Daytripping to some of the coolest rock formations along the Sonoma-Mendocino Coast
As a geologist prowling the landscape along the coast for the past 40 years, I'm often stopped by folks asking what I'm doing. At the time I might be chipping away with my rock hammer, measuring the dip of the rock strata or collecting beach sand. A geologic conversation commences, and people ask all sorts of questions including: Where can we see the San Andreas Fault? Are there fossils around here? How fast is the bluff retreating? Once I start answering, the next question is often, “Why don't you write a book?” Hence, “Shaping the Sonoma- Mendocino Coast - Exploring the Coastal Geology of Northern California” (River Beach Press February 2017). The last third of the book is a road log, which utilizes highway mile markers for a self-guided geology tour along a pretty spectacular 85-mile stretch of coastal terrain from Bodega Bay to Elk.
My suggestion for novices is to explore this complex region in smaller sections as scenic day trips - I personally can't cover even a mile in four hours because I find so much to look at (no wonder it's taken me 40 years to write the book). But here is a suggestion for a road trip of a day or two. We've provided highway mile marker numbers that appear along Highway 1 to help you on your journey and designed a trip that can be done in a day or two, depending on how fast you drive and where you stop, with some geologic points of interest that are visible and easy to get to.
Where faults converge
Starting out on your journey, you can head to the coastline of Bodega Bay from the back roads of Petaluma or Sebastopol. Traveling these roads will bring you through open valleys with farmlands and few trees (most are eucalyptus). Strange rocks poke up out of the landscape, but little else betrays the underlying geology. These rocks are metamorphic nobs or clinkers caused by the folding and faulting of the underlying sedimentary rock strata in the formation of the coastal ranges.
The area around Bodega Bay has a long diverse history, both geologically and historically. From a geological perspective, several important faults coalesce here in the bay. The San Andreas Fault, having left Point Reyes, comes briefly back onshore through the bay, with the other important one being the San Gregorio Fault.
As you drive along the coast, you can observe that the Pacific shoreline is much different from other coastlines, such as those along the East Coast where the land slopes more gently into the sea with long, sandy beaches and the nearest mountain or ridge is many miles inland. By contrast along our local coast, we encounter a bluff edge with little or no beach area. The bluff height ranges from 30 feet to more than 200 feet in height. Our beaches are short stretches of sand, which is primarily found in coves. These coves and rocky points produce our spectacular scenery, including sea stacks, sea caves, small waterfalls, sand dunes, rocky beaches and sandbars blocking the mouths of major streams.
First stop is highway mile markers SON 9.0 and 9.16, entrances to Doran Regional Park on the Bodega Bay estuary and bay-mouth sandbar, which guards the bay from incoming storms. Just north of the park entrance is Bird Walk Dunes with a couple of picnic tables and a nice introduction to the sand dune environment. If you're ready for a restaurant stop, there are several in the area (my favorites are Lucas Wharf and Inn of the Tides located on the east side of the bay). Incidentally, the San Andreas Fault is located under or immediately adjacent to these restaurants. The fault moves in periodic jumps of a few inches to several feet, but in 100 to 300 year intervals, so the odds are good you'll make it through your meal shake-free.
‘Hole in the Head'
The next stop is traveling out to Bodega Head. In the northern part of the town, turn west onto East Shore Road, and you'll cross a large sand dune area leading to the granite rocks of Bodega Head. The San Andreas Fault cut these dunes in two or three places in the 1906 earthquake, but the sands quickly covered the traces and people rebuilt new homes over the dunes.
The next point of interest is at the south end of the road before it turns west and climbs the head. Stand here and take in the site once slated for a nuclear power plant - blocked by environmentalists in the '60s. What remains of this failed attempt is a massive hole. Originally dug to house the reactor, it is now called “hole in the head,” a double entendre we'll leave to your imagination. Filled with water, it's now a large pond relished by wildlife.
If you keep on going over the head to look at the ocean, you'll see the only granite outcrops from Bodega Bay north to the Gualala Block that are along this coast, and there are some marvelous trails around the head. These are accessible from the parking lot, one to the south and one to the north.