Archive for the ‘oakland rocks’ Category

More of the “Leona rhyolite”

4 April 2015

(Starting now, I will be putting up images 600 pixels wide instead of 450. Hope you like that as much as I do.)

A big chunk of the high hills consists of the “Leona rhyolite,” shown as the pink unit labeled Jsv on the geologic map below.

Jsv-geomap

To help you orient yourself, here’s the same area in Google Maps. The area I’m featuring is around the asterisk near the bottom of the geologic map.

Jsv-Gmap

“Jsv” stands for Jurassic silicic volcanics (or keratophyre), the kind of sticky, explosive lava and ash that island arcs are made of. Similar examples today would probably be the Greek islands. The original rocks have been rather thoroughly altered since their birth around 165 million years ago, but they still stand out among Oakland’s rocks. I described them previously here in the former Leona Quarry and here in Leona Canyon. The experts are still arguing over these rocks, and for now I will spare you the details. But basically, they aren’t really rhyolite so the name isn’t used officially any more.

This is the view north across I-580 to the northern side of the quarry scar. The high valley is the headwaters of Chimes Creek, and I continue to be fascinated by the idea of standing up there in that perched catchment.

chimesCktop

A property on Sunnymere Avenue has a nice boulder of this stone.

Jsv-boulder

And a yard on Columbian Drive uses the stone for landscaping. You may already notice how consistent the color of this stuff is. I think that may be the best way to identify it around town. Some of the Tertiary sandstone has a similar honey color, but it weathers into evenly colored, rounded forms whereas the Leona turns craggy and mottled.

Jsv-yard

And here’s a hand specimen found at the end of Field Street.

Jsv-hand-FieldSt

This appears to be Oakland’s oldest rock.

Trees and serpentine

29 March 2015

There’s a stretch of Castle Drive, up in the Piedmont Pines neighborhood, lined with huge trees. On the Walk Oakland map, it’s even labeled “Colonnade of Eucalyptus.”

CastleDrTrees

These give me mixed feelings, as tree removal projects have aroused lately elsewhere in Oakland.

First, there’s the experience the trees provide. For one thing, you basically can’t walk here, so the colonnade is not a realistic attraction for walkers. Its main effect is a momentary diversion for drivers, who really don’t need one at this location.

Second, there’s the effect on the surroundings. As you climb up in this valley, the trees emerge as a very tall fence that blocks the view of the hills and the city and the bay.

Third, there’s the geologic setting. This part of the roadway runs along a very steep 40-degree slope through pure serpentinite, visible in the small landslide scar on the right side of the photo. Serpentine rock is poor footing for these massive trees. The trees may seem like they’re buttressing the roadway, but when they inevitably tip over in a storm or earthquake, they’ll uproot it instead, forcing the locals to drive up and down Ascot Drive for many months.

But how about that rock? Here’s a hunk of it that spilled across the road.

castleserpfoot

And here’s a hand specimen. I love this stone, but roadbuilders don’t.

castleserphand

It’s not my problem, since I don’t live there, but I think the best thing to do is to turn this colonnade into a line of ground-level stumps. The root systems would bolster the soil for another decade or so, giving the city time to plan and execute a properly engineered roadway. And bollards set in the stumps would preserve the trees’ most useful current function of keeping cars out of the canyon.

Trees are supposed to be wonderful stockpiles of carbon, sequestering it from the atmosphere. For me, that argument shouldn’t apply to individual trees or even individual groves of trees. What do we do, in the long run, with the carbon in trees—pile the trunks in pyramids? Carbon is best stored in the soil, where it provides excellent tilth and maintains a thriving ecosystem that resists fire and drought. It’s like circulating money in an economy: do you hoard it in vaults or spread it around among people ready to use it as a medium of exchange? Humans have spent thousands of years degrading the world’s soils, and I’d rather we begin to restore them.

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.

pinetop3

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.

mills-bluschist2

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.

mills-bluschist3

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.


Follow

Get every new post delivered to your Inbox.

Join 8,796 other followers