Fallen trees

30 May 2016

Naturally, the people of a city named Oakland cherish their trees.

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But trees are living things, and every one must die at some point. After years of drought plus one wet winter, I’ve noticed a lot of downed trees. This one was in Leona Heights, an oak. When oaks topple, that’s the end of them.

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This one, a bay laurel, was in Joaquin Miller Park. Fallen bay trees commonly reroot, which is a handy thing in their preferred moist habitat.

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Fallen trees may interest field geologists because as their roots rip out, they expose the bedrock in areas with thick vegetation. Thus information from “tree throws” can help in mapping the rocks. For instance, I inspected the rocks uprooted by this tree . . .

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. . . and noted that they were typical mudstone of the Joaquin Miller Formation. The park is the formation’s type area, after all.

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Another treefall farther downstream exposed serpentinite. (The two very different rock types are separated by the Chabot fault.)

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Sometimes data points like these are the only way to get a handle on steep, vegetated terrain.

Research-type geologists consider “tree throw” a significant actor in the evolution of wooded landscapes (here’s a taste from a recent conference).

Down by the water, especially along the smaller creeks, fallen bay laurels often span the stream and root on both sides. Sometimes it’s charming, like here above Dunsmuir House, but it’s usually a hassle if you’re trying to walk along the streambed.

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To hydrologists — geologists who specialize in streams — fallen trees are a crucial part of stream systems. Natural streams always contain some amount of dead wood, because it lasts for years before it rots away or burns up.

Wood doesn’t act like sand and gravel. It doesn’t wash away easily, it sticks around and snags stuff. So making a realistic model of stream behavior requires a sophisticated understanding of how this “wood load” affects the water. Hydrology journals publish a steady trickle of papers on this topic.

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Most of us would rather that trees stayed standing and streambeds stayed clear. That only happens in landscaped parks. To know nature, you have to learn to appreciate snags.

Upper Arroyo Viejo: My first Oakland fossil

23 May 2016

Two weeks ago I wrote about the upper part of Arroyo Viejo, in Knowland Park, and said that I hadn’t walked the whole section exposed along the stream. Soon afterward I returned there and did the deed. I had a special goal of locating fossils in the Knoxville Formation.

This time, instead of following deer trails along the hillside, I went up the streambed, starting at the conglomerate outcrop I showed in that post. This is a closeup.

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It was pretty easy going for the most part. There have been no recent landslides into the creek, and in general you can see that the creek has been cutting down leaving tree roots–and bedrock–exposed. Most Oakland creeks aren’t this vigorous.

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Most of the section is coarse-grained rocks, sandstone and conglomerate. Here’s an outcrop that exposes conglomerate on the left and sandstone on the right, all steeply tilted.

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Researchers from both the US Geological Survey and UC Berkeley have found and collected fossils near here. (In fact I think they were all the same person, Jim Case, who did his Ph.D. work here in the early 1960s and then expanded it for the USGS in Bulletin 1251-J.) Many rocks have microfossils in them, which are useful for experts but not very thrilling for the average amateur geologist. What caught my interest was that these were macrofossils, the remains of regular everyday-sized organisms.

To find macrofossils, you want shale. There were chunks of it in the streambed, but outcrops were scarce.

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The fossils were reported as being shellfish of the genus Buchia and unspecified belemnites. Buchia was a group of chunky, oblong bivalves much like clams that lived during Jurassic and Cretaceous time. Belemnites were squidlike creatures with internal shells that lived around the same time. You’ll see polished specimens in any rock shop.

I didn’t come prepared for fossil hunting, with the right hammers and chisels and so forth; I just wanted to see if I could spot some on the ground. Rocks like this, I gave a searching inspection.

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Then, there it was in the middle of the streambed–a belemnite mold.

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A mold is the space that once held a fossil. Here the stream managed to dissolve the calcite shell and pluck it out of the rock. So this isn’t much of a fossil, and belemnites aren’t useful for zeroing in on the age of a rock the way Buchia is, either. But it’s proof of concept. I was as thrilled as when I was a kid grubbing brachiopods out of the Devonian shales of upstate New York, once upon a time.

Where my rocks go to die

15 May 2016

For many years I saved and collected rocks. This was especially true during my years at About.com, when I put together a large set of photos and explanations to help people learn about rocks.

When About.com dropped my contract in 2014, I’d reached “peak rocks.” My office had rocks everywhere, and my closet had still more. I’d even started putting rocks back where I collected them.

(I’d normally add a bunch of links to my old site to document those statements. However, I learned last week that About.com will take down its Geology site entirely in yet another desperate attempt to gain altitude. Some of my articles have already disappeared from Google searches. Soon my work of 17 years, and the work of my hapless successor, will disappear except from archive.org, the internet’s “Wayback Machine.” So fuck ’em.)

Some of my rocks are sentimental favorites. Other rocks, I’ll look at it and realize I can’t recall where it’s from or decide that’s too far away to visit again, and I decide to set it free.

My go-to place for that is Devil’s Slide, the sedimentary version of Orodruin, where Frodo Baggins went to destroy the One Ring. There the Pacific lies ready to grind every stone into sand. And above the old Route 1 roadway, now a county trail, are spectacular exposures of what the sand will become in the geologic future.

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I went there last week. On a somber day Devil’s Slide is very fitting for this purpose. Marine haze shuts out the world. The coast is under noisy, vigorous attack. The whole place is falling into the sea, which makes the textbook “rock cycle” a visceral reality. You feel a bit wary about the roadway itself.

Offshore is Point San Pedro, made of the same rocks. Geologists have determined that those rock beds have been overturned.

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Anyway, here’s where I stand. I take my rocks and fling them toward the waves. Most of them fall short, but that’s OK, they’ll make it down to the surf soon enough.

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One rock was this piece of coal I once found in the yard, dating from the days when our homes had coal-fired furnaces. I think it was from Utah, because California coal wasn’t this high quality.

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Given the popular feelings about shipping Utah coal through Oakland, it was the right time to make a statement by destroying this specimen.

As a geologist, I adore coal. It’s a fascinating storehouse of carbon from the distant past. As a respecter of history, I honor coal’s fundamental role in the Industrial Age. Coal saved the forests from being turned to charcoal. When I was young, my family once burned coal in the house. But its time is over. Except for scientific research and historical reenactments, coal should be left in the ground.

I support Oaklanders’ efforts to stop commerce in coal. I just don’t endorse every statement they’re making. Specifically, coal dust from rail shipments won’t cause asthma or cancer; it’s only a nuisance. Smoke from burning coal is what causes asthma and cancer, and that will happen somewhere else. I oppose burning coal, and causing asthma and cancer, anywhere on Earth.

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All I ask is that people do the right thing for the right reasons. It’s important for the long term.


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