Exploring the Bay’s explosive history in Sibley Volcanic Regional Preserve
Sibley Volcanic Regional Preserve is surely the East Bay’s most unique park, with its open, rolling hills, rocky cliffs and layered portrait of the Bay Area’s geological history. In the Miocene epoch, when the first apes were evolving into the first hominins, a volcano erupted here; in the 10 million years since, that volcano has been pushed, split and tipped onto its side by the movement of tectonic plates.
In the 1930s, a quarry sliced into the remnants of the volcano and revealed a cross-section of basaltic lava and compacted ash, the now-vertical strata still legible in the sides of the rubbly crags. In short, this volcano has been through a lot—but one can still stand on its tilted shoulder and look out along the beautiful Wildcat Fault, where pieces of the original volcanic vent and lava flows created the ridges and peaks on the horizon.
Geologists come to Sibley to study the innards of the extinct volcano, and interpretive signs walk amateurs through the most interesting geological features. Take a park map and go on a self-guided tour to learn about the basaltic dikes, tuff-breccias, red-baked cinder piles, air-fall tuffs, amygdules and other volcanic relics strewn throughout the park by ancient eruptions.
And, for a look at a more recent artifact, see if the mysterious spiral labyrinths nestled in the valleys of Sibley can be found. The first was built by artist and professional psychic Helena Mazzariello in 1989 and allowed to remain by the East Bay Regional Parks District in an endearing breach of official policy.
Sibley’s hills aren’t too steep, and the park’s 928 acres are dog-friendly. The open terrain is good for spotting wildlife, so keep an eye out for black-tailed deer, gray foxes, coyotes, golden eagles and the gentle, grazing cows that like to gaze steadily back from the trailside.
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