Thanks, Oakland

On a gorgeous day two years ago (it was 11/29) I made a field trip to Corona Heights, across the bay in that other city, and shot this view of my home town. Soon I turned it into the banner image for this blog. I thought I’d offer a larger version—just click the photo for the 1600×835 size. It doesn’t get the whole city, only from Hiller Highlands down to Crestmont, but it gets the essence: green hills, wide lowlands, a handsome downtown, a busy harbor.

San Francisco is a fine place (it certainly has us beat for slickensides), but Oakland is where I’ve been happy to live for almost 20 years. I feel like I’m just getting to know it.

Thrust and fold

The same day I was up at Redwood looking for the bent trees, I ran across this fine example of a thrust fault right next to the Huckleberry Botanic Reserve.

Today I finally got around to putting it in my gallery at About.com—not as a thrust fault (I already have a good one), but as an example of a drag fold. Looks pretty good there, almost textbook quality. But here’s a secret: look at this view of the fault.

I can’t figure out what the double curvature means! I can’t figure out the relationship of the fault to the folds. I feel like a freshman in his first field course. A real geologist would crawl all over this, including the hillside on top, until everything was clear. But I tell myself, the key to being a good scientist is to admit when you’re mystified because enlightenment comes that way. A certain set of people can visualize things ten times as complicated, and I hope one of them will pipe up.

By the way, my spread about Oakland’s geology is in the new Oakbook, the printed one. Go get one for free.

Breccia

Here is a nice example from the Oakland hills of breccia, a kind of rock that consists of broken pieces of rock. There are several different ways to make a breccia, and not having a lab and a petrographic microscope I can’t specify what made this. But considering that the hills have been raised by vigorous tectonic action accompanied by earthquakes, it’s easy to imagine some seismic event opening a fracture, shattering the rock, and allowing mineral-charged fluids into the space. Other ways involve dissolving the rock until it collapses, slumping while the rock is young and poorly consolidated, and simply cementing together a pile of landslide debris or volcanic deposits.

Because breccia really signifies an activity rather than a material, geologists would rather think about brecciation when they see a breccia.

UPDATE: I should mention that I have lots more breccia pictures on About.com: start here and try a search on “breccia” for more.

Soil creep

I was up in Redwood Park this morning looking for this. Not the woods or the sun or the fine cool air, lightly scented with fall leaves. A textbook publisher wanted a photo of bent trees like these, which curve because they grew on a creeping slope. The sandstones across the Oakland hills weather into clean, fine sand that doesn’t have much strength. Thus the slopes in the redwood groves are as steep as a sandpile. Redwoods favor the sandstone because it retains moisture, and they seem to be fine with the angle of slope. But other trees root themselves differently, helpless against the very slow, steady earth movement called soil creep, and saplings may have to correct their stance as the ground shifts.

If compression across the Hayward fault didn’t keep pushing the hills up, they wouldn’t have these intimidating slopes but would soften into something more like the hills of Moraga.

The valley here is precious for being the habitat of the rainbow trout’s type population, the community of fish from which the species was first officially described. That doesn’t mean that the trout here are higher in genetic diversity, or bigger, or more special in any way except their fortuitous encounter with a biologist. But the park is taking good steps to safeguard the stream anyway, and I am so proud of East Bay voters for continuing to ensure funds for the regional park system. Developers take care of themselves; utilities and municipalities do too. Only the people, united, can take care of their common lands.

Flint nodules (almost)

As you drive up Route 24 toward the tunnel, look to the right, and you’ll see a rocky ridge facing the Hiller Highlands neighborhood with a power line on it. One set of towers is right above the intersection of routes 13 and 24. This spot is at the next set of towers; you reach it via the fire road from the North Oakland Regional Sports Center on uppermost Broadway. Few people visit here except the usual partiers, but the view is great. The rock here is mapped as undifferentiated Late Cretaceous rocks of the Great Valley Complex. It seems to be mostly shale, but the visitors have helpfully gathered the lumps of harder, flinty rock in it to make this fire ring.

This broken stone shows a dark, siliceous surface that reminds me of the flint nodules that are common in many limestones. On the other hand, it’s just as likely to be a concretion. What differentiates a nodule from a concretion is that the material in a nodule is cleanly separated from and often different material from the surrounding rock, whereas a concretion is merely a strongly cemented portion of the material around it, in which sedimentary structures are uninterrupted. But if there is no structure to be uninterrupted, then I need to check these more carefully before giving them a name (a good reason to come back). Also, nodules tend to have a lumpy surface whereas concretions are smooth.