The Mills College lobe

19 April 2015

The big alluvial fan of old Pleistocene gravel making up Oakland’s most unusual topographic feature—the Fan—is cut by stream erosion into eight lobes, which I’ve numbered from west to east. Lobe 7 is entirely inside the grounds of Mills College, as shown here on the geologic map.

lobe7-geomap

Seminary Creek passes the west side and Lion Creek the east. These streams are responsible for carving the hill away from the rest of the Fan. Of all the bits of the Fan, only this hill and Pill Hill stand isolated from adjoining bedrock. I’m unaccountably fond of both.

Here’s an impression of its topography from Google Maps. The numbers mark the locations of the photos that follow.

lobe7-topo

The hill is densely wooded, for the most part, which limits views of it and views from it. I’ve done what I can, but it’s hard to take in by eye.

I have walked the hills and streets on all sides of this feature, and so far I haven’t found any spot where it stands out in a photo. You have to go there and walk it to get a sense of it. This walk goes clockwise around it. We start where Kapiolani Road is bumped off its straight line by the hill and a footpath takes you up its flank.

lobe7-rise

As I said, most of the hill is wooded, but along the west flank there are spots with a view. The next two photos look from the top of the hill over Seminary Creek’s valley (the creek is culverted here) toward the populated slopes of Maxwell Park hill, lobe 6.

lobe7-to-maxwell1

millshill-over-SemCk

Near the north end of the hill is a footpath down the hill’s western slope to a little-used parking lot along MacArthur Boulevard.

mills-SemCkvalley

From there you can see a fair bit of the hillside.

mills-lobe

You can continue through the woods along Seminary Creek, then take the path past the little graveyard where the Millses, Cyrus and Susan, are buried.

the-millses

Rocks of the Crusher Quarry

11 April 2015

This former quarry in Laundry Farm encompassed the ground now occupied by Belfast and Bermuda Avenues, just south of Horseshoe Canyon and Leona Heights Park. One source, the Laundry Farm map, shows something called the Hotel Mine in this area but I have found nothing about that mine, only information about the Crusher Quarry. It was operated by the E. B. & A. L. Stone Company around the turn of the last century.

Here’s where we start, at this fire road just north of Bermuda Avenue off Mountain Boulevard.

crusher1

The square object is the upturned west end of a long, arcing concrete ramp. It slopes down to ground level and levels off beneath some oak trees.

crusher2

Then it turns up again, as if aiming at the old quarry face.

crusher3

Here’s the broken upper end.

crusher4

This was once the working middle part of a cable tramway, a set of steel cables that carried huge buckets back and forth between the rock face and the crusher. The ramp therefore describes a roughly catenary curve corresponding to the natural sag in the cables.

Onward to the high working face of the quarry.

crusher5

This is highly fractured “Leona rhyolite” that needed little processing because it was already naturally half-crushed. The name of the quarry may be typical 19th-century American irony.

crusher6

The stone has acquired a typical blushing orange hue because it releases a lot of iron, which oxidizes and hydrates forming thin crusts on exposed surfaces. I think that the notable deposits of red and yellow ocher in this area, which were widely known among Bay Area native tribes, arose from many thousands of years of uplift, fracture and weathering of this rock.

crusher7

Higher on the hillside above the quarry, you can find lots of natural outcrops. They show signs of working in places. I think the stone has a nice presence.

crusher8

You’ll see sheltered spots with the glimmer of various shades of ocher; also evidence of wildlife.

crusher9

On the way back out, take time to see what’s blooming in the raw land the quarry left behind. These are called red maids.

crusher10

And the fine gravel even supports an underground ecosystem that produced this emerging mushroom.

crusher11

The locals enjoy and watch this place, so there isn’t any tagging or bottle-smashing to speak of. Climbing is dangerous; stick to Berkeley’s rock parks for that. Visit discreetly and leave the place cleaner than it was when you came.

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.

Triple creek junction

17 March 2015

A while ago I featured the casting ponds at McCrea Memorial Park, in the valley of upper Lion Creek. I didn’t poke further downstream at the time, but since then I have. The creek runs alongside a pair of small concrete “trout ponds,” cunningly made with cobble-lined runnels that would send a lifegiving trickle through them if there was enough streamflow, which there isn’t, to sustain fish, which there aren’t any of.

leonack-troutponds

The lower pond did have enough water in it to interest a mallard couple. The male kept watch on me as the female gorged on duckweed.

To the left of that photo, just over a low ridge and behind a fence, the iron-stained waters of Sulfur Mine creek exit a pipe.

sulfurmineck-outlet

Farther down, it joins Lion Creek and the combined watercourse enters a tunnel beneath the Warren Freeway. I haven’t gone into it, but people clearly do.

sulfurmine-lion-ck-junction

If you cross the freeway on the little-used pedestrian overpass, you might expect to find the creek on the other side. Instead you’ll see Horseshoe Creek coming out of its canyon in Leona Heights Park and entering its own final conduit.

horseshoeck-mouth

Where the two creeks meet must once have been a cheerful place. Today it must be a black and dismal one, visited only by rare daredevils. If the tunnels aren’t screened, perhaps they can crawl all the way down to where Lion Creek reemerges, at Lake Aliso at Mills College, in a wretched feat of urban spelunking.


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