Archive for the ‘the hayward fault’ 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 . . .

claremontcircle1

. . . 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.

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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.

A reconnaissance of San Leandro geology

14 March 2016

San Leandro is a much smaller city than Oakland, but it has its share of interesting rocks and features. ‘Twas a cloudy day when I visited, but the worst day geologizing is better than the best day working. Here’s the geologic map with the photo locations numbered on it.

SLgabbrogeomap

The purple area marked Jgb is underlain by the San Leandro Gabbro, of Jurassic age, a crystalline rock similar to granite that belongs to the Coast Range Ophiolite. It’s about 160 million years old and was once a deep-seated part of the oceanic crust. Unfortunately the color, while it follows the official U.S. government geologic color guidelines for Mesozoic plutonic rocks, makes the map hard to read. The blob of brown marked Jpb represents Jurassic pillow basalt, which I thought would be very interesting to see. And the solid black line down the middle of the map is the Hayward fault — it’s solid black because the fault is very well mapped there.

The San Leandro Rock Quarry has been closed for a few years.

SL-quarry

The land is for sale — 58 acres of it, right on the Hayward fault — but I didn’t feel up to impersonating a possible buyer, so it was off limits. But the view the other way is pretty cool, overlooking the gorge of San Leandro Creek below the Chabot Reservoir dam. It’s the biggest canyon between Niles Canyon and Wildcat Canyon and pretty intimidating.

SLCreekcanyon

A little ways west on Lake Chabot Road, where it meets Astor Drive, is a saddle in the hillside where the Hayward fault crosses the road. A steep gulch descends to the north along the fault trace. To the south, the Bay-O-Vista Swim and Tennis Club has nestled on the fault unscathed for almost 60 years.

SL-bayovistaclub

We’ll visit the fault on the other side of the club. But first, the gabbro! It’s exposed in various places in the Bay-o-Vista neighborhood, where it’s mostly shattered from being next to the fault for millions of years.

SLGabbro-exposure

Gabbro is made up mostly of dark pyroxene and light plagioclase feldspar. Like granite, it likes to weather into decent soil. The excavations of residential areas are helpful in bringing it into view. And up close, this gabbro is pretty.

SLGabbro-specimen

Studies of this area using airborne gravity meters and magnetic instruments suggest that this gabbro extends well north and south of here in a big slab about 3 kilometers thick lying between the Hayward and Chabot faults, tilted almost straight up and down. This figure is from a 2003 study led by Dave Ponce of the U.S. Geological Survey.

SLgabbroprofile

In Oakland, the gabbro shows up in stringers and blobs along the fault as far north as Chimes Creek. I’ve picked up pieces on Eastmont hill by the reservoir. But the geophysical study suggests that it underlies a much larger area as far as Merritt College, beneath the surface rocks.

The gabbro is strong enough that it bends the Hayward fault slightly off course. But during the 1997-98 El Niño, a big hunk of hillside gave way just below the place where I shot the outcrop. Two homes were lost.

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The top of the slide displays some pretty rotten stone.

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Farther south, Fairmont Road swings around the county juvenile justice center past the Hayward fault. This is looking north from there up the fault trace.

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Our active faults grind up the rocks so fine that they’re easily eroded into gulches, gullies and valleys, and that’s what this one is. It last ruptured on October 21, 1868, so any trace of that is long gone. It takes careful trenching studies to find it. We’ll have to wait until the next big one to see where it decides to rip up the ground.

Oakland panorama

29 February 2016

Even though it’s called Leap Day, February 29 feels like a good day to stop and look around. Here’s a fine panorama from 10 February 2015, the first day I explored the Ridgemont neighborhood high above the old Leona Quarry (1200 px). The view is almost due south.

ridgemont-pano

Most of the California landscape is oriented like this, northwest-to-southeast, thanks to its plate-tectonic position between the North America and Pacific plates.

What are we looking at? Here’s an annotated version, with the following landmarks noted.

ridgemont-pano-anno

1: Mission Peak, overlooking Fremont and San Jose.

2: Knowland Park hill with its green meadows.

3: Fairmont Ridge, overlooking San Leandro.

4: Loma Prieta.

5: Mount Umunhum and the spine of the Sierra Azul.

6: Coyote Hills on this side of the bay, Stevens Creek canyon on the other, where route 17 leads to Santa Cruz.

7: King Estates Open Space, looking its emerald best.

8: Black Mountain overlooking Mountain View (and hey what a coincidence).

All of these are great places to visit.

The red Vs mark the Hayward fault, left to right: at the foot of Mission Peak, in the saddle of Fairmont Ridge, in the Oakland Zoo and running down the valley of Arroyo Viejo past Holy Redeemer College.


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