Archive for the ‘Oakland geology views’ 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).

claremontcircle2

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.

claremontcircle5

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.

claremontcircle6

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.
claremontcircle9

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).

claremontcircle10

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

Vollmer Peak and the Bald Peak Basalt

11 April 2016

At 1905 feet above sea level, Vollmer Peak is the highest point on San Pablo Ridge and in the Berkeley/Oakland Hills. It doesn’t stand out from below — you know it by the two widely separated towers on it, to the right of Grizzly Peak — but it sure stands out when you’re on it. Here’s a view of Grizzly Peak from its upper flank.

griz-from-vollmer

Vollmer Peak used to be known as Bald Peak, which accounts for the name of the rock unit that holds it up. The Bald Peak Basalt is the youngest volcanic rock in the Oakland area, the well-defined reddish blob in the geologic map labeled Tbp (for Tertiary Bald Peak).

vollmer-geomap

I haven’t seen a lot of this rock, but it’s described as “massive basalt flows.” Here’s an example from nearby Chaparral Peak. Notice the dark color and the light-brown weathering rind, both of which are typical.

BPbasaltfromchap-peak

Its age, about 8.4 million years, is distinctly younger than the lava flows you’ll see at Sibley Volcanic Reserve, which are part of the Moraga Formation (Tmb) and date from 9 to 10 million years ago. The two volcanic units are separated by sedimentary rocks of the Siesta Formation, and apparently the Bald Peak and Siesta interfinger with each other in outcrops in the upper part of the Siesta Valley.

It’s nice and quiet up there, and the wildflowers are in progress. The peak used to be unforested, like all the high hills, and it has remnant populations of many different meadow plants.

Here’s a shot overlooking Briones Reservoir on a moist day. In clear weather the Sierra Nevada takes precedence.

vollmerviewE

And here’s the view southeast. There’s a lot to point out in it.

vollmerviewSE

Left to right on the horizon, we see the flank of Mount Diablo, the twin humps of Las Trampas Ridge and Rocky Ridge with Bollinger Canyon between them, the Diablo Range hills beyond the Livermore Valley, and Round Top on Gudde Ridge. The green valley in the middle is Wilder Valley (or Gateway Valley) in Orinda, now being developed. It’s the counterpart of Siesta Valley on the north side of route 24, which we can’t see because we aren’t high enough. The tree-studded hilltop in the middle is Eureka Peak.

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.

SLslideface

The top of the slide displays some pretty rotten stone.

SLslidescarp

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.

SL-faultvalley

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.


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