Our native forest

8 February 2016

I spent Saturday in Napa Valley on a field trip with the Northern California Geological Society, visiting the ultrapremium Promontory vineyards west of Yountville and the merely superpremium vineyards of Harlan Estate west of Oakville.

High-end wine is a culture obsessed with the character of land, and Bill Harlan talked a lot about his land as we tasted the 2009 releases from both sites. What struck me most was his emphasis on the forest around the vineyards. Both properties are enclaves of vineyards in a forested setting. This view of woods-encircled vines is from the top of Promontory.

promontory-op

Harlan spoke of airs and influences while, given my recent reading, I thought of wild yeasts and soil microbiomes. I also thought of Oakland’s woods, which have been on my mind lately as I’ve explored the Leona hills.

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To some Oaklanders, all trees are good trees. To me, the oak forest is what belongs here, and eucalypts should be exterminated from our wild lands. I’ll spell out several reasons, mostly familiar, plus a new one about living in Anthropocene time. But first, a clump of oaks in the Leona hills.

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History: Once our oaks were the basis of a thriving native economy, and their acorns are still here in abundance for true locavores. Eucalypts were planted in our hills in a failed attempt to make a killing in timber. The poet Joaquin Miller, spellbound by a Victorian ideal of Arcadia, covered his grassy Hights with thousands more. Neither wave of tree-planting was done responsibly. We owe that legacy of heedlessness the same respect we owe pampas grass, French broom, foxtail grass and other pest species. Here’s a mono-stand near the Caldecott Tunnel.

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Aesthetics: Our native oaks hug the hills in fleecy blankets; eucalypts hide them in scraggly tufts. Compare the two types of forest, side by side at Lake Chabot. The very air is different.

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Accessibility: In my search for rocks, I go off trail and across country. Eucalyptus country is typically impassible, the footing noisy and slippery. Oak country is typically parklike and quiet, like this grove in the Leona hills — although it’s friendlier to poison oak, I’ll give you that.

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Ecology: Oaks make food and mulch; eucalypts make trash.

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The Anthropocene: The future’s climate is up to us. It’s vital that we pull CO2 out of the air. Trees are touted for doing that, but planting and managing forests is a sophisticated practice we’re just beginning to understand. Forests are more than just groves of fixed carbon. By far the majority of a forest’s carbon is in its soil, not above the ground.

A newly published study argues that more than two centuries of reforesting the European landscape hasn’t made a lick of difference to its CO2 balance. Because the new forests were managed for timber, the wrong kind of trees were subsidized.

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Luckily, we don’t harvest timber in Oakland any more, so we can do things the right way. That means nurturing the biomes that suit the region: native oaks, redwood, chapparal and coastal wetland. Their intricate webs of underground organisms, tuned to our rocks and climate over thousands of years, soak up carbon better than any other strategy.

Well-chosen eucalypts are splendid specimen trees, but they should be phased out in the wild. If we feed our wildland soils correctly, Oakland can do its share to fight global warming. And it will be better in all the ways I’ve mentioned.

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Restoring our land will take time, sustained funding and the same dedication that Bill Harlan gives his vineyards. If we take our ownership of Oakland’s land seriously, we can’t slack.

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.

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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.

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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.

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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.

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

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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.

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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.


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