Archive for the ‘Oakland rocks’ Category

Chimes Creek headwaters

1 February 2016

I’ve mentioned how tempting the uppermost catchment of Chimes Creek looks, perched above the Leona Quarry scar:

chimesCktop

Finally got up there last week. Access is difficult and not for casual visits.

Here’s the valley in Google Earth, looking obliquely at it. At first glance it looks natural, but it’s heavily engineered.

Chimeshead-view

I’ll show photos going from top to bottom, between the two dots on the above image. The valley above the upper dot was filled in by the Ridgemont developers, using material cut from the ridge to its left. This is the view downhill from the position of the upper dot.

Chimeshead-1

The floor of the valley here is crisscrossed with concrete ditches, which converge about a third of the way down the transect at a culvert. Along the way you pass a large outcrop. All the rock here consists of the Leona “rhyolite.” The outcrops are tempting, but the slope is steep and treacherous.

Chimeshead-2

Here’s the culvert, possibly the only one in Oakland without graffiti, joined from the right side by the concrete ditch. I think it must carry runoff from the Ridgemont streets. But what’s that ugly orange?

Chimeshead-outlet

Why, it’s acid drainage from one particular part of the subsurface here. I would love to see the geotechnical reports from the time they built this development.

Chimeshead-pipes

This is one of many places where I’ve seen “yellowboy” in the Leona; the most notorious is the old sulfur mine, of course, and I noted another last week. By now I think that every excavation in this rock unit, old or new, should be treated as a potential hazard.

The next couple hundred feet downhill from here is a lovely tree-shaded, undisturbed steep cascade over large boulders. Here are just two of them.

Chimeshead-3

This beautiful rock is extensively slickensided (polished by underground movements) and coated with the iron hydroxide minerals that result from natural weathering. It is not stained orange by the pollution from upstream; in fact the water at this point is only slightly milky. It’s very much like the rocks in the Redwood Road boulder pile.

Chimeshead-4

And what to say about this one? It’s an unusual lithology within the Leona. My impression just from eyeballing it was that it’s an autobreccia — a ground-up body of lava or tuff consisting of lumps of the original rock in a matrix of pulverized (and relithified) rock. But that’s not the only possibility.

Another outcrop up on the valley wall exposes a slickenside that covers a good square meter.

Chimeshead-5

At the base of the cascade is more engineered land, a small catchment housing a screened culvert entrance. Chimes Creek is trapped here and conveyed beneath the old quarry and across the freeway to emerge in the Millsmont neighborhood. As I say, it’s engineered land, but it’s planted with trees and rather pleasant. It also catches runaway rocks before they can take out a townhome down below.

Chimeshead-6

The view up from the lip of the catchment shows the two outcrops and the shape of the land.

Chimeshead-7

I can’t wait to return for a closer look, though with so many other places to see it might be a while.

Clay outcrop in Horseshoe Canyon

25 January 2016

The gorge of Horseshoe Creek, in Leona Heights Park, is unusually grand for its size. Its rugged rocks, mostly Leona “rhyolite,” are pretty homogeneous though.

horseshoeclay-1

So when a spot along the stream caught my eye with its color — reddish red and bluish gray — I went off the trail and checked it out. Notice that the surface is cut into the hillside.

horseshoeclay-2

This streamside lump looked just like concrete. But there was a lot of it, in different states of preservation and age, so I took it as a natural deposit.

horseshoeclay-3

It was hard, but a little higher up I was surprised to find soft material. Not just soft, but pure clay.

horseshoeclay-4

The high-silica lava and volcanic ash that makes up the Leona should weather into kaolinite (white china clay), especially under acidic conditions. We have that combination in parts of the Leona that are rich in pyrite. This mineral, with the formula FeS2, reacts with air to form iron oxyhydroxides and sulfuric acid (here’s a brief treatment).

