Archive for the ‘Oakland blueschist’ Category

Lake Chabot’s north shore

23 December 2019

Oakland has a lot of ground to cover, and it can take a while for me to return to places I’ve been before. In this case, it’s been four years since I traversed the trails on the north side of Lake Chabot, between the reservoir and the golf course.

The land is much the same (though I’ll point out some differences). It’s my frame of reference that’s changed.

Here’s the Google Earth view of the area. My walk started near the dam and went up the trail (the Bass Cove Trail) along the west edge of the lake, then back down on the unmarked, unsigned trail running just west of all the chaparral.

The latter trail is basically an access road for the power line that runs through here over the hills. The land ownership is mixed, but there are no barriers.

For reference, here’s the bedrock map of the same area. (It also shows the access road snaking along the power line.)

Jsv is the Leona volcanics, KJk the Knoxville Formation, and the slightly darker green on the right is the Joaquin Miller Formation. The blue section labeled KJfm is mapped as Franciscan melange. I’ll show you some of these. What’s changed in my frame of reference is that I’ve learned there are conflicting interpretations of that blue section. I found no smoking-gun evidence in my visit, so you’ll be spared the details in this post, but my eyes were peeled in a way they weren’t before.

I like a lot of things about this time of year. For purposes of geologizing, the footing is firm and quiet even off the trail. Also, the rain has washed the outcrops clean.

Of course, rocks like this are still covered with lichens so you can’t be quite sure what they are. The best-exposed rocks are in the streambeds, like this blueschist boulder.

All three of these are in the melange unit. For purposes of my enlightenment, it’s the matrix between these blocks that’s crucial, but none of that was visible. Just have to keep looking.

The sandstone of the Joaquin Miller Formation is nicely exposed in the rain-washed roadways.

It’s a pretty pure sandstone; there are spots in the trail where rainwater has washed the eroding stone downhill into sandy drifts.

Sand is nearly eternal. It can be recycled time and again in the rock cycle for hundreds of millions of years. But that’s another post.

And off the trail I was pleased to find some excellent examples of the conglomerate at the base of the Knoxville Formation, which is otherwise mostly shale.

The rounded cobbles in this exposure are largely composed of the Leona volcanics, proof of the genetic link between the two bodies of rock and a clue to the geography of ancient California during the Late Jurassic.

Enough bedrock. Other things I like about this time of year are that it’s cool, the air is clean and fragrant, the colors are distinctive and the light is interesting. It’s a primo time, if you ask me, to walk the high hills, and this part of town offers good views of Fairmont Ridge and the lake.

Even a peek at the Hayward hills and Mission Peak beyond.

The parks district has been visibly sprucing things up in the park. And along the power line, it’s obvious that PG&E has been at work too, taking the fire risk seriously by reducing the fuel load.

They’ll probably shred this plant material and leave it on the ground, but I would favor some good old-fashioned controlled burning here in the fire-friendly chaparral. They’re even making a little headway against the eucalyptus, which besides its fire hazard tends to shed limbs. Eucs make fine specimen trees, like the one across the way, but lousy forests when they aren’t well tended — take a look at Australia these days.

Lake Chabot and the surrounding parks are a special part of Oakland. Get yourself out there; let your mind roam free.

Even at the bottom of the year, there’s a lot of good light.

The Dunsmuir-Chabot trail

1 October 2018

The most remote part of Oakland will be opened to public access fairly soon, when the East Bay Regional Parks District finally gets around to constructing a trail between Dunsmuir Ridge and Lake Chabot. I made my way into part of the route a few weeks back. It’s interesting and inviting territory, set above an untouched oak-filled stream valley with Fairmont Ridge beyond.

The land has divided ownership, with parcels belonging to the city, EBMUD and the East Bay Regional Parks District. They’re all public agencies, so the bureaucracy must have been difficult. Here’s the setting, as laid out in a 2009 EBRPD map.

The solid blue line is a trail in Anthony Chabot Regional Park that’s been closed for years. The dashed blue line is the proposed Dunsmuir Heights to Chabot Regional Trail.

This summer the EBRPD board was shown this map of the approximate route. Again, the solid line is an existing (deteriorated) roadway and the dashed part will be built from scratch. The photo at the top of this post is the view from the EBMUD water tank at top center. The part I’ll be showing is between there and the “P” mark at the city golf course.

A couple more maps to help you see what’s here. First is the bare land as shown in Google Maps terrain view.

The centerpiece of the trail’s route is the valley in the lower right quadrant. Note its depth and steepness. The permanent stream in that valley has no formal name, so I will hereby dub it Chabot Creek. And finally here’s the same area in the 1947 topographic map, which shows the old roads that will become the trail.

