The hills of Berkeley and El Cerrito include our bodies of volcanic rocks which might be incorrectly mapped as the identical stuff Oakland has. They’re the 2 pink patches (and a tiny speck between them) on this a part of the geologic map.
These rocks are coloured pink and labeled “Jsv” on the map, however they aren’t the Leona Volcanics, accurately mapped and labeled Jsv, in Oakland. This submit is in regards to the northern patch.
The explanation they’re mapped improper is that we’ve realized extra about them because the late Nineties when the USGS’s Russ Graymer assembled the map (Miscellaneous Subject Research MF-2342, the idea of almost all my map visuals). The rocks had been carefully studied quickly after and decided to be not Jurassic metavolcaniclastics, however Miocene rhyolites: not some 160 million years outdated however extra like 11 million. Now they’re known as the Northbrae Rhyolite. The 2 rocks, conflated for a lot of many years, are literally fairly distinct within the discipline.
Present you what I imply. Right here’s the Leona, from Leona Canyon:
And right here’s the Northbrae, from Grotto Rock Park in Berkeley:
Some stories say a small a part of the Berkeley patch actually is Leona, however I gained’t weigh in on that till I see it myself. It’d properly be.
The distinctive rocks of Berkeley’s Nice Stone Face Park, Indian Rock, Mortar Rock Park and so forth are very onerous and robust, with little or no granularity and bearing robust indicators of circulation (circulation banding). Their outcrops are very sturdy, rugged however rounded on the identical time. Damaged items typically have a waxy luster. They’re well-known for spawning the American mountain climbing motion within the Nineteen Thirties.
I needed to see for myself whether or not the El Cerrito rocks are the identical because the Berkeley rocks. The El Cerrito hills supply nice views within the spring, and the northern patch of rhyolite is a beautiful place to discover. Take the 7 bus line up there, mild out and go searching. It was a superb early-spring day once I visited a number of weeks in the past.
Let’s zoom in on that geologic map.
After which even nearer to this hyper-local map made in 2011 by John Wakabayashi.
Map included within the Hillside Pure Space Geology Stroll information (obtainable from this web page).
That second map was simply what I wanted. I need to make it clear, although, that each maps are good maps. Russ’s older map was produced as a database for a big area, and he needed to sacrifice a number of attention-grabbing element to create a computationally tractable and bodily printable map. However even John’s newer map is a image, an train in visualization and an interpretation of scattered, incomplete proof. He checked out each rock within the space, in fact, however he additionally analyzed the terrain. He drew his strains primarily based on all of that, plus he’s an Oakland native who is aware of the world from a lifetime of expertise, AND he had the benefit of more moderen research, together with the revelatory digital elevation fashions of the Hayward fault space. The strains on his map, representing contacts between totally different models, aren’t seen on the bottom, at the least not the place I used to be. I walked the entire roads inside the mapped rhyolite, and every thing contained in the pink discipline is certainly the identical stuff.
And it’s not just like the Berkeley rhyolite. Superficially, it’s additionally mild coloured, nevertheless it’s granular, extra like a tuff — a rock fabricated from volcanic ash. And it acquires a honey-colored movie of iron oxides because it weathers, just like the Leona volcanics. I’ll proceed to name it rhyolite, although.
Nowhere did I discover any of the rounded, polished surfaces of the Berkeley outcrops. Although reality be informed, true outcrops are scarce on the bottom.
What is perhaps interpreted as circulation banding on a weathered floor seems to be merely delicate variations in grain measurement.
The photographs above are all from the western physique of rhyolite. The jap physique is its personal form of enjoyable, not as a result of there are any nice outcrops however due to what the residents have achieved alongside Brewster Drive. Right here’s an publicity of bedrock subsequent to boulders of it integrated right into a driveway wall.
Others have been extra bold.
The upshot is that the 2 our bodies of rhyolite, in Berkeley and El Cerrito, are comparable however not similar. I don’t know if the northern set of rocks have been dated, but when they’ve and the dates match these from Berkeley, then it’s affordable to interpret them below one title, representing two totally different features of the identical small volcanic heart. However that is perhaps improper. Though the 2 our bodies of rhyolite are close to one another at this time, they could have been far aside once they had been erupted. We might by no means know, however extra analysis may slender the uncertainties.
In the meantime, it’s a pleasant place to go to. Murieta Rock is close by, too.
