Lake Aliso

30 December 2014

Mills College occupies a geologically interesting part of town. It owes its stimulating geomorphology to the confluence of three streams under the influence of the Hayward fault. I plan to write several posts about it. Here’s the segment of the geologic map that includes the campus.

mills-college-geomap

The main strand of the fault runs just left of the “Jb” symbol. The narrow lobe of tan, symbolizing Pleistocene alluvium, is where Lion Creek turns from its southward course and cuts across a low ridge of Jurassic basalt (Jb) to cross the fault. I have to say that I haven’t yet found any basalt there, so treat the map with caution. After every large earthquake, whenever and wherever the ground is uplifted the creek, momentarily dammed, gathers its strength and cuts its way through to maintain its right of way. But a flat spot in the streamcourse persists above the fault trace, and there may be a tectonic element at play there too, downdropping the spot in a sag basin. In any case, that wet spot is where the college’s administrators erected a dam to create Lake Aliso, a picturesque basin that was also useful (1) as a water supply for landscaping purposes and (2) for regulating the creek in an attractive state of flow, neither flood nor trickle, as it traverses the campus.

Old photos show the lake as a fine place for boating and pageants, but sediment has inevitably filled it in. Today it’s trying to return to marsh, and from there it aims to retire as a nice meadow.

lake-aliso

But we made the lake, and we can maintain it with enough money and machinery. Here’s Lion Creek, such as it was, at the lake’s inlet, which must date from the building of freeways I-580 and Warren.

lake-aliso-inlet

My visit was a few weeks before the December rains but after November’s whistle-wetting, so the water was cloudy with fresh sediment and possibly some of that ugly runoff from the old sulfur mine. Right now Lion Creek should be closer to roaring.

The other end of the lake is an earthen dam, including this spillway.

lake-aliso-outlet

It demonstrates one of the basics of managing streams of any size: If you block a stream, it will silt up its bed on the high end and start eroding its bed on the low end. Another way to think of it is that when we mess with a stream, it usually backfires in the long run. The guidance of a licensed geologist with some expertise in hydrology can help forestall the inevitable.

There is some loose rock around, most of it looking like this.

lake-aliso-rock

Although they may just be landfill, I assign these stones to the “Jsv” unit—the metamorphosed volcanic rocks that make up the high hills.

Our local fill

23 December 2014

There’s a little corner of Lake Merritt that the improvers haven’t gotten around to, on the north shore by the pergola. Here the concrete walkway gives way to a stretch of old fill.

lakemerrittchert

The original wetland that became Lake Merritt was known as San Antonio Slough. From Oakland’s earliest days, the locals kept trying to “reclaim” it by turning it into dry land, just as they did all around the bay. The whole waterfront is reclaimed land. The basic technique was to haul dirt and rock and rubbish down to the water, shove it in and tamp it down. In Gold Rush San Francisco they’d use abandoned ships for fill, but Oakland’s founding fathers had advanced beyond such crude strategems.

Some of this material came from the holes dug for building foundations, but it also came from quarries in the local hills ranging in size from little borrow pits to big enterprises like the Blair Quarry (now Dracena Park) in Piedmont. Not just stone, either—Oakland had abundant gravel nearby, too.

If they weren’t trying to fill it in, the makers of Lake Merritt were trying to elevate its mucky shoreline and civilize it. The rocks in this part of Lake Merritt appear to be good old Franciscan chert, possibly from the “phthanite” diggings that Walter Blair exploited in today’s Moraga Canyon. It made quality fill, hard and clean and compact. I don’t know how long it will stay visible as we continue to civilize the lakeshore. Visit it some time when you’re on a walk around the lake and the ground is washed clean. The more we kick it and scuff it and curse it for stubbing our toes, the more its polish gleams.

Grizzly Peak

20 December 2014

Grizzly Peak is the highest point in Oakland, at 1754 feet elevation (sources differ). As you approach it on Grizzly Peak Boulevard, it seems to loom quite high.

grizpeak1

That’s an illusion caused by the eucalyptus forest. As you get closer, you start to see through the trees.

grizpeak2

And from the bay side, the peak has a mohawk look because the trees are stripped off its northern half.

There’s a vague trail up the south side. Even in its true contours, Grizzly Peak is a steep little climb, and the thick layer of leaves is slippery. I’d rather the eucalyptus trees weren’t here, but they do offer a lovely privacy.

grizpeak3

And underfoot are rocks! The peak is mapped as the Moraga Formation, a set of lava flows from 9 to 10 million years old. This is the stuff connected to the volcano at Round Top.

grizpeak4

The top of the peak has a broadcasting tower of some sort, with a fenced-in support building at its foot. There used to be a lookout tower here, and a benchmark nearby attested to its elevation. Mount Diablo is almost exactly due east—not that you can see it through the damn eucalyptus.

You could walk up the access road instead. Either way, you can’t get any closer to the peak per se than this.

grizpeak5

That route offers nice views, and it takes you past a lot of broken rock, if you have your heart set on a specimen.

grizpeak6

Most of the rock is like this—weathered and fractured. There’s no easy way to tell what causes the strong layering, as this rock has been tilted almost vertical and then eroded by the fog, rain and earthquakes of the Berkeley Hills.

grizpeak7

It’s hard to expose unaltered bits. What’s there is a medium-gray, featureless stone that geologists typically call andesite until they can study it in the lab.

