Grotto Rock Park

6 November 2017

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.

Advertisements

Orinda’s 1204 Hill

30 October 2017

As far as I know this hill has no name, but it’s a highly visible part of Orinda. You pass it on Route 24 between the Wilder exit and downtown, as seen in this Google Maps perspective view. The USGS topo maps give it an elevation of 1204 feet, so I’ll call it 1204 Hill.

It’s part of San Pablo Ridge, but the west branch of San Pablo Creek cut a deep gorge through the ridge while it was being uplifted, forming a classic water gap. That’s clear to see in the 1915 topo map, made before anything significant was developed there.

During the mid-20th century, at least four quarry pits ate their way into this hill, one on the north side and the others on the south. Now it provides a bit of seclusion to the exclusive luxury Orinda Wilder development. I paid the hill two visits this month, and to me it offers seclusion from Orinda Wilder. Here it is, as seen across Wilder Valley last summer.

A fire road encircles the hill for your visiting convenience. It offers good views all around. This is looking north across 24 to grassy Eureka Peak along with wooded Vollmer Peak behind it. Both are also part of San Pablo Ridge.

And in the other direction is the last, southernmost lump of San Pablo Ridge, separated from 1204 Hill by another branch of San Pablo Creek.

Looking west, Route 24 approaches the wall of the Oakland Hills. Eureka Peak, at the right edge, and 1684 Hill on the left, part of Gudde Ridge, form the sides of Siesta Valley.

Both peaks are made of the same lava of the Moraga Formation. The formation is folded like a taco shell under Siesta Valley, a geological feature called a syncline, and hey why don’t we look at the geologic map now. This is the same area shown in the topo map.

“Tmb” is the basalt lava of the Moraga Formation — the taco shell — and “Tst,” the Siesta Formation, is the filling. That red line with the little arrows is the axis of the syncline. The light-yellow unit “Tms” is sedimentary rocks, also in the Moraga Formation, that were laid down between eruptions of lava. To the right of the black toothed line — the Moraga fault — the rocks are much younger and I won’t mention them again.

The quarries were excavating freshwater limestone from the Siesta Formation, used to produce cement and soil amendments, and basalt from the Moraga Formation, used for crushed rock. Here’s the lava.

Where lava flowed onto wet ground, the underlying clay got baked and the steam oxidized the lava, turning both materials red. It’s not always easy to tell what’s what.

There are bits of light-gray limestone here and there. You can tell by the way a drop of acid fizzes vigorously on it. I didn’t take a picture, but I did stop to shoot examples of mineralization in the lava. They might be copper compounds, or phosphates, or several other possibilities that aren’t easy to identify just by eyeballing.

There’s also a little conglomerate that looks for all the world like the Orinda Formation. It goes to show that a geologic map is a simplification of a wacky, complicated Earth. However, I don’t quite trust the stone because it’s loose, not a proper outcrop.

Climbing the hill from the fire road is a scramble. There are only subtle trails, the kind that deer make. But on top of the hill there’s the stubs of a former water tower, and a stone spiral with a little cairn at the center, the kind that locals make. And outcrops.

The last thing to mention is the quarry scar. It displays the structure of the Moraga Formation nicely.

But the exposures are unapproachable and dangerous.

I’m not sure what the developers plan to do with this hill. Probably nothing except watch it closely. The closest street to the quarry scar has home sites only on the far side of the road, and there’s a wide ditch between the road and the quarry that will catch falling rocks.

I’m gonna keep my eye on this interesting spot.

Serpentine and pebbledash on Broadway

22 October 2017

The intersection of Broadway and 20th Street features strong buildings on all four corners. We all know the I. Magnin building (built 1930) and the Capwell (Sears/Uptown Station) building (built 1929) facing it. Across Broadway, we have the metal-clad urban spaceship of the Kaiser Permanente Division of Research building (built 1982) and finally the dark cube of the Golden West Tower Building (built 1968).

