Some East Oakland stones

31 January 2015

Oakland’s rocks aren’t all in the ground. They’re in our yards and homes, too. Here are a few presented in the order I found them lately.

There’s a house on 60th Avenue that stopped me in my tracks, its walls studded with stones. A neighbor down the block told me “Oh yes, those are wonderful! We looked at that house when we were buying in this neighborhood. The owner’s daughter runs a preschool across the way.” Click that one for an 1100-pixel version.

morse-at-60th-450

A few weeks later I visited Best Avenue, high in Maxwell Park, and was arrested by the front yard here. Sometimes rocks, like people, look best with painted faces.

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A block away is another property treated by the same decorator.

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And just yesterday this basket of painted stones seized me by the eyes. It’s at the Free Oakland UP gallery and workshop, in the Loard’s plaza at Coolidge and MacArthur.

oakland-up-rox

Lion Creek in Mills College

29 January 2015

Lion Creek was once an Oakland landmark, named for the mountain lions and their habitat high in its catchment. Today it sneaks its way, mostly underground in culverts, to the bay. Didn’t use to be like that. Nowadays, the only place the creek is cherished and well cared for is on the Mills College campus. Here’s a brief tour from top to bottom.

At the top (west) end of the campus, Lion Creek feeds Lake Aliso, as I mentioned a few posts back. I was astonished on my last visit to find that the lake has been drained! This is the view from the dam. It’s a temporary calamity for the lake, but now the creek is visible.

aliso-drained1

I’ve been trying to reach someone at Mills to tell me the story without success. Maybe one of you knows. In this close-up, you can see that the water is yellow, I assume from the rehabilitation work going on upstream at the former sulfur mine.

aliso-drained2

And maybe the lake is drained just to let that crap wash out to sea.

The creek is, to my mind, the aorta of the campus, the way Strawberry Creek is at UC Berkeley. The old oaks and bay trees are still in place as the creek meanders graciously west.

mills-lioncreek

Just before the old west entrance, the creek is culverted, curving left past a small lobe of the Fan on its way out.

mills-lion-ck-exit

And that’s it for Lion Creek, except for a little exposure along 64th Avenue by Bond Street, until it hits the old Coliseum Gardens property in its remade bed.

Pine Top, Mills College

20 January 2015

At the back of Mills College is a steep little hill called Pine Top. It looms especially high over Lake Aliso. The geologic map shows it as consisting of Jurassic basalt.

mills-college-geomap

As I walked up Pine Top Road to reach it, I saw only what looked like nondescript sandstone, but I didn’t look closely. Rocks may be altered, and the Hayward fault running through here is notorious for swapping splinters of rock from one side to the other, so who knows.

As you round the hill, the view opens across Seminary Avenue, which runs up the valley of Chimes Creek, to Millsmont hill.

pinetop1

At the top is this big old stone stage, where it’s easy to imagine the young women of Mills assembling for all kinds of ceremonial occasions.

pinetop2

What’s harder to imagine is the view that this place once commanded. All the trees are so mature, both pines and eucalyptus, that it’s frustrating.

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On the other hand, people live on all sides now, so privacy is more important. And the freeway behind the hill is so noisy that any ceremony you could think of would be spoiled by the din.

As I said, there was no obvious evidence of basalt exposed along the road to Pine Top. So what was this lump of conglomerate doing there behind the stage?

pinetop-cgl

Just another reason to come back and poke around the flanks of this hill.

The Mills College high-grade blueschist block

14 January 2015

Longtime commenter artisancrafts reminded me, in a comment to my last post, that there’s a nice exposure of blueschist at the north edge of Mills College. Yesterday I easily located it by following his/her directions. First there’s the old railbed that once ran to Laundry Canyon. This stretch of it, which once continued down through the Fan parallel to High Street, used to be part of Courtland Street.

mills-railbed

At the spot in the distance where the sun shines across the roadbed is this lovely exposure, about 2 by 3 meters.

mills-bluschist1

It’s clearly worn by water, and a little concrete-lined ditch running along the uphill side of the roadbed feeds it. Following it upstream takes you right up to the 580 freeway abutment, where it veers north in a culvert. The stream is a mystery that I won’t try to solve today.

Back to the rock. Getting closer to it, you may not believe your camera. Clear blue skylight can do that.

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This closeup, showing tightly folded lamination in the cleft on the right edge of the first shot, is a truer indication of its color thanks to my camera’s flash.

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It’s classic blueschist, the largest outcrop of it I’ve seen in Oakland. Let’s call it blueschist-grade melange, what’s usually referred to in Franciscan circles as a high-grade block, and I’m very pleased to know that we have one in town.

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.

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

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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.

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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.

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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.

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