If you look at the upper part of Chabot Creek valley, you’ll see it turns sharply from southeast to southwest as you go downhill. On the Google map, though, the streamcourse is interrupted by a flat area. That’s landfill made of waste from the Cypress Viaduct, which collapsed in the 1989 earthquake. It’s sterile and weedy, but the view is nice.

At its edge is a curious structure, visible in Google Earth, that turns out to be a spillway, made for the event of a large rainstorm during a very wet winter. Presumably the landfill was capped with a layer of clay to stop any contaminants from leaching into Chabot Creek, and thus the site would fill with rainwater quickly and have a risk of spillover.

If you looked closely at the second map, you saw the intriguing pointers to an “old foundation” and a “1936 WPA rock chimney.” The chimney is a massive stone fireplace, suitable for a hunting lodge. But the building it once occupied is gone. Foundations around it show that it was a group facility of some kind. I’m hoping that local historians can say more about it in the comments.

A stone in the entryway is carved with the date 1935, so the map is slightly in error.

In any case, the stonework is indeed classic WPA masonry, of the same vintage, material and durability as the Woodminster Cascades in Joaquin Miller Park.

And speaking of stone, what are the rocks like around here, you ask. Here’s what’s mapped in the area.

“Jsv” is the Leona volcanics, “KJk” is the sedimentary Knoxville Formation, and the blue field is Franciscan melange, the same body of rock underlying Knowland Park. When I visited, I checked out a roadcut right where the Franciscan and Knoxville meet and found an assortment of rocks.

All of these are appropriate for the Franciscan, but the brown sandstone could just as easily be from the Knoxville. A return visit is in order during the upcoming wet season, when the ground is firmer. I hope to see more signs of the Franciscan “knockers” so well exposed in Knowland Park — and on the golf course, like this blueschist knocker cropping out in the rough.

The plans for the Dunsmuir Heights to Chabot Trail are supposed to come up for public comment this fall, with the work to be completed by 2021.

Franciscan landscaping

9 July 2018

This house in Piedmont caught my eye not long ago. Homeowners who live in conspicuous places do their neighborhoods a service by making their properties shine. I appreciated the care the owners of this home displayed not just in their plantings, but also in their choice of rocks.

The site (110 Scenic Avenue) is in the middle of the block of Franciscan sandstone that underlies most of Piedmont and some adjacent parts of Oakland. The massive sandstone, of an unobtrusive tan color and undistinguished structure, makes a serviceable setting for some of the Franciscan’s other, more colorful rock types.

The exposure of bedrock is discreetly patched with concrete, which may well conceal rock bolts set into the hillside. The section of concrete on the right side, below, is surfaced with the same blue serpentinite found at Elks Peak in Mountain View Cemetery, the old pit at Serpentine Prairie, around Butters Canyon, and elsewhere.

There are several basins built onto the exposure. The bluish high-grade metamorphic river rock is carefully chosen, too. It comes from outside the Bay area, most likely somewhere on either flank of the Sacramento Valley.

And just beneath it is this little jewel of high-grade blueschist.

Of course a geologist’s first focus is on the stone. But the true beauty of a yard like this is how the rocks converse with the plants set among them over the course of a California year. I’ll be back to see that.

Museum-quality rocks from Oakland

30 January 2017

I keep saying that Oakland has geological features worthy of being put in textbooks. Today I’m here to show you that Oakland has rocks worthy of being in museums, and I’ve put them there.

In 2012, I was asked to put together a set of teaching rocks for the Chabot Space and Science Center. After all, other planets are made of rocks, right? It took some doing, but some of the rocks were easily available within Oakland’s borders in roadside exposures. The conglomerate of the Orinda Formation was one.

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The red chert from the Franciscan Complex was another.

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And of course there was our serpentinite.

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All told, I made five sets of 15 rock types for the kids.

The next year I got a request from Las Positas College, in Livermore, for a boulder of blueschist. Turns out this little college teaches geology, because every citizen will benefit from a course, and students can get a head start on a 4-year degree there. I struggled one out of this streambed, where it wouldn’t be missed.

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They installed it in their teaching garden as Rock J, on the left. It’s small compared to its mates, but that thing weighs a ton because high-grade blueschist is pretty dense.

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My reward included a visit backstage to see their cool collections.

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Then last year, I got a note out of the blue from the under-construction Maine Mineral and Gem Museum asking my help in building their collection. Maine is well known for its gemstone and mineral mines, but the state has no blueschist. I went to a quiet outcrop where it’s just lying around.

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Got two nice boulders and couldn’t choose between them, so I sent them both. They told me one will go on display and the other will go in their teaching collection.

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None of these are precious collectibles or gemstones. They’re just cool and educational.

I’ve pretty much stopped collecting rocks for myself because I’m not important enough. But museums are important enough.