One of the nice things about taking the bus is that while you wait, you can look around like architects and urban planners do, as if the cityscape were a stage set. And so, waiting for the 33, I finally took notice of the building wall that I’ve walked past hundreds of times.

It’s chips of dark serpentine, embedded in cement and polished. People in the building trade must know exactly what this is called, but I can’t crack their code. Here’s a closeup.

So all this time, the Golden West Tower Building has been giving the serpentine-clad I. Magnin building a nod, a salute, a shoutout, a heart-tap. I thought that was cool.

That day the 33 took me to Piedmont, where I recognized the same material in Bufano’s “Bear and Cubs” sculpture in Crocker Park, featured here previously.

The other element in that Broadway streetscape is the sidewalk. You’ve all seen it.

This is what’s called a pebbledash finish. Concrete is laid down, then pebbles are pressed into it. When I look at it I think, “What a lovely Franciscan color scheme,” because the reddish and greenish mixture of metamorphosed argillite and chert is so typical of our coastal Northern California rocks. I also think fondly of the red-and-green gravel of Rodeo Beach.

Earthquake apps and post-quake observations

16 October 2017

Seems like disaster is in the air every October. This year the catastrophe is wildfire in the North Bay, bringing up memories of our own turn in the line of fire this very week in 1991.

Yesterday the local paper published an article by Seung Lee that explicitly linked the October fires of 2017 to the Oakland Earthquake of 20XX, not here yet but sure to come.

Lee pointed to some promising smartphone apps that could save us lots of anguish and maybe even lives. Of course, MyShake came first. It’s an Android app (IOS version pending) that turns your phone into a crude but effective, networked seismograph-plus-earthquake-alarm. I’m watching MyShake closely and will let you know when iPhones can participate.

Lee mentioned several apps to help with communications, offering flexibility in the face of degraded cell service and bringing more superpowers to your smartphone. They include Zello, which turns phones into crude but effective walkie-talkies; FireChat, which enables phones to network without internet or cell service by using peer-to-peer technology; and NextRadio, which turns last-generation phones into FM radios. Needless to say, a portable charger belongs in your purse and go-bag — fully charged.

But I’m also writing to point out some possibilities for us to help science after a large East Bay earthquake, once you’ve taken care of yourself and those nearby. Lots of geologists will show up, doing different things. Some will be inspecting damage as consultants. Some will be there doing science on behalf of state and federal agencies. Professors will come with their students, teaching them real-life lessons in disaster response and collaboration with other scientists. You can give them a hand during the aftermath. And some more things you can do of your own initiative.

If you see ruptures in the ground, collect some data. Measure their offsets, photograph them with date and time stamps, and insert objects for scale like a coin or lens cover or your own hand. Do these things quickly before city crews come in to fix the damage. Document offsets in buildings, too. Repeat these observations as the days go on, because afterslip — continuing fault motion once the shaking is over — is a new and lively topic among earthquake scientists.

Ambitious amateurs can practice the structure-from-motion (SfM) technique, by which a series of photos taken from all sides can be turned into accurate three-dimensional models. This yielded dramatically good images after the 2014 Napa earthquake. If I were a maker type, I’d do this. Any practitioners out there?

Monitor your local streams. As earthquakes rattle the ground, they shake down the material of the hills as surely as you’d shake down a jar of coffee beans. When this happens, groundwater gets pushed aside, and a sudden rush of water fills the streams for a while. The Napa earthquake of 24 August 2014 did this all over the North Bay, which I posted about at the time, and Oakland was affected, too. An Eastmont Hills homeowner whose property has a tiny backyard stream valley, dry most of the year, told me that he saw water rise in it within hours of the Napa quake, and the stream ran high for about two weeks.

The Oakland Earthquake of 20XX will be a Katrina-sized event, much worse than the Loma Prieta quake (another October surprise that happened 28 years ago tomorrow). The more we’re aware, the more we prepare, the less likely that our local Katrina will be our Katrina catastrophe. And let’s hope it picks a different month to strike.

In housekeeping news, I’ve pulled earthquake-related posts into their own category, mostly separate from posts about the Hayward fault.