I think it took us all by surprise to learn that Tilden Park contains several fossil localities and has a rich history with the UCMP. Don Savage, a former professor of paleontology and past chair of the Department of Paleontology at Cal, found a gomphothere jaw by Inspiration Point off Nimitz Way in 1961 and John C. Merriam collected the type specimen of Eucastor lecontei from deposits near Vomer Peak.
Underlying the beautiful rolling hills of the park are terrestrial deposits of the Miocene. The oldest of these deposits are the Claremont Formation containing chert and siliceous shale layers deposited 14 to 16 million years ago in a deep marine basin. Overlying this formation are the alluvial-fluvial mudstone, sandstone and conglomerate deposits of the Orinda Formation that originated from a higher, mountainous region west of the East Bay. You can see clear views of the Orinda Formation just east of the Caldecott Tunnel on Highway 24. The Moraga Formation overlies the Orinda Formation. This basaltic flow erupted from a volcano at Round Top in the Robert Sibley Volcanic Regional Preserve, just south of the Caldecott Tunnel, about 9-10 million years ago.
There are a number of lava flows visible from Highway 24, east of the Caldecott Tunnel, that are several feet thick. Many of them have a red zone or baked contact at their base where the hot lava contacted with the wet and cool alluvial deposits of the Orinda Formation, oxidizing the sediments. These volcanic deposits are resistant and now form the ridges of the Berkeley Hills and San Pablo Ridge. Some of the lava flows dammed rivers causing the formation of lakes. Deposits from these lakes formed the Siesta Formation composed of fine-grained light gray sediments. These soft rocks are easily eroded and have resulted in several landslides. Capping these deposits is another lava flow called the Bald Peak Basalt (9 million years old), visible at Vomer Peak in Tilden Park. All of these rock layers were folded due to tectonic activity. This created a large north to south plunging syncline that encompasses Tilden Park.
Photos courtesy of Nick Matzke, Tony Huynh, and Lucy Chang.
Please note that a collecting permit and official permission is required to collect, or even pick up, any vertebrate fossil or fossil fragment in any of California's State and National Parks. Other public lands, including city parks and open spaces, may have similar regulations. Best to check in with the appropriate land use office wherever your adventures take you to inquire where the best spots are to see fossils in the field and what is and is not permitted while hiking and exploring our fossil heritage in these natural preserves.