The San Francisco Bay Area is home to many spectacular rock formations and rock types, ranging from the subducted serpentines and jades of the Franciscan Formation to the Great Basin sediments of the central valley and the deposits of Antioch petrified wood. Golden Gate Bridge – San Francisco
And, as if we never could ask for more, field trips in the bay area are some of the most pleasurable events. The mild Mediterranean climate is, well, just perfect. And most areas are accessible with a normal vehicle.
Of course, the Bay Area is an urban area, and much of it is not accessible for collecting because it is either privately held or in public parks. That said, there are places reasonably close or well worth the drive — you just have to know. We’ll lead you to some of the best, read on.
In addition, there are the beaches! Casual collecting for agate, jasper, carnelian, and whale bone, and just plain old “beach combing” have been long permitted at most public coastal areas.
In further addition, for the geologist, or amateur geologist there are untold non-collecting areas of extraordinary formations and very unusual rocks with a truly historic story, all preserved in extensive local and national parks. Really, the bay area is a treasure trove of plate tectonics, inland seas, subduction, and mountain building, and of course the most famous of earthquake faults, the San Andreas Fault.
Just pick up a book, and go for a walk. Find places where tectonic forces have uplifted and displaced the roadways, where the shale is folded into waves, and where serpentine scraped off the earth’s mantle is exposed on mountain tops. The Bay Area is a plate tectonic text book.
Local Collecting Sites
Agate Beach (at Bolinas, CA)… for agates, jaspers, tar babies, shells, and whale bone.
Agate Beach for agates, jaspers, tar babies, shells, and whale bone. Agate Beach is located just west of Bolinas CA at the end of Elm Road. Parking is available. From the parking lot, go north along the beach. It is best accessed during low tide.
Although the directions from Google Maps seem unambiguous, getting there is more of a problem. The locals have a trick of turning the street signs and otherwise confusing your way to the beach. Just stick with your GPS… and some luck.
Bolinas is 60 miles west of Concord and 30 miles north of San Francisco. Limited hotel accommodations are available.
Dillon Beach (near Tomales, CA). for agates and jaspers
Dillon Beach for agates and jaspers. The beach is 4.5 miles west of Tomales CA. Parking is available. It is best accessed during low tide. Beach wave energy is high throughout the year. Watch for sneaker waves and be careful when collecting near the surf zone.
Dillon Beach is north of the entrance to legendary Tomales Bay where Hitchcock’s movie The Birds was filmed in Point Reyes. I say this because the long Tomales Bay is a massive conduit for cool pacific air, creating tremendous winds and kicking up the sand in the afternoon. Collecting here is often like working in a sand blaster.
Dillon Beach is about 65 miles north of either Concord or San Francisco. Camping for tents and RVs is available. Motel accommodations are available in Tamales.
By the way, Point Reyes Beach, just south of Dillon, in the Pt Reyes National Seashore is also a favorable collecting area. But it’s not easy get to Point Reyes from Dillon Beach because you have to circumnavigate Tomales Bay — about 35 miles. Because Point Reyes is a National Park, advanced approval from a park ranger is advisable. By the way, Point Reyes Beach is about 75 miles from Concord and almost as windy as Dillon Beach. Parking and a cafe is available near the beach.
As an interesting geologic sidelight, the Point Reyes block is an anomaly having traveled hundreds of miles north from parts of Baja California
Point Reyes Beach (13 miles west of Point Reyes, CA). for agates and jaspers (see Dillon Beach)
Russian Gulch (near, CA)… for jasper and actinolite
Russian Gulch for jasper and actinolite. Russian Gulch is located on the Russian River (This is not the Russian Gulch State Park in Mendocino County)
Black Butte Reservoir and Stony creek (near Elk Creek, CA). for jasper, agate, petrified wood
Black Butte Reservoir and Stony Creek for Jasper, agate, petrified wood. Black Butte is located near the town of Elk Creek. Follow Stony Creek for jasper.
Coulterville, Ca. for Mariposite and Serpentine
Coulterville for Mariposite and Serpentine. Coulterville is located on Highway CA-49 at CA 132. Serpentine can be found on most road cuts along CA-49 from Coulterville south to Mariposa and north nearly to Moccasin Reservoir. Mariposite can be seen with its bluish green coloring just west of Coulterville.
This is a note from GemTrails”Visited the Mariposite site at the intersection of Hwy 132 and 49 yesterday. The exposed Mariposite is in the road cut on the NW side of the intersection and is impossible to miss. (A number of guides mention a roadcut on the SW side of the intersection, but there is no cut present.) Be prepared for some hard rock work (chisels, sledge, etc.) Here’s a tip: there is a lot of fractured Mariposite buried at the foot of the deposit. ”