By Constance Taylor
So, we all know about the Hayward Fault. If you didn’t grow up somewhere around Oakland learning about it while doing earthquake simulations with trays of jello in science class, you’ve almost certainly heard about it as an adult through the gestalt of local news (for anyone who’s new to the area, you can learn more about our resident fault here).
But what about the rest of Oakland’s geology?
Keep in mind that geology is a jigsaw puzzle, not a road map. So said Andrew Alden, our geologist guide and author of the very excellent Oakland Geology blog, as we sat almost directly atop the fault at Montclair Playground. We were there to learn about the Cretacous-era geology in the neighborhood, and see some of the 70-80 million year old rock formations that were already ancient by the time mammoths and giant ground sloths got around to munching on Oakland’s prehistoric vegetation.
The three major rock units on our agenda were the Oakland Conglomerate, Shephard Creek Formation, and Redwood Canyon Formation. These three units are all part of the Great Valley Sequence, one of the major rock formations in California; it forms the western wall of the Central Valley and underlies its entire extent.
Our first stop was the Oakland Conglomerate unit, part of it exposed as a wall of light brown sandstone near an unofficial, hand-made skate park with ramps dug into the hard soil. Andrew mentioned that all sedimentary rock is defined by grain size, not composition of materials, and a “conglomerate” is a geologic term meaning sandstone with pebbles and larger stones in it. This unit was created by sediment from the ocean floor and deposits from mountain rivers, and the larger cobbles are a mix of volcanic and granite rocks. “The Oakland Conglomerate is unusual because it stains red from the iron in it,” he explained. “But no one’s spent much time studying this unit, so we don’t really know a whole lot about it.”
A hundred feet away from where we were standing, the formation changed to the shale of the Shephard Creek Formation, created between 83 and 71 million years ago. Shale is made mostly of clay, which isn’t as strong as the sandstone of the Oakland Conglomerate.
Our last major unit was the Redwood Canyon Formation, a thick unit of sandstone. This rock, like the other units we saw, was created deep in the Pacific Ocean, where subduction zones and tectonic activity 100 million years ago generated enormous landslides that created thick sandstone beds.
Near the end of the walk we saw a “turbidite” landslide bed of the Shephard Canyon Formation sitting atop flaky claystone consisting only of tiny particles, which give this rock a distinctive look as well as texture. “Go ahead and nibble on a small piece,” advised Andrew, pointing to the claystone. “It breaks up very easily, and it has a creamy feel in your mouth because there’s no grit in its fine sediment.” Who knew that eating rocks could help you identify them?
For a far more in-depth description of the geology of this area, check out Andrew’s blog of this same Wild Oakland walk, titled “Shepherd Canyon: Type localities of Oakland rocks”.