Grizzly Peak is not a place to stay long, but it seems that there are those who love it.

grizpeak8

Local global science

16 December 2014

cider2014

I don’t spend all my time out among Oakland’s rocks. I also take advantage of the Bay area’s opportunities to learn about Earth science. Every year, for instance, I attend the Fall Meeting of the American Geophysical Union, held without fail in San Francisco since 1968. I started in the mid-1980s, and it’s where I’m spending this week. Last year I also started attending the annual sessions of the Cooperative Institute for Dynamic Earth Research, or CIDER, held at Berkeley on the UC campus. It’s a geology geek’s gathering that marked its tenth anniversary last Sunday. UC professor and seismologist Barbara Romanowicz, on the left, is the prime mover; unfortunately I didn’t catch the names of the other two people. CIDER uses an NSF grant to bring together senior researchers and “junior” scientists—grad students and postdocs—who pick a few meaty deep-earth topics and set up a summer workshop to attack them. Last year’s puzzles involved the chemistry of the Earth’s mantle and the nature of the core. This coming year it will be the relation of the solid Earth to climate change, a conversation long overdue among specialists.

Anyway, that meeting was last Sunday and they let me be a fly on the wall. It’s one more thing that makes Oakland the navel of the world.

Another hematite boulder

29 November 2014

Some of Oakland’s busiest places are little visited and little noticed. This set of boulders sits at the intersection of MacArthur and Foothill Boulevards, up by the Eastmont Transit Center.

macarthur-foothill

The stuff of these boulders appears to be the hematite ore of upper Leona Creek. It almost looks like they would fit together in a larger rock.

Who knows how they got here. Somewhere in the city’s maintenance yards there must be a stockpile of Oakland boulders, kept handy for deployments like this where you want to keep cars off the dirt and on the road.

Oakland City Hall: Stone and structure

24 November 2014

A few weeks ago I set foot inside our City Hall—for the first time, I’m embarrassed to say. I hope you will step inside before you’ve lived here 25 years, like me. I’ve always known we have a gorgeous building, and now I’m amazed. C’mon in.

cityhallcorner

First we’ll have a look at the structure’s famous seismic retrofit. See the light-colored strip at the foot of the walls? That’s a steel apron that covers an air gap all the way around the building. That gives the structure room to shimmy and sway on its fancy rubber feet during a severe earthquake. You can see it better by the side door, on 14th Street.

cityhall-skirt

Notice the bellows-style barrier on the building below the street level, filling that air gap.

When you go inside there’s a little exhibit space that has, among other interesting objects, this cutaway model of the base-isolation pads.

cityhall-pad

There are more than a hundred of these under the building, each one the size of a cafe table, made of thick rubber and lead plates. In the early 1990s, when City Hall was retrofitted, no one had ever used this technology at such a scale before. Since then many other precious historic buildings have used it. Hearst Mining Hall on the UC Berkeley campus is one.

OK, now comes the luscious stuff. The interior is beautiful in the way people favored a hundred years ago. Here’s the grand stairway leading from the front door up to the City Council chambers.

cityhall-balustrade450

Click the image for a large version. The balustrade is translucent marble on top, ceramic tiles on the sides. The stairs are marble. The ornamentation on the walls is plaster.

Here’s a skylight on the upper floor, edged in black marble.

cityhall-skylight

And this thing is a large lighting fixture that illuminates the rotunda. Click that photo for a large version. It was futuristic in 1914, and it’s still futuristic today.

cityhall-light450

There are other, smaller fixtures elsewhere on the ceiling that are worth searching for. The bronze ring suspended above the big ball depicts personifications of the planets—eight of them, from Mercury to Neptune, just like today.

Claremont chert, and other Oakland rocks, in Sunol

16 November 2014

The other day I took a field trip to see the construction site of the new dam for the Calaveras Reservoir. (The old dam is being replaced because it’s not strong enough to endure a big earthquake on the Calaveras fault, which runs essentially right through it.) It was a cool time, with fossils and big construction vehicles and engineers and grout. As the dam people were orienting us that morning, they passed around specimens of the major rock types in the area, and here was a fine chunk of chert from the Claremont Formation.

Claremont-in-Sunol

The stuff in our Oakland hills is white, because its carbon content was been leached out. This deep specimen retains the organic remains of ancient plankton that make the Claremont, like its larger sibling the Monterey Formation, source rocks for petroleum. (I showed you an exposure of similar stuff down at Alum Rock Park a while back.)

What the heck, I’m not planning to publish these anywhere else: here’s a cobble from a conglomerate down there that’s been stretched and fractured by activity on the Calaveras fault. It was exactly like the examples in our own Oakland Conglomerate.

calaverascobble

I think this one was from the Berryessa Formation (which is also found up at Alum Rock). Considering the looseness of the definitions of these formations, I feel safe in correlating them, although Crittenden defined the Berryessa as lying above the Oakland. Anyway, the phenomenon is the same.

One major feature of the construction project is an enormous cut made into the hill on the west side of the dam site. Basically, they discovered an active landslide there and decided to excavate the whole damn thing. We got to wander out along this exposure and hunt for fossils in the Temblor Formation, or at least rocks mapped as the Temblor.

calaverascut

There were a fair number of big ol’ scallop shells to be found, usually in pieces. Also some coaly bits of fossil wood. This is a rare exposure of an actual bedding plane, as marked by a pavement of scallop shells in what looks like their growth position.

calaveraspectens

We also learned a lot about the recent project that rebuilt part of the Hetch Hetchy Aqueduct that runs through here. Once the work was done, this tunnel-digging machine became surplus.

calaverasdigger

The guy asked us if we wanted to bid on it. I thought it had the makings of a good Burning Man stunt.


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