Archive for the ‘Oakland geology walks’ Category

A circumambulation of Claremont Canyon

25 April 2016

A few weeks ago I took a strenuous ramble to accomplish a small thing — returning a stone to its home. The journey took me through some beautiful and interesting country, and the whole thing was the point.

The blue line of my route started from its farthest western point, where the 49 bus dropped me off by the Claremont Resort. From there I went up the ridge forming the northern side of Claremont Canyon, across the canyon’s upper reach to save time, and down the ridge on the southern side. The elevation change was more than 1000 feet, so this was not an idle stroll.

claremontcircletopo

The canyon is one of our finest examples of a wineglass canyon, a landform typical of major faults, in this case the Hayward fault. In case a photo image says more to you than a topo map, here’s a grab from Google Maps showing the canyon.

claremontcircle-earthview

Now look at it in relation to the Hayward fault, which runs straight across the bottom of this image through Cal Stadium and right behind the Claremont Resort.

claremontwineglass

Movement on the fault has lifted the eastern (hills) side relative to the western (bay) side, which keeps Claremont Creek cutting down hard where it meets the fault. The result is that the mouth of the canyon is restricted to a narrow, steep-walled breach while the upper part of the canyon is free to spread out sideways as it erodes. This shape resembles the narrow stem and wide bowl of a wineglass, hence the name.

OK, what about the rocks? Here’s the geologic map with the photo localities shown. You see the Hayward fault cutting the lower left corner.

claremontcirclegeo

We start out (1) through an unexpected little exposure of the Leona rhyolite (Jsv), with the Chabot fault defining its eastern edge. Here’s the stone with its typical rusty tint . . .

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. . . and here’s the view north of the dramatic contact between the Leona and, on the right, mudstones of the Great Valley Sequence (Ku, for undivided Cretaceous rocks).

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Higher up (2), the sandstone and related rocks of the Great Valley Sequence appear in abundance. In Shephard Canyon and farther south, these rocks are subdivided into several formations.

claremontcircle3

The bedrock seems to support chapparal rather than forest, although maybe that’s only a function of the recent history of fire here.

claremontcircle4

On the horizon, left to right, we see bare 1684 Hill, Radio Tower Hill and the adjacent knob across Grizzly Peak Boulevard, and dark Round Top just peeking over the ridgeline.

Still higher (3) we can spot a fresh landslide running from a Grandview home down onto its neighbors.

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Up around 1300 feet (4) is this exposure of a fault that has pinched across these strongly bedded rocks. By now we’ve crossed a contact into much younger strata.

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And right around the corner appears the Claremont Chert in all its typical glory. This is directly above the formation’s type section along Claremont Boulevard.

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Achieving the top of the canyon (5) gets you the reward of one of Oakland’s finest views.

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Finally I got to my destination (6), the exposure of Claremont Chert at Radio Tower Hill. That’s where I put back the specimen I collected there some 10 years ago.
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If you manage to find it, you can be its next temporary owner.

The trudge back down will work your quads pretty hard, but you get nice views of where you’ve been (7).

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I hope it gets easier with repetition, because I want to come back.

Rocks of the Chabot Reservoir northside

30 November 2015

The hike on the Goldenrod Trail from the Grass Valley staging area, where Grass Valley Road meets Skyline Boulevard, down to Chabot Reservoir is a lovely walk. On the geologic map below, it’s the dirt road between the two O’s on the right side.

GrassVly-Chabotgeomap

When I walked here the first time, a few weeks ago, the idea was to check out the Franciscan Complex — shown as blue in the geologic map — where it crops out along the lake. On the east side is the Joaquin Miller Formation, which is a straightforward sandstone here.

GrsVly-JM-Fm

And on the west side is the Knoxville Formation, which is a straightforward shale here.

GrsVly-Knx-Fm

Nice rocks: brown, crumbly. Trees like the soils they make. They don’t stand out. In between is something completely different: blue and green metamorphic stuff. You’ll see it in boulder piles.

GrsVly-KJfrox

You’ll see it in knockers.

GrsVly-KJfknocker

And you’ll see it along the lakeshore. The other two formations leave plain old sand, which the birds seem to prefer, but the Franciscan gravel is worth looking at close up.

GrsVly-KJfbeach

The cool weather is a great time to explore this part of town, even if you don’t care about rocks.

GrsVly-Chabotres

The hill here is Fairmont Ridge — its forested back side. If you’re used to seeing it from anywhere else in Oakland, you won’t recognize it. And that’s what makes this a getaway.

Knowland Park: On geologic maps

21 September 2015

I enjoyed yesterday’s Wild Oakland outing in Knowland Park. It was hot, but a gradually freshening breeze made it a dry heat, and we spent a good bit of time in the shade for discussions.

My goal was to explore the relationship between the ground of the actual Earth, in its strange and opaque reality, and the maps that geologists make of it. Here’s the set of places we visited, in Google Earth — we started at the end of Snowdown Avenue.

Knowlandgeomapwalk

There were obvious things, like big outcrops, and subtle things, like the texture of the dirt in the roads, that yielded information to us about the rocks underlying various places as we walked. But that collection of observations wasn’t always easy to reconcile with the bold, definitive-looking patterns on the geologic map (800 pixels; see the same five localities on it).

Knowland-MF-2342

I tried to explain that a geologic map, especially one of Oakland, is as much an act of imagination as it is of observation. The rocks aren’t very well exposed, the different rock units are hard to describe and each one includes a lot of variety.

The Leona “rhyolite” (Jsv) and the Knoxville Formation (KJk), to the extent we could see them, were easy to distinguish after a bit of exposure to them. But the major rock unit we encountered, the melange of the Franciscan Complex (KJfm), is really a meta-rock unit, a mixture of blocks (“knockers” in the local geo-parlance) of very different lithology. It’s a meta-rock unit in the same way you might call a package of frozen mixed vegetables a meta-vegetable. So that’s not an easy concept to grasp, but I think the group enjoyed getting a taste of the subject.

Notice that almost all of the contacts between different rock units on the map are shown as bold, dashed lines. These all mark faults — fractures where the rocks on either side have been displaced — and none of them are visible on the ground. They are inferred. We’re sure they’re there because our knowledge of rocks in general, and these rocks in the Bay area, leads us to that conclusion.

That may seem like arm-waving, and it is. Geologists have a joke that the way to make us shut up is to tie our arms down. Geology, more than most branches of science, is a tentative discipline. Geologists hold that tentativeness close. Consider how this area was first mapped 100 years ago by Andrew Lawson, professor at Berkeley and highly regarded then and now. The excerpt below is from USGS Geologic Atlas Folio 193, published in 1914. (800 px)

Knowland-Lawson1914

It’s barely recognizable. We’ve learned a lot since then, but there’s no guarantee we know it all, and geologists of 2115 may have a sympathetic chuckle at our mixture of certainty and puzzlement today. Someone asked me what has changed in California geology since John McPhee wrote about it in Assembling California forty years ago, and I said the basics are still sound, but in some important topics our ideas have changed greatly. In the progress of geology there is no prospect of an end.


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