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.

These stops alone, with a quick run into a store or two for taffy, kites, wine, trinkets, or groceries can easily chew up half a day. If you have more time, turn east on Bay Hill Road, which is a narrow winding road bypassing Bodega Bay. It has marvelous coastal views, as well as views eastward over the coastal ranges - you can also see nobs and clinkers sticking up here adjacent to the road that connects back to Highway 1 on the east end of Cheney Gulch.

To continue your rock star tour, head farther north up Highway 1, proceeding past the Salmon Creek Bridge to SON 12.40, which has several parking areas overlooking the long and sandy Salmon Creek Beach.

Continuing north to SON 13.21, the sea stacks here are marvelous and photogenic. The stacks consist of hard spots in the Franciscan Formation. During the folding, faulting and uplift of the coastal ranges, the squeezing of the rocks along fault zones produced greenschist metamorphic clinkers. The surrounding rocks are sedimentary sandstones, siltstones and shale, with the hard pods of metamorphic rocks. Various mineral crystals, including garnets, epidote, mica and others are found within these clinkers.

In Sonoma County, some of the best stops for sea stacks are at SON 22.20, SON 24.42, and finally at the PG&E marker 45.35 at Salt Point State Park. Here, long trails circle the meadows around paleo-sea stacks on the coastal terraces and descend to the bluff edge. This is a very quiet area blocked from road noise, which allows one to contemplate nature and enjoy observing birds, flowers and other wildlife.

Bowling Ball Beach

If you keep driving north past Schooner Gulch, you'll see the Galloway Formation exposed in the sea cliff. Dips range 40 degrees to 66 degrees to the south-southwest. The rocks consist of thin-bedded sandstones, mudstones, and siliceous shale. The predominant color is a light, whitish gray. These rocks are exceedingly soft and subject to minor landslides. In strong winds, their particles rain down on the beach. Various large concretions up to several feet in diameter are eroding out of the bluff face. The so-called “bowling balls” are aligned along a series of beds at the foot of the bluff.

Bowling Ball Beach (MEN 11.41) is accessible from the northern trail. The descent from the bluff to the ocean is difficult at the base and includes a section of rope ladder but seems to not be a problem for most people. The Bowling Balls are only visible at low tide and completely out of water only during very low tides. The beach is still present at high tides except during strong storms.

MEN 12.40 offers another alternative access to Bowling Ball Beach along a good trail, although the entrance is barely visible unless you stop at this marker. When you arrive at the beach, turn south and walk for approximate a half mile to the bowling balls. The bluff rock face is nearly vertical here and consists of soft siltstones, mudstones and shale. On windy days, bits of shale and mudstone rain down on the beach, attesting to its rapid erosion. You may notice a couple of larger slides which have slid off the bluff, but wave action has quickly carried away most of the landslide debris. Compared to many areas along the bluff, this area exhibits a decided lack of faulting. In fact, there are only three faults along this entire section of bluff.

Before you reach the bowling balls, you will encounter many round boulders from 12to 18 inches in diameter. These appear to consist of siltstone and mudstone, quite similar to the rest of the bedrock. On closer examination, dark spots and branching-like structures make up a considerable part of each boulder. These are worm burrows. I wonder why they occur in these cobbles - possibly they were nests where the worms collected. Worm burrows are scattered elsewhere in certain layers of rocks, but not as concentrated as they are in these cobbles.

Devil's Punchbowl

How can the rather unique set of holes in the bluff near the Point Arena Lighthouse be explained? These openings are up to 50 feet in diameter and up to 50 feet deep. These are located within a few feet of the bluff edge, but also appear to be developing as far as 150 feet from the bluff edge. The underlying bedrock in the area consists of a series of thin-bedded sandstones, siltstones, and shale belonging to the Point Arena member of the Monterey Formation of Miocene Age. They were deposited approximately 20 million years ago. These rocks have been folded, faulted, and uplifted above the ocean by movement of the Pacific Plate along the San Andreas Fault. Since deposition in the Santa Barbara area in a deep ocean basin, this section of the plate has moved approximately 300 miles northward to its present location.

Ocean waves beat against the coastline and erode the least-resistant rocks, forming the current headlands, coves and islands. Numerous sea caves were carved out, especially in the Point Arena area. The sea caves commonly occur along small adjustment faults, or in intensely fractured areas.

Take the MEN 17.0 turnoff from Highway 1 to Lighthouse Road to the Point Arena Lighthouse located two miles north of the city of Point Arena. To see the sinkholes and sea caves, stop and park at the coast 1½ miles from the highway turnoff. You can walk south along the coast up to a mile leading to the old Coast Guard Station. The holes in the bluff - such as the Devil's Punchbowl at the Point Arena Lighthouse, and Satan's Hole and the Witches Cauldron Hole located a half mile to the south - are the result of roof collapses of the developing sea caves.

The sea caves allow waves to erode and penetrate into weak areas with thick coverings of sand and gravel left as coastal terrace deposits. The resulting debris from the erosion is carried out of the caves by retreating waves. Once the cave has eroded the ceiling bedrock, exposing the overlying weak coastal terrace deposits, gravity causes the sand and gravel to fall into the caves. Ocean waves quickly remove this material. It's interesting to note that the sinkhole might be quite large, 50 feet in diameter, but the cave opens to the ocean by way of a small hole 10 feet or so in diameter at the base of the bluff. Ultimately, the holes will be connected to the ocean as the roofs fall. Start about a half a mile from the parking area. The sea caves along the path are incredible. Offshore is a small island, Sea Lion Rocks, with a couple of good sea caves. The rock strata on the offshore island are nearly horizontal in contrast to the steeply dipping rocks onshore. Some of the best spots for viewing sea stacks in Mendocino County can be found at MEN 9.76, Greenwood State Beach in Elk, and at our journey's end, MEN 35.50 at Cuffeys' Cove cemetery one mile further north.

Whether exploring a stretch of Highway 1 for a daytrip or getting out on foot for a weekend or more of hiking excursions, the drive will take you past a number of small towns and a range of restaurants and other dining options, as well as overnight accommodations from rustic campgrounds to well-appointed vacation house rentals, bed-and-breakfasts and motels.

As always with coastal excursions, err on the side of caution. Driving along the coast while sightseeing can be distracting, so please pull over in safe areas to take in the sights. On the coastal trails, beaches and cliffs, mind all warning signs, the changing tides and all weather or surf advisories. With that in mind, rock on.

Thomas E. Cochrane is a professional geologist who's lived and worked on The Sea Ranch since 1988. His just-published book, Shaping the Sonoma-Mendocino Coast – Exploring the Coastal Geology of Northern California (River Beach Press 2017) is available at all Copperfield's locations, Sonoma Outfitters, as well as Amazon.com. To learn more and see a roster of retailers and upcoming events, visit RiverBeachPress.com.