In preparation for the 4-mile walk I’m leading on 18 November for the Berkeley Path Wanderers, I’ve been visiting some of the unique and wonderful rock parks in north Berkeley. Grotto Rock Park will not be on the route, so I’ll feature it here.
It’s a little park on Santa Barbara Road at Indian Rock Avenue, just the size of a large lot, that preserves a nice outcrop of the remarkable Northbrae Rhyolite. The first thing you’ll notice about it is its very light color.
This is volcanic lava. Unlike the black, low-silica basaltic lava we know from the Hawaiian volcanoes, rhyolite is light colored and very high in silica. That makes it very stiff, even at the highest temperatures. Rhyolite lavas tend to form domes, like the Inyo Domes just south of Mono Lake or the central peaks of the Sutter Buttes.
After the Northbrae Rhyolite erupted, about 11.5 million years ago, the silica in it permeated the rock and turned it exceptionally hard and solid. It’s just about the best rock there is for climbing. Even the littlest toeholds will bear your weight.
Grotto Rock displays a typical texture of rhyolite — flow banding — that arises as the viscous lava flows like taffy. The name “rhyolite” in fact means “flowing stone” in scientific Greek. It’s also very bare. There’s only a little lichen growing on it because it has few nutrients, being mostly quartz.
Most of Berkeley’s rock parks feature the Northbrae Rhyolite. Yes, the rock is beautiful, but the developers gave the land to the city because the rock is so indestructible the lots couldn’t be built upon. Nevertheless, nature was powerful enough to round the corners off these bodies of lava.
Landslides could have done that, but surely the nearby Hayward fault did its part in rubbing these rocks smooth.
In non-geological news, Grotto Rock is said to offer better views than Indian Rock.
Generations of California geologists, including some quite eminent ones, couldn’t tell that the Northbrae Rhyolite is utterly different from the Leona volcanics of Oakland. Going through the literature on these rocks will teach you humility. It was a re-entry grad student at Cal State Hayward, a climber named Lin Murphy, who straightened everyone out about 15 years ago.
I’m started to get excited about the walk.
Oakland Geology 
6 November 2017 at 10:35 am
The rubbing seen in the last two pictures, as per John C. and the Senior State Archeologist, E. Breck Parkman, and others is animal, in particular, mammoth, rubbing. The prime example is just south of Jenner and is widely accepted as mammoth rubbing. (Published as Rnacholabrean Rubbing Rock on California’s North Coast, Calf. State Parks, Science Notes #72) John C. has found a lot similar animal rubbed, polished, rock in Berkeley and Tilden. Whether it was done by mammoths or bears or….is an open question mostly argued based on height above presumed grade.
There are however, some excellent examples of slickenseide or slickensides, on the west side of Indian Rock, along with a number animal rubbed surfaces. The don’t hold a candle to the world class slickenside at Red Rock Park in S.F. but are still quite wonderful. For those unfamiliar with the term who are reading this: slickensides form when competent rock fractures during earthquakes in such a way as to make a rather polished and planar surface. Partial melting may be involved, I don’t know. (The San Francisco example is in radiolarian chert.)
Of the animal rubbing, note that it is only small scale, up to a couple of feet, projecting surfaces that are polished. The polishing of rock surfaces by animal rubbing is a worldwide and continuing phenomenon.
John c. also found small amounts of cinnabar at Remillard Park.
6 November 2017 at 5:08 pm
I doubt that there’s any animal polish at Grotto Rock, because the orientation of the rock is not favorable the way it is at Indian Rock and elsewhere. That’s why I didn’t mention the possibility here.