Archive for the ‘Oakland geology puzzles’ Category

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.

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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.

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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.

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

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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.

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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.

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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.

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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.

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

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

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

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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 . . .

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and the glorious Franciscan metachert.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

Chimes Creek and the Hayward fault

7 February 2015

Chimes Creek is the second of the three streams in Mills College. It is said to get its name in reference to the college’s church bells. The sound would have traveled up the creek bed to the meadows behind Millsmont ridge. Today the freeway noise drowns them out. Here’s how it looked to the mapmakers of the U.S. Geological Survey in 1897—it’s represented by the dashed blue line in the middle. Below that is the same patch of land in Google Maps as of today.

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chimescreekmapnow

The land has been changed substantially in the last 118 years, but the creek continues to drain its catchment. Let’s look at the changes from the top down:

  • The headwaters have been filled and paved and are now occupied by Viewcrest Drive.
  • The Leona Quarry removed all the overburden below a short stretch of the upper creek, exposing bare rock.
  • The flats beneath have been leveled and developed, and the creek is culverted.
  • Seminary Avenue has been widened and straightened, putting more of the creek underground.

All of these changes have added to the runoff seeking to enter the creek while constricting its course. A stream will respond by running higher and faster and eroding its banks.

I haven’t yet visited the highest part of the catchment. Here’s a look at it down Altamont Avenue.

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The original creekbed is high above the left edge of the quarry, and the creek ran toward the lowest part of the foreground. Next is the view one block over, at Delmont Avenue and Hillmont Drive looking north. The creek comes out of its culvert behind the houses on the left.

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I should note that the Hayward fault is mapped running right up the valley to this spot. That’s an important detail that no one seems to acknowledge. For my purposes in this post, it means that Chimes Creek is probably cutting downward through fault gouge, the finely ground material that faults make all over California.

Farther downstream, this is looking across the creek valley at Nairobi Place. The sides are quite high here because the stream cuts downward rather strongly.

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The presence of the Hayward fault also explains why the right (opposite) bank of the creek valley is elevated above its surroundings—it’s not a levee, but rather a pressure ridge. Farther downstream along Oakdale Avenue, the valley is at its deepest.

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The lots along Hillmont Drive, across the creek, are being undermined as the invigorated stream does its work.

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I’ve love a good look at this material, but I’ll probably never get the chance. The geologic map shows this area as the northernmost splinter of the San Leandro Gabbro.

The creek enters a culvert under Seminary Avenue here . . .

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. . . and emerges here on the grounds of Mills College for a couple hundred feet. Then it enters its last culvert and joins Lion Creek underground.

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The Chimes Creek Neighbors site has thorough documentation of the human squabbling over this much put-upon watercourse. The neighbors know it as a permanent creek, although the 1897 map showed it as intermittent except for its lower reach on the Mills College campus. I suspect that the land-use changes of the last century have turned it into a permanent and more powerful stream.