Archive for the ‘Oakland geology puzzles’ Category

The Dimond Canyon water gap

15 May 2017

In a city full of geologic features, Dimond Canyon stands out as a classic example of a water gap. But it can be hard to see, even from the prime viewpoint of Leimert Bridge.

Let’s abstract ourselves by studying the overhead views shown in maps. Google Maps, with the terrain view turned on, is where I like to start.

Compare Dimond Canyon, cutting straight through the bedrock ridge of the Piedmont block, with Indian Gulch (Trestle Glen) on the left, a conventional stream-cut valley that fans out against the ridge.

For a starker view of the topography, I like to consult old USGS maps like the 1897 Concord quadrangle, made before most of the area was built up and dug into.

Here we can see that the ridge reaches the same elevation on either side of the canyon — without the canyon cutting through it, this would be a continuous crestline.

There is no sign, either, that Dimond (“Diamond”) Creek cut its way through by headward erosion. That would have left tributaries fingering off on either side, like those visible in the contours above Indian Gulch. Indeed, the single little tributary in the canyon is actually a hanging valley that has to descend steeply as it meets Dimond Creek — not as spectacular as those in Yosemite Valley, but with the same basic configuration.

Finally, we can look at the bedrock evidence in the geologic map.

The whole area around the canyon is mapped as Franciscan sandstone (Kfn), with no hint of faulting or other structure that might have favored the formation of a canyon here. Consider the well-developed valleys above the canyon, guided into existence by the rock-crushing Hayward fault, or the more subtle topographic features where the southern edge of the Franciscan bedrock meets old alluvium.

What we have here, then, is a genuine water gap — a deep pass in a mountain ridge with a stream flowing through it. Geology textbooks will tell you there are two ways to make one. One is for a river to uncover an ancient ridge as it strips the countryside of its sediment cover. The classic case is the Delaware Water Gap in Pennsylvania. The other is for a river to sit there, doing its thing, as the land rises up around it. The Central Valley has good examples at the foot of Del Puerto Canyon . . .

. . . and the Berryessa water gap west of Winters.

What’s odd about the Dimond Canyon water gap is that it’s being carried along the Hayward fault. Every few hundred thousand years, then, it gets itself a new headwater catchment. Today its catchment is Shephard and Palo Seco Creeks. Once upon a time, though, it must have carried the waters of San Leandro Creek. Coming up: Temescal Creek.

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Rubbing rocks

13 March 2017

Rocks interact with animals of all kinds. Obviously, lizards and voles and snakes and woodchucks live on rocks and/or dig under them. Humans paint on rocks and move them around and blow them up. Today, however, I’m going to talk about animals that scratch themselves against rocks — rather, rocks that animals have rubbed for thousands of years.

Last week John Christian, a sharp-eyed and inveterate walker of our hills, showed me this outcrop next to the Little Farm in the Tilden Nature Area.

Not much to look at unless you get up close. When you do, you’ll see that it’s covered with moss and lichens, except for some oddly smooth bare spots on the outermost surfaces.

Some of these are smooth enough to gleam in the sun.

These features are well known in buffalo country, elephant country and other places around the world. Large herbivores deal with the mites and lice and other irritants in their skin by rubbing themselves against anything scritchy they can find, preferably after a nice wallow in high-quality mud or at least a good roll in the dust. These marks, in a word, are sandpapered onto the rocks.

This outcrop appears to have gotten its smooth spots from that cause. But the cover of lichen and moss shows it hasn’t been used in a very long time. Today, deer have plenty of trees to use, but historically — and prehistorically — most of coastal California was treeless because the Indians kept it that way with regular fires. However, deer aren’t tall enough to make most of these marks. Now during the ice ages, though, this was a treeless cold savanna that supported herds of elk and mammoths and ground sloths and horses and bison and camels. Could those extinct animals really have buffed these boulders?

The best case for that is on the Sonoma coast just south of Jenner. I wrote a piece about the “mammoth rocks” there, which you’ll have to pick through on the Internet Archive because the folks who paid for it threw it away. The archaeologist who discovered the site has also written it up. John and I both know that site, but I never thought to look around our own hills whereas he did. When we hiked a little farther down Wildcat Canyon and he showed me a polished boulder of the same blueschist found on the Sonoma coast, I had a shiver of recognition.

Berkeley is justly famed for the rock parks in its boulder-strewn northern hills. They, like the Little Farm outcrop, feature the Northbrae Rhyolite, a particularly tough volcanic rock that you have to climb to appreciate. Even the tiniest fingerholds are as solid as steel.

