Sibley Volcanic Regional Preserve

The preserve contains a 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 Central Coast range volcano. Lava within the vent has been dated by UC Berkeley at 9.5 million years old. - wikipedia


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