There may be a pod of rock here with a different texture or composition from its surroundings, which might account for the purity of the clay. But I don’t actually know how pure the clay is. The way to tell would be nibbling it. Maybe on my next visit.

I think that a gradient in pH, plus interactions with air and surface water, explains the transition from gray to white to red clay as you go from depth to the surface.

horseshoeclay-5

Without a lab, there’s not much I can say about it, although geologists with more experience probably know this stuff cold. If so, speak up. There was another piece of evidence at the scene, though: a bit of leaking “yellowboy” from the floor of the streambed.

horseshoeclay-6

It means there’s a little bit of acid drainage here, not up to the level of the ex-sulfur mine just south of here. More like a geologically slow bit of natural acid drainage. It will be interesting to watch this spot during this wet winter.

Dunsmuir Ridge revisited; Irvington gravels visited

18 January 2016

Six and a half years ago, I put up a post about the special little gravel patches on Dunsmuir Ridge. On Tuesday I finally returned there to put back the rocks I collected. That’s extremely fussy of me, but a principle’s a principle and my living space refuses to stretch with the rocks I bring home.

I didn’t include a geologic map in my earlier post, so here it is now.

dunsmuirridgegeomap

The gravel patches are shown in light tan, inside the “Jsv” area (which you’ll recognize as the Leona “rhyolite”). The straightest black dashed line is the Hayward fault.

Winter is a good time to walk Oakland’s hills.

dunsmuirv-1

The Leona makes up the country rock, distinctive yet somehow nondescript.

dunsmuirv-2

As you climb through the woods, the stones in the road turn rounded, and variegated. That’s the odd, out-of-place gravel.

dunsmuirv-3

Here’s a hunk of the laminated Claremont chert, well outside its domain.

dunsmuirv-4

As I encountered each lithology, I pulled out the appropriate cobble from my 2009 raid and reintroduced it to its cousin: the celadonite-bearing Leona, more abundant in the gravels than in the field . . .

dunsmuirv-5

and the glorious Franciscan metachert.

dunsmuirv-6

When all the stones were dispersed, I went up to the hilltop where the gravel had been so abundant in the roadways in 2009. I knew what to expect.

dunsmuirv-7

This part of the hilltop is no longer fit for science, but I’m not bothered. Real science would be done by careful excavation in undisturbed places, and those are still available. Meanwhile, you could say those rocks are now cherished stones. Rainwashed and sundried, they look their best now. Enjoy them while you’re up there.

dunsmuirv-8

My second purpose in revisiting Dunsmuir Ridge was to compare its gravel to the official Irvington Gravel down in Fremont, which I did that afternoon. Here’s the geology, from the Alameda County bedrock map, USGS Open-File 96-252.

warmspringsgeomap

It’s not a great map for field use — the geographic base layer is quite poor — but you get the gist. I-680 curves east through the northern tip of the gravel, and the original quarry is right there where the “25” is. The ex-quarry and the streambed along the north edge of the gravel are preserved in Sabercat Park.

It was an ordinary gravel quarry until the 1940s, when a treasure trove of Pleistocene fossils was extracted from the site.

More fossils are surely there, thousands of them, in the hillsides.

dunsmuirv-9

I took a good look at the gravel cropping out. There was a general resemblance to the Dunsmuir Hills patches that I wouldn’t have noted had I been a naive visitor. But for an initial reconnaissance visit, this was cool.

dunsmuirv-10

Fossil hunters adore the rain, because it gently washes dust and dirt away from even tiny fossils, allowing them to stand out on the clean surface. So I closely inspected a few spots just in case there were, like, Pleistocene mouse teeth exposed.

dunsmuirv-11

Just some pretty Franciscan chert again. Parks are off limits to collectors, so I would have merely photographed anything I found. I’ve come to believe that finding is a purer pleasure than keeping, anyway.

The Northern California Geological Society had a field trip here 10 years ago. Many wonderful Irvington fossils can be seen and touched at the Children’s Natural History Museum in Fremont.


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