Archive for the ‘Sausal Creek watershed’ 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.

Fallen trees

30 May 2016

Naturally, the people of a city named Oakland cherish their trees.

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But trees are living things, and every one must die at some point. After years of drought plus one wet winter, I’ve noticed a lot of downed trees. This one was in Leona Heights, an oak. When oaks topple, that’s the end of them.

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This one, a bay laurel, was in Joaquin Miller Park. Fallen bay trees commonly reroot, which is a handy thing in their preferred moist habitat.

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Fallen trees may interest field geologists because as their roots rip out, they expose the bedrock in areas with thick vegetation. Thus information from “tree throws” can help in mapping the rocks. For instance, I inspected the rocks uprooted by this tree . . .

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. . . and noted that they were typical mudstone of the Joaquin Miller Formation. The park is the formation’s type area, after all.

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Another treefall farther downstream exposed serpentinite. (The two very different rock types are separated by the Chabot fault.)

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Sometimes data points like these are the only way to get a handle on steep, vegetated terrain.

Research-type geologists consider “tree throw” a significant actor in the evolution of wooded landscapes (here’s a taste from a recent conference).

Down by the water, especially along the smaller creeks, fallen bay laurels often span the stream and root on both sides. Sometimes it’s charming, like here above Dunsmuir House, but it’s usually a hassle if you’re trying to walk along the streambed.

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To hydrologists — geologists who specialize in streams — fallen trees are a crucial part of stream systems. Natural streams always contain some amount of dead wood, because it lasts for years before it rots away or burns up.

Wood doesn’t act like sand and gravel. It doesn’t wash away easily, it sticks around and snags stuff. So making a realistic model of stream behavior requires a sophisticated understanding of how this “wood load” affects the water. Hydrology journals publish a steady trickle of papers on this topic.

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Most of us would rather that trees stayed standing and streambeds stayed clear. That only happens in landscaped parks. To know nature, you have to learn to appreciate snags.

Fruit Vale

27 June 2013

The valley of Sausal Creek below Dimond Canyon made a natural site for orchards: a nice flat floodplain with decent soil and a permanent stream off on the western side. Also, the valley is straight to a degree that strikes me as unusual, which is handy for laying out blocks of land. It may or may not have been filled with oaks—I have a copy of an old print titled “Oaks of Oakland” that purports to be from this area. In any case it has a classic shape with a flat floor and steep sides formed by the Oakland alluvial fan (the Fan). I’ve shown the high, landslide-prone western side before; here’s the eastern side. This is the view from the Fruitvale freeway exit looking up Harold Street, where the valley wall is pretty dramatic.

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Farther down, the valley wall fades away well before you get to Foothill Boulevard, which everywhere marks the edge of the Fan. Here at Fruitvale Boulevard and Bona Street, the valley wall is already lower and more subdued.

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It looks like I’ll name this lobe of the Fan the Patten lobe. The valley of Peralta Creek is just over the hill. It’s interesting to speculate why the Peraltas put their rancho buildings there rather than here.


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