Archive for the ‘Oakland geology walks’ Category

Oakland geology ramble 1, Leimert to Redwood

6 June 2016

For a while now I’ve been envisioning geological rambles around Oakland — walks (hikes, really) that aren’t loops, but traverses. They rely on public transit, because that’s mainly how I roll. You can walk them in either direction. My ultimate idea is to work out a network of rambles that will cover the whole town. You could combine them into epic outings. This is the first ramble. It’s a little more than 4 miles.

The west end of the route is on Park Boulevard at the Leimert Bridge. The 18 bus line will get you there from the MacArthur or either downtown BART station. Starting elevation is ~375 feet. Here’s the street route (1000 pixels):

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And here’s the corresponding geologic map:

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Briefly, the route takes you past Franciscan sandstone of the Piedmont block (Kfn), then crosses the Hayward fault into much older mixed rocks of the Coast Range ophiolite (basalt (Jb), serpentinite (sp), Leona “rhyolite” volcaniclastics (Jsv)) and a bit of Late Jurassic mudstone of the Knoxville Formation (KJk). (Search this site for more about all those rocks.) Remember to leave the stone alone.

Oakmore Hill looks pretty intimidating as you cross the Leimert Bridge. Part of that is because of Dimond Canyon below. The bridge is about 125 feet above Sausal Creek.

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Buy your fuel and water in the charming little Oakmore commercial district. Then make your way to Braemar Street along the top of the hill. Take any route you like. The intersection of Arcadia and Melvin, directly above the E in “Oakmore,” is a good shady spot to regroup and refresh.

Along the way you’ll see exposures of the sandstone.

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Braemar Street is nice and level. Truck along right to the end and enter the footpath like you do it all the time. As you descend the steps, look across the fault-line valley to the bare slopes of Joaquin Miller Park. That’s where you’re headed.

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Closer to hand, you’ll see that the rock has changed. This appears to be the Jurassic basalt, unit Jb.

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On the way to the freeway overcrossing at Lincoln Avenue, look at the lay of the land.

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The active trace of the Hayward fault isn’t precisely mapped here, but it runs from about the lower middle edge on the left side to the horizon directly behind the large tree (note the LDS Temple spire on the right edge). The next time the ground breaks, you’ll see it very clearly here.

Cross the freeway and take Woodminster Lane to Woodside Glen Court, where the road ends at a backdoor entrance to Joaquin Miller Park at about 700 feet elevation. Things get pretty steep here, and they’ll stay steep.

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The exposure appears to be either Leona rhyolite or Franciscan sandstone; the important thing is that the bedrock changes abruptly as you enter the woods into the area mapped as serpentinite.

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Specifically, this is blueschist, the electrifying high-grade metamorphic rock that’s intimately mixed with greenish serpentine rock throughout this map unit. Enjoy the trail, which is the little-traveled west end of the Sinawik Trail, as you puff your way up to about 950 feet at Lookout Point. Stop a bit and check out the high-grade boulders there. (You’ll want to stop anyway.)

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This is where I show the route forking. It’s easier to go right, either on the trail or on Sanborn Road, going downhill to Joaquin Miller Road and across it to Butters Drive. I took the high route, up what I call Visionary Ridge, because I was returning two pieces of basalt to the locality where I got them. I thought better of that plan as I passed the park’s native plant nursery, where I added them to the little border at the bottom of this photo.

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The hillsides here are pure serpentinite and worth a close look. The high route continues along the ridge crest, around 1100 feet, to Joaquin Miller Road, where you cross and take Robinson Drive to where it meets Butters Drive at about 1025 feet. The high route will save you a loss and gain of 200 feet, but you’ll miss Butters Drive.

Butters Drive starts in some of Oakland’s most spectacular serpentine/blueschist ground, and it’s landscaped too. (See more photos from a 2015 visit here.)

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Continue past the hairpin turn into the headwaters of Peralta Creek in the Butters Canyon private preserve. Here the rock along the road is mapped as Leona rhyolite.

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The intriguing thing is that right across the creek the rock is Knoxville Formation, a unit that’s generally shale and hence easily eroded. I think this contact is exploited by the creek to dig the canyon so locally deep. You can get a good look at the Knoxville right above the intersection with Robinson Drive, where the high and low routes meet again.

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Now the route plunges about 300 feet down Crestmont Drive and through Oakland’s largest area of serpentinite. Take in the prodigious exposure at Crestmont and Kimberlin Heights drives.

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The west edge of the serpentinite zone is a thrust fault, which means the rock here is quite pulverized. This part of the hike has several interesting exposures that I’ll let you discover on your own. The very easternmost end of Crestmont Drive goes through Leona rhyolite, which you’ll see in boulders.

When you reach Redwood Road, truck on downhill to Campus Drive at about 650 feet elevation, where the 54 bus comes by regularly. It’ll take you to the Fruitvale BART station or connect you to major lines on MacArthur, Foothill or International boulevards.

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.

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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.

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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.

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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.

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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.

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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.

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.

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

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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.

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You’ll see it in knockers.

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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.

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

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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.


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