite, schist, and marble respectively. The magma eventually cooled to form the granitic rock we now see on the Salinian Block. The granitics and metamorphosed country rocks were exposed after they were uplifted and the overlying material eroded away.

The age of the original sedimentary rocks is not known. They must be older than the granitic rocks that intruded them 80 to 90 million years ago. Rocks similar to the original limestones, shales, and sandstones are found in eastern California, the Mojave Desert, and northern Mexico; they are Paleozoic (probably about 350 to 450 million years old). Thus, it is likely that the Bay Area’s metamorphics are also Paleozoic in origin, very old indeed.

You have to look carefully for these oldest rocks because not many have survived transport and erosion. The most accessible exposures are on Point Reyes, occurring with the granitics that altered them. One good site is at the south end of McClure’s Beach, where beautiful quartzites and schists are among the large boulders lying at the foot of the cliff. These rocks also appear at Heart’s Desire Beach (Tomales State Park), near Mount Vision on Inverness Ridge, and at Teachers Beach just north of Inverness. There are also small remnants of these ancient metamorphics on Bodega Head and Montara Mountain (in Pacifica). Visit them and pay your respects to these elders of the Bay Area’s rocky foundation.

About the Author

Doris Sloan's articles on the geology of the Bay Area have been appearing in Bay Nature since our first issue. Her book Geology of the San Francisco Bay Region was published by UC Press in 2006.

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