The hills of Berkeley and El Cerrito contain bodies of volcanic rocks that are incorrectly mapped as the same stuff Oakland has. They’re the two pink patches (and a tiny speck between them) on this part of the geologic map.
These rocks are colored pink and labeled “Jsv” on the map, but they are not the Leona Volcanics, correctly mapped and labeled Jsv, in Oakland. This post is about the northern patch.
The reason they’re mapped wrong is that we’ve learned more about them since the late 1990s when the USGS’s Russ Graymer assembled the map (Miscellaneous Field Studies MF-2342, the basis of nearly all my map visuals). The rocks were closely studied soon after and determined to be not Jurassic metavolcaniclastics, but Miocene rhyolites: not some 160 million years old but more like 11 million. Now they’re called the Northbrae Rhyolite. The two rocks, conflated for many decades, are actually quite distinct in the field.
Show you what I mean. Here’s the Leona, from Leona Canyon:
And here’s the Northbrae, from Grotto Rock Park in Berkeley:
Some reports say a small part of the Berkeley patch really is Leona, but I won’t weigh in on that until I see it myself. It might well be.
The distinctive rocks of Berkeley’s Great Stone Face Park, Indian Rock, Mortar Rock Park and so on are very hard and strong, with very little granularity and bearing strong signs of flow (flow banding). Their outcrops are very sturdy, rugged but rounded at the same time. Broken pieces often have a waxy luster. They’re famous for spawning the American rock climbing movement in the 1930s.
I wanted to see for myself whether the El Cerrito rocks are the same as the Berkeley rocks. The El Cerrito hills offer great views in the spring, and the northern patch of rhyolite is a lovely place to explore. Take the 7 bus line up there, light out and look around. It was a fine early-spring day when I visited a few weeks ago.
Let’s zoom in on that geologic map.
And then even closer to this hyper-local map made in 2011 by John Wakabayashi.
Map included in the Hillside Natural Area Geology Walk guide (available from this page).
That second map was just what I needed. I want to make it clear, though, that both maps are good maps. Russ’s older map was produced as a database for a large region, and he had to sacrifice a lot of interesting detail to create a computationally tractable and physically printable map. But even John’s newer map is a picture, an exercise in visualization and an interpretation of scattered, incomplete evidence. He looked at every rock in the area, of course, but he also analyzed the terrain. He drew his lines based on all of that, plus he’s an Oakland native who knows the area from a lifetime of experience, AND he had the advantage of more recent studies, including the revelatory digital elevation models of the Hayward fault area. The lines on his map, representing contacts between different units, aren’t visible on the ground, at least not where I was. I walked all of the roads within the mapped rhyolite, and everything inside the pink field is indeed the same stuff.
And it’s not like the Berkeley rhyolite. Superficially, it’s also light colored, but it’s granular, more like a tuff — a rock made of volcanic ash. And it acquires a honey-colored film of iron oxides as it weathers, similar to the Leona volcanics. I’ll continue to call it rhyolite, though.
Nowhere did I find any of the rounded, polished surfaces of the Berkeley outcrops. Though truth be told, true outcrops are scarce on the ground.
What might be interpreted as flow banding on a weathered surface appears to be merely subtle variations in grain size.
The photos above are all from the western body of rhyolite. The eastern body is its own kind of fun, not because there are any great outcrops but because of what the residents have done along Brewster Drive. Here’s an exposure of bedrock next to boulders of it incorporated into a driveway wall.
Others have been more ambitious.
The upshot is that the two bodies of rhyolite, in Berkeley and El Cerrito, are similar but not identical. I don’t know if the northern set of rocks have been dated, but if they have and the dates match those from Berkeley, then it’s reasonable to interpret them under one name, representing two different aspects of the same small volcanic center. But that might be wrong. Although the two bodies of rhyolite are near each other today, they might have been some distance apart when they were erupted. We may never know, but more research might narrow the uncertainties.
Meanwhile, it’s a nice place to visit. Murieta Rock is nearby, too.
Oakland Geology 