Robert Sibley Volcanic Regional Preserve

Robert Sibley Volcanic Regional Preserve is located in the Berkeley Hills of the East Bay region of the San Francisco Bay Area, California.

The park is in Alameda County and Contra Costa County, east of Oakland. It can be entered from Oakland on Skyline Boulevard, or from Contra Costa County on Old Tunnel Road.

The park was one of the first three parks established by the East Bay Regional Parks District (EBRPD) in 1936. It was originally named Round Top Regional Park. Round Top (elevation 1,761 feet/537m) is an extinct volcano in the Berkeley Hills. It is home to several stone labyrinths of recent origin. The park was renamed after the second president of the EBRPD, Robert Sibley, shortly after his death.

The preserve contains a Pliocene epoch volcanic center that, about 10 million years ago, produced most of the lavas that underlie the East Bay ridges from Inspiration Point in Tilden Regional Park to Moraga. Geologists refer to this local volcanism as the Moraga Volcanics. Subsequent compressive strains produced by various local faults including the Hayward Fault folded the lava-bearing rock formations, tilting the Round Top vent complex on its side.

Folding, erosion, and a quarry operation exposed a cross section of the great volcano, providing an excellent means to study a California Coast Ranges volcano. Lava within the vent has been dated by UC Berkeley at 9.5 million years old.

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