Oakland’s rocks spill over the city boundaries. Previously in this space I’ve presented two examples (1, 2) from down Fremont way. This week I tracked down an odd bit of color on the geologic map and found a splendid lump of one of our most distinctive rocks, way up north.

It turns out to be a really nice park. And the best place to see the Leona volcanics in a natural outcrop. And the Hayward fault, too.

The Leona volcanics are the rocks in the hills around Leona Heights, Merritt College and Redwood Road. They’re light-colored, with rare shots of pale blue-green, and are usually covered with a reddish coating of iron oxides.

The geologic map shows them mostly in that part of East Oakland, displayed in a shocking-pink color. It also shows the same colors in parts of Berkeley and El Cerrito — in error; those actually are the unrelated, much younger Northbrae Rhyolite. But there’s another bit of pink even farther north on the map that I had to check out. It’s way up where the Hayward fault nips the tip of San Pablo Ridge and the Berkeley Hills run aground at the mouth of Wildcat Canyon. Just a tiny bit of pink.

^{Qls, landslide; Qhaf/Qpaf, sand and gravel alluvium; Tor, Orinda Formation; KJfm, Franciscan melange; spm, serpentinite melange}

Mappers have disagreed on what it is, but I always start with the map released in 2000, USGS MF-2342.

It’s not hard to get there, just catch the 72 bus line up San Pablo Avenue at the El Cerrito del Norte BART station and walk up McBryde Avenue. The outcrops are all preserved inside Wildcat Canyon Regional Park at the park headquarters, right where Wildcat Creek exits the hills. The rock in all the stonework around the park, starting at the entrance, is promising.

Eastablished in 1909 as the private Grand Canyon Park, this 64-acre piece of streamside land was described at the time by the Richmond Independent as “a paradise where one may revel in dreams of joy forever.” It had a spring, a large dance floor, space for athletic events and pathways to “shady dells and cozy nooks beyond number.” A small hotel, the Grand Canyon Chateau (now a private residence), was erected in 1911 on the low hill just north of the entrance and soon became a hot spot for all-night dancing and drinking — and opposition by the Good Government League. There was a short-lived dam that created a swimming hole. The city of Richmond acquired the land in 1923 and renamed it Alvarado Park in 1925 after Juan Alvarado, the last Mexican governor of Alta California, who lived nearby. It’s been part of the regional park district since 1985.

The stonework is Depression-era work supported by the federal Works Progress Administration and its California predecessor. The stone may not have been local though it matches the local rock; it may have come from the same source used by the Morcom Rose Garden in Oakland, but the outcrop is unmistakably the real Leona. It lives here and welcomes visitors.

Because the rock is in the shade by a perennial stream, moisture supports lichens and moss all over its surface. But underhangs, and gentle buffing by a hundred years of people’s shoes and butts elsewhere, reveal its reddish patina.

Time for a closer look at the map. My photos are from the northern lobe of pink. It consists of two portions defined by three faults, one on each side and one up the middle. The dashed lines mean that these are inferred, not visible on the ground. From what I could see, the faults and the divisions are real. The western division includes the little hill the former Chateau stands on, and the eastern one includes the outcrop shown above.

The fault on the right is certainly real; it separates the volcanics from very different rocks just a few feet away, along the stream.

^{Franciscan melange, unit KJfm}

Both inferred faults also coincide with offsets of Wildcat Creek, which are the main evidence suggesting that these faults are active today. They are marked “G3, rs” on the USGS Hayward fault map meaning “weak geomorphic evidence, right-offset stream.”

^{From USGS MF-2196}

Two of these three little faults are consistent with ongoing creep shown by offset curbs, which may not have been visible when the map was compiled in 1992. This is the one in the middle, at McBryde and Arlington:

and this is the one on the left, on McBryde just west of Marin Avenue, north and south sides.

As for the southern lobe of pink, I saw no evidence of it, although the views are good.

In fact, the only bedrock I could find there was serpentinite, right where the map shows pink volcanics.

So we can’t quite trust the map for the rocks — maybe the authors had better evidence than I do — but it seems reliable for the faults. The next surface-rupturing earthquake here should be quite informative.

Just for completeness, here’s the digital elevation model for the same area.

^{From Northern California GeoEarthScope Lidar Hillshades KMZ (via opentopography.org)}

It supports what I’ve found, but is tantalizingly vague about anything more.

In Oakland, which has more rock types than any other city in America, the Leona volcanics may be our oddest rocks. They’re certainly the oldest, born some 165 million years ago as an island arc on the tectonic Farallon plate in the Panthalassa Ocean, a thick pile of volcanic ash shot with layers of basalt lava. As they slowly sank underwater and drifted around beneath the proto-Pacific sea, hot fluids cooked the rocks and infused them, here and there, with sulfur minerals that attracted human attention. And that has made all the difference.

The Leona volcanics eventually drifted to North America and were plastered onto its western edge. There they survived a lot of tectonic movements, only to be shredded sideways by the Hayward fault. Most of them remain in the hills of Leona Heights, but bits trail off as far as Hayward to the south and as far as southern Berkeley to the north — plus this little outlier.

This entry was posted on 24 November 2025 at 7:55 am and is filed under Hayward fault, Leona volcanics, Outside Oakland. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.