Ten million years ago, lava from an active volcano flowed through the East Bay hills, with flows reaching from Tilden Park’s Inspiration Point to Moraga. Today, Round Top volcano – rising 1,763 feet above sea level – is Sibley Volcanic Regional Preserve’s most prominent feature. Dedicated in 1936, the preserve is one of the East Bay Regional Park District’s oldest parks. The area was once the center of vast quarrying operations, and today the preserve is best known for the mysterious labyrinths that have appeared at the bottom of the quarry canyons.
This walk of about 2 1/2 miles takes you past two of the labyrinths. The preserve is a popular dog-walking spot, so you will see lots of dogs, mostly well behaved and on leash. There are also a fair number of cows, so watch where you step.
Round Top Loop Trail
Pick up a brochure and map at the visitor center. Inside are numbered stops if you want to do a self-guided tour. Also at the visitor center is an interpretive display showing various types of volcanic rocks. Take the uphill path on your right (as you’re facing the Visitor Center). The trail is woodsy, with lots of eucalyptus, pine, bay and coast live oak, and then opens up for views across the valleys to the east.
The main road soon curves to the left up to the water tower. Here, stay to the right on a smaller, but still paved, path. Don’t take the first marked cutoff to the right, which puts you on the Bay Area Ridge Trail into Huckleberry Preserve. Instead, take the second cutoff to the right to stay on the Round Top Loop Trail, a smaller, somewhat rocky path. Just past a cattle gate, the forest disappears and you are in grassland and rolling hills. When you see a thin footpath on your right, climb the short distance to the hilltop to see views that on a clear day can reach to the Sierra Nevada.
Continue on the Round Top Loop Trail to a T intersection. Straight ahead is a fenced overlook where you can gaze down into a deep quarry. Once this was the guts of Round Top Volcano. However, quarrying removed the lava and created this enormous pit, creating a wonderful opportunity for geologists and others to see and study a volcano’s interior.
The largest and most elaborate of the labyrinths is visible from the peak, and it’s the only one that we know for sure was not made by extraterrestrials. It is the Mazzariello labyrinth, created by East Bay resident Helena Mazzariello in 1989 as a gift to the world. It is a true labyrinth, meaning it has only one path to the center rather than a maze, which offers several choices. You can take the path down to the labyrinth or walk (carefully) along the quarry top, peering into its depths.
To see a second labyrinth, return to the Round Top Loop Trail until it curves to the left to complete its loop. Continuing straight on the Volcanic Trail, which was once the quarry haul road, enjoy the views of Tilden Park. When the Quarry Trail intersects to the left, you will see a numbered post, matching the numbered stops on the self-guided tour. The second labyrinth is at No. 5 (No. 7 on older maps). Look down the quarry at a delicate, heart-shaped maze, smaller and perhaps more mysterious than the first.
You can continue along the Volcanic Trail to see the remainder of the numbered stops on the self-guided tour or return to the Round Top Loop Trail, bearing right to complete the loop and return to the parking lot.
By car from San Francisco, cross the Bay Bridge, and head east on Interstate 580 toward Highway 24. Merge onto Highway 24 east toward Walnut Creek. From there, exit on Claremont Avenue (Exit 3) and turn left. Past Ashby Avenue, stay on Claremont Avenue (don’t veer left onto Claremont Boulevard), and continue on Claremont Avenue until it intersects with Grizzly Peak Boulevard. Turn right and stay on Grizzly Peak until it ends at Skyline Boulevard. Turn left on Skyline and then immediately left into the Sibley parking lot.
Photo: Taylor Hanigosky ©