What, and where, are the oldest rocks in the Bay Area?
by Doris Sloan 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 t
