Archive for the ‘Oakland rocks’ Category

Bike trails, right wrong and ambiguous

15 February 2016

One day a year ago, as I set out to investigate the old Crusher Quarry, I was standing off the fire trail looking at something when a rattling sound came from up the hill. Two mountain bikers burst out of the woods in a spray of dust and gravel and jerked to a halt nearby, rear wheels tipping off the ground. “That was intense!” I heard one say. Then they rode a few yards down the fire trail to Mountain Boulevard, where someone in a pickup had just arrived to meet them, and a minute later they were gone, with no one the wiser except for me.

Where they’d come from, tracks ran up a delta of dirt to a narrow trail.

biketrail-1

It was really steep, and slippery too (that was a dry winter, you’ll recall). A false move might get a rider impaled on young acacia stumps. And if another biker had hurtled down upon me I had precious little space to duck. I climbed the trail a bit nervously. None of us really belonged there.

biketrail-2

Bikers get an intense ride here. An intense ride is a legitimate thing. But isn’t all they see here just a blur of trees during their minute of white knuckles? What I saw was untreated erosion on the ground, an unpermitted trail in the city’s Leona Heights Park, and a community of scofflaws for whom it’s their personal secret.

After a while I struck off the trail to continue my own intense hike, and that was that. Then the other week I found the trail’s top end, near the Merritt College parking lots.

biketrail-3

The trail is pretty nice up there. Not exactly legal, but not in the city park either. Useful. I even approve of it. I saw access to some nice countryside with splendid views, a well-built trail that suits the slope, and a community of avid cyclists for whom it’s their personal secret.

Once again I got sidetracked, so I haven’t traveled the whole trail yet. Clearly, though, the top and bottom segments, Jekyll and Hyde, meet at the fire road in Leona Heights Park. It would be really nice if the lower part were converted to foot traffic only, with erosion control and occasional steps. Mountain bikers could take the fire road down to McDonell Avenue.

At Leona Canyon Open Space Preserve, less than a mile south, “the park is an ideal place for hiking, running, biking, dog walking, and similar activities.” The intense, pellmell riding experience can be had on either the Artemisia or Pyrite trails there — just not the thrill of living outside the law.

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.


Follow

Get every new post delivered to your Inbox.

Join 10,835 other followers