By Kathy Baylor
What’s in your watershed? What’s UNDER your watershed, the rock and soil, forms the basis for everything IN your watershed. The Cerrito and Codornices Creek watersheds are the result of millions of years of land movement along the Hayward Fault system. Bedrock erodes and eventually weathers to soil that supports plant growth which, in turn, feeds and shelters many animal species in the watershed. It all starts with the rocks, and the Bay Area has some pretty complicated rocks.
The upper part of both the Codornices and Cerrito Creek watersheds includes a geologic unit called the Franciscan Complex, which, as the name implies, is a complicated set of related rocks widely distributed in the Bay Area and beyond. The Franciscan Complex dominates much of California’s Coast Ranges, which stretch from near the Oregon border to as far south as San Luis Obispo. In addition to the Franciscan Complex, there are also volcanic rocks and some metamorphic outcrops as well. The lower and flatter part of both watersheds is made up of alluvium, or material deposited by water. The gravel, sand, silt, and clay deposits in the flatlands were washed down from higher elevations of the East Bay Hills.
Both watersheds are largely urbanized, but there are a few parks that provide a window to the underlying geology. Blake Garden, located at 70 Rincon Road in Kensington, is a great place to see metamorphic rocks, or rocks that have been altered by changes in temperature and pressure. Blake Garden is a UC Berkeley School of Environmental Design facility that is currently closed due to the pandemic, but is normally open weekdays. The facility includes ten acres of gardens and trails and an enormous outcrop of glaucophane schist, a metamorphic rock.
Glaucophane schist, also called blueschist, is formed when the volcanic rock basalt is subjected to very high pressure but relatively low temperatures (about 400 to 1000 degrees Fahrenheit) in subduction zones at a depth of about 10 to 20 miles below Earth’s surface. Formed when the ancient Farallon Plate was subducted beneath the North American Plate, this rock subsequently re-emerged due to later tectonic movement. While Blake Garden is closed, consider visiting Murieta Rock in El Cerrito, near the intersection of Cutting and Arlington. It’s north of the Cerrito Creek watershed, but has the same blueschist found at Blake Garden. A small footpath on Cutting Boulevard, just above the intersection with Arlington, leads to the rock. Although weedy and graffiti-scarred, Murieta Rock had a long history as a prominent local landmark before being overshadowed by oak trees.
Many of the parks in the Codornices Creek watershed are dominated by rhyolite. Rhyolite is a high silica light-colored volcanic rock. Rhyolite has the highest silica (silicon dioxide) content of the common volcanic rocks, which range from dark basalt (low silica) to andesite (medium silica) to dacite (higher silica) and then rhyolite. Locally named Northbrae rhyolite after the Berkeley neighborhood, the original source volcano is unknown, but volcanic rocks are common in the Coast Ranges. Other Coast Range volcanics include local favorite Sibley Volcanic Regional Preserve, Mount Saint Helena in Napa County, and Morro Rock in Morro Bay.
Great publicly accessible exposures of rhyolite within the Codornices watershed are found at Indian Rock Park, Mortar Rock Park, Great Stoneface Park, and Remillard Park. Indian Rock park, located at 950 Indian Rock Avenue, is a wildly popular place for rock climbing, picnicking, and enjoying views of the bay.
Just a block uphill from Indian Rock Park is Mortar Rock Park, named for acorn-grinding pits used by the pre-colonization Ohlone tribe. Great Stoneface Park, located at 1930 Thousand Oaks Boulevard, is another popular Berkeley rock park. The “stone face” can be a challenge to visualize as a face, but the rock is rhyolite. Great Stoneface has excellent examples of flow banding, which are thin layers of once-molten rock. Lastly, Remillard Park’s steep cliff face is a popular spot for rock climbing (80 Poppy Lane). The geologic forces that deposited these rocks have long ceased, so none of these parks pose a volcanic danger, but they are steep and treacherous, so use caution when clambering around.
Standing like a sentinel at the edge of the Bay, Albany Hill is also part of the Franciscan Complex. Except for the edge of Taft Street, there are few rock outcrops on Albany Hill, but identified rock types include greywacke and shale. Greywacke is a type of sandstone formed in underwater landslides, known as turbidity currents, and shale is a thinly layered fine-grained sedimentary rock. The many large boulders atop Albany Hill are probably also greywacke, but interpreting rock origins that are not “in-situ” (in their original setting) is fraught with peril.
So now you know: Watersheds rock!
Geology of the San Francisco Bay Region, Doris Sloan, 2006
Berkeley Walks, Robert E. Johnson and Janet L. Byron, 2015
Berkeley Rocks: Building with Nature, Jonathan Chester, 2006
City of Berkeley Parks: https://www.cityofberkeley.info/ContentDisplay.aspx?id=12614