At Indian Rock, generations of climbers haven’t buffed anything smooth. Nevertheless you’ll see two kinds of smooth spots. There are slickensides, formed where rocks slide against each other.

And then there’s this wide, vertical rock face that looks like it might be very inviting to an itchy mammoth or ground sloth. And it’s polished.

A few other Berkeley rocks display the same kind of evidence, if you look closely. But I had to see if Oakland can boast it too. Thinking like a mammoth, I visited my favorite Oakland blueschist outcrop in Knowland Park to reconnoiter. It looked very mammoth-friendly, including a good site for a wallow in the headwaters of Upper Elmhurst Creek.

But no such luck. Every surface of the outcrop was rough and rugged as can be. The same with this notable serpentinite knocker farther down the stream valley.

I blame Oakland’s rocks. We don’t seem to have anything as tough as the Northbrae Rhyolite, capable of retaining a polish for tens of thousands of years. But I’ll keep my eyes open; you never know.

The Idaho connection

13 February 2017

I’ve been getting into the weeds as I work on my book manuscript about Oakland’s geology (tentative title, Deeper Oakland). Where did Oakland’s rocks come from? Specifically, how did they get from where they formed to where they are? This problem is particularly vexing for the older rocks with Mesozoic ages. The western edge of North America has been built, unbuilt, shifted, rebuilt and disassembled for hundreds of millions of years.

Generally the pieces have been carried northward by the vagaries of plate tectonics. Rocks that were once Californian now sit as far north as Alaska, and likewise rocks that live here now come from as far south as Mexico.

The rocks in lower Shepherd Canyon (the Shephard Creek and Redwood Canyon Formations) belong to the huge set of sandstones and related rocks underlying the Central Valley — the Great Valley Group — but are separated from them. How they broke off and how they wandered to where they sit today are, as we say, poorly constrained. One clue may be within the sand itself.

Last year a paper in Geology laid out an intricate case that the sediment making up one part of the Great Valley Group was eroded from an ancient set of rocks in Idaho, the Lemhi subbasin of the Belt Supergroup. Papers dealing with the assembly of the North American Cordillera usually have gnarly figures, because the story is so complex, and this one, the product of an all-California team of geologists led by Stanford’s Trevor Dumitru, was no exception.

idaho-gvg-connection-figure

The whole scenario is based on microscopic grains of zircon, which lurk in many sandstones because the mineral is extremely durable. Luckily, we can determine the age of zircon grains because they’re superbly suited for the gold standard of dating techniques, the uranium-lead method. Think of them like pennies with dates on them.

So there’s a big body of rock in the Lemhi subbasin full of a unique combination of zircon ages, including a bunch around 1380 million years old. At one point during Cretaceous time, around 80 million years ago, a mountain-building episode pushed these rocks into a knot of high peaks, which eroded into sand that was carried by rivers in at least four directions. Dumitru and his coauthors duly gave these hypothetical ancient rivers names, because that’s one of the perks of doing historical geology.

Sandstones containing Lemhi zircons, with their telltale 1380 Ma peak, are found in Wyoming, Oregon, Washington and California. In the Bay Area, the paper identifies them in sandstone from Del Puerto Canyon, west of Patterson in the Central Valley. Apparently a huge pulse of Lemhi sand poured down the “Kione River” and filled the whole sedimentary basin for a while. (Basins keep sinking as they fill, because the sediment load depresses the crust.) That sandstone is mapped as the Kione Formation, a portion of the Great Valley Group that’s been considered mysterious because the sand clearly didn’t come from its usual source, the nearby (ancestral) Sierra Nevada and Klamath Range.

The point of all this is that the Oakland rocks I’m talking about date from this same period! If only we could get a zircon researcher to check them out, we might learn a little something. I mean, Dumitru dated rocks from Albany Hill, Stinson Beach, Bolinas Ridge and Sutro Baths (localities H, G, F and E in the inset map) among other places. He dated rocks from the Novato Quarry terrane of the Franciscan complex, the same unit our own Franciscan rocks belong to. He dated rocks from the Sierra de Salinas Schist, down near Monterey.

sierrasalinasschist

I love it; it has its own cool story. So why not visit Oakland too? I guess rocks on this side of the Hayward fault aren’t as important for the bigger story. But you never know until you find out, right?

By the way, I will be speaking at the upcoming East Bay Nerd Nite, Monday the 27th; watch its Facebook page or website for details. The topic is, “Are Rocks People?”

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.

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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.

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.