Archive for the ‘Oakland rocks’ 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 either the MacArthur or the 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.

Conglomerate in the upper Arroyo Viejo streambed

9 May 2016

Oakland has three different bodies of conglomerate: from youngest to oldest they’re in the Orinda Formation (of Miocene age, maybe 10 million years old), the Oakland Conglomerate (Late Cretaceous, maybe 80 Ma) and the Knoxville Formation (Late Jurassic, about 165 Ma). Recently I’ve been getting fixated on the last one. I hope these photos from upper Arroyo Viejo will help you understand why.

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Above I-580, Arroyo Viejo runs along the road as you drive up Golf Links Road and into the woods of Knowland Park. Halfway through, it takes a sneaky dogleg to the left, then another left turn through this tunnel under Elysian Fields Road. Beyond is the stream’s source, now in the Sequoyah Country Club golf course.

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Between those two left turns, the stream valley exposes the Knoxville Formation. I haven’t walked the whole section yet, just visited both ends. The lower end has wonderful conglomerate outcrops.

High on the valley walls, the rock is heavily vegetated. It gets that way because between the pebbles and cobbles of the conglomerate, the matrix is a fine-grained sediment that supplies nutrients to plants and offers space to their roots.

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As you get close to it, the conglomerate reveals the abundant well-rounded cobbles beneath its green coat. Geologists like their rocks clean, but nature prefers them this way. You have to admire them.

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For the best exposures, look down into the streambed.

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A little bit downstream is a section of creek choked with boulders of this stuff, some as big as sofas, that I showed you a few months ago. Those monster rocks weren’t carved out of the streambed by the creek. Instead, they rolled down the valley walls, which are very steep (35 to 40 degrees) as most are in Oakland.

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The valley does have some landslide scars, so we know that those happen. But mostly I blame earthquakes.

Both events provoke the stream into washing the boulders away. Landslides create instant dams, which build up the water pressure in the lakes that pond above them. When the stream bursts through, usually in a matter of days, it makes short work of the rock pile. Earthquakes, for their part, give the whole underground a shakedown and cause a weeks-long surge of water afterward that likewise gives the boulders a good head start.

In between these disturbances, maybe once in a century, rare cloudbursts pour enough water into the watershed to roll the biggest rocks downstream and grind down the streambed an inch or so at a time.

Given enough time — and geology always provides that — the boulders break down into pebbles and clay and wash out to sea, eventually to become new rocks. The Knoxville Formation conglomerate has waited some 165 million years to start that journey. The Earth is almost thirty times older.

Rocks and views of Fairmont Ridge

2 May 2016

Fairmont Ridge is the grassy upland that forms the backdrop to San Leandro. As it happens, East Bay Regional Park District owns much of it as part of Lake Chabot Regional Park. It has some rocks, which I’ll show first, and also some fine views.

Here’s the aerial view of the ridge from Google.

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And the geology of the same area is here.

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We’ll look at rocks from three different units: the green area is underlain by the Knoxville Formation, a shaly sedimentary unit; the light-brown area labeled Jpb is basaltic lava; and the pink area labeled Jsv is Leona rhyolite, which you’re familiar with by now from Oakland.

The Knoxville is well exposed around Lake Chabot. Here, to the east of the access road at locality 1, it appears to be strongly sheared, suggesting that its contact with the structurally underlying Leona and basalt is a fault. This view is facing north, parallel to the contact.

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The basalt unit is formally described as pillow basalt, the kind of balloon-shaped flows you’d find where lava erupts beneath seawater. But these rocks have been shoved around a lot since they were erupted in Late Jurassic time, and I have yet to see decent pillow morphology in any exposures. Still, the outcrops, like this one at locality 2, are picturesque.

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The windbreak of giant, mature eucalyptus is visible in the photo. This is a naturally breezy park, and the line of trees offers some welcome shelter.

Across the ridge on the Bay side, there are more outcrops of the basalt. Around locality 3 it’s well displayed.

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If you pay attention, you’ll see bits of this rock with polished surfaces, or slickensides, on them. These are caused by motion on faults, which rubs rocks against each other. Here and there, proper outcrops enable us to see that the faults are oriented vertically and parallel to the ridgeline. I interpret these as forming recently as these rocks were folded and tilted upright by motion related to the Hayward fault.

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The peak of Fairmont Ridge is fenced off, but an informal trail leads north along the east side of the fence to locality 4. (Poison oak will very soon make it impassible.) That’s where this typical specimen of Leona rhyolite was.

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But sometimes rocks are just rocks. Lift up your eyes from the hills and sit a spell. You can gaze upon the Bay side . . .

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. . . or over the reservoir toward Mount Diablo.

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Looking due east is a nice prospect of the ridge known as The Knife, overlooking San Ramon.

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Its high point is named Wiedemann Hill, elevation 1854 feet, and I have a growing fixation with it.