Sibley Volcanic Regional Preserve: volcanoes, pits, outcrops, knobs and tuffs

The Sibley Volcanic Regional Preserve in the Berkeley-Oakland hills, California, is named to honor Robert Sibley, a founder and director of the East Bay Regional Park District (EBRPD) [1,2]. The preserve includes the 1.6-miles-long Round Top Loop Trail, circling around the extinct volcano, after which this trail is named. Round Top has an elevation of 1,763 feet [3,4]. Originally the preserve was called Round Top Park and mostly limited to the area around this peak. Today, the preserve extends further northwest beyond Quarry Road to a couple of ponds and to the site of the Stephen A. Lewis bench, from where one can view Siesta Valley across Highway 24.

The most interesting features in the preserve are those left over from a 10-million-year-old volcano as well as the exposed rocks and sediments telling the geological history of the uplifting of the Berkely-Oakland hills by tectoniic forces along the Hayward and Moraga earthquake faults. The above picture shows the Round Top quarry pit, discovered by quarry operations and now exposing the interior of the Round Top volcano. At times the pit turns into a small “crater lake.” Currently, a spiral labyrinth has been crafted onto its tuff-breccia floor. Northwest of this crater pit, you'll find various numbered sites of rocks and outcrops, whose formation history is explained in the preserve flyer by referring to diverse volcanic phenomena.

Getting to the Sibley Preserve entrance, visitor center and trail junctions
The preserve entrance is located east of Skyline Boulevard, just 0.1 miles south of the Grizzly Peak Boulevard/Skyline Boulevard junction. From Tilden Park in Berkeley follow the winding Grizzly Peak Boulevard southwards, past the intersection with Fish Ranch Road until you reach Skyline Boulevard. Turn left and find the park entrance to your left. From Highway 24 take the Fish Ranch Road immediately east of the Caldecott Tunnel to get to Grizzly Peak Boulevard after driving 0.8 miles. Turn left and follow Grizzly Peak Boulevard to Skyline Boulevard as described above.
Trailheads are found on the left and the right side of the unstaffed visitor center. They are the beginning and end of a loop trail. After hiking or horse-riding either half of this loop, you get to the Round Top Loop Trail and the southward Bay Area Ridge Trail/Skyline Trail. The Round Top Loop Trail connects with further trails in the preserve, including the Volcanic Trail and Quarry Trail.

References and more to explore
[1] Sibley Volcanic Regional Preserve flyer: East Bay Regional Park District, P.P. Box 5381, 2950 Peralta Oaks Court, Oakland, CA 94605-0381.
[2] East Bay Regional Park District Map: www.ebparks.org/parks.
[3] East Bay Regional Park District > Sibley Volcanic Regional Preserve: www.ebparks.org/parks/sibley.
[4] David Weintraub: East Bay Trails. Wilderness Press, Berkeley, 1998; pp. 252-255.