Archive for the ‘Franciscan rocks’ Category

Bedrock in the headwaters of Trestle Glen Creek

13 November 2017

Surprisingly, the town of Piedmont has its share of woodland trails — well they’re paved sidewalks, but they’re unused, covered with duff and overhung with untended shrubbery. On a weekday afternoon, you can walk quietly on miles of these soft paths and encounter only a handful of property workers. It’s in that spirit that I recommend a geologizing stroll around the highest part of Trestle Glen Creek’s eastern watershed.

The shaded-relief map below shows the creek valley in eastern Piedmont, with Dimond Canyon on the right edge. The area I’m featuring is the triangle just right of center bounded by Crest Road, Pershing and Estates Drives and Hampton Road.

And here’s a closeup just to display the street names. During weekdays, the 33 bus stops at Lexford and Hampton, where the two valleys in this little watershed join.

The geologic map shows that this area is solidly within the block of Franciscan sandstone (Kfn) that underlies most of Piedmont. The hilltop above it consists of Franciscan melange (KJfm) that includes bodies of chert (fc), notably the one on Pershing that I’ve called the best bedrock in Oakland.

The neighborhood is gracious. This view looks up Huntleigh Road, which runs on the valley floor. As I traversed the streets, I used sidewalks that almost never feel a human foot. At times it was easy to imagine being in a Tolkien novel.

Lexford Road, in its own valley, is more secluded and more whimsical architecturally.

For the geologist, these streets are valuable because they aren’t as tightly landscaped as in most of Piedmont, and the Franciscan bedrock can be seen and studied at leisure in several places where the road builders exposed it. That’s unusual for this town.

Plenty of hand specimens are available too, if that rings your chimes.

There are even a few empty lots here. Unlike the existing homesites, these are especially challenging due to the steepness of the terrain and the strength of the rock — not just on the surface, where the weathered sandstone has fractured into rubble, but also deeper down where foundations would need to be dug into the hard, unweathered bedrock. When this lot was cleared recently, it had shed enough rubble to nearly cover the sidewalk.

A large house has been proposed here for many years, and the record of the intricate wrangling needed to invite and address everyone’s concerns is mind-numbing. However, the record does include geotechnical reports that give us a glimpse underground.

What’s on the site now is this set of what are called story poles, which serve to outline the planned building.

Geologists acquire a certain ability to see the ground through everything growing or sitting on it. It’s an ability to visualize the landscape as if it were covered with story poles instead of vegetation and structures. This bit of watershed is a good field site to practice.

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Mountain View Cemetery: The Bay area’s best landscape

1 May 2017

Although I’m tempted just to let the photos in this post stand on their own, let me make a case that Mountain View Cemetery offers the best landscape in the Bay area.

First there’s the cemetery itself. The managers have been putting a lot of effort into improving the ground — see the excellent new stonework and gravel path in the first photo — and this winter’s abundant rainfall has abetted it by giving the hills a coat of green that ought to last longer than usual before turning gold, then brown.

Unlike your typical cemetery, Mountain View is very large and occupies the rolling terrain of the Piedmont block, consisting of Franciscan melange. In the photo below, all of the land in sight lies within the cemetery’s property.

Long ago the operators arranged for Cemetery Creek (headwaters of Glen Echo Creek) to fill three ponds, where the water can be parceled out over the dry season to help keep the turf lush. Right now they’re brim full and support a few waterfowl. The open hilltop on the left side will be turned into a new section of graves under a proposed plan.

The Franciscan outcrops or “knockers” in the cemetery’s hills echo the finished stone displayed so touchingly in the grave markers. Many historical Oaklanders famous and obscure rest here. A random walk in any direction will bring up names that ring a bell if you’ve spent significant time in the East Bay. Yes, that’s what cemeteries are for, but without the graves this territory would be just another busy park. The dead ensure that the living visitors stay on their best behavior.

And this is important — the delights of the cemetery don’t stop at its edge. The east side has had its huge hedge of overgrown eucalyptus removed, opening the crest of the high hills and the well-tended neighborhoods of Broadway Terrace to view.

A few eucalyptus trees remain on the hilltop hosting the cemetery’s high staging area. In manageable numbers, their trunks are attractive as they frame views of tempting places.

With the view east restored, there’s now a postcard vista in every direction you look. To the west you can see the Golden Gate, San Francisco, St. Mary’s Cemetery and Oakland’s outer harbor.

To the southwest is downtown Oakland and the ridges of the San Mateo Peninsula. These views, as well as those to the north and south, will always be unspoiled. From Mountain View, one can take in surroundings that encompass a large share of the greater Bay area from the midst of a setting that’s both attractive and historic.

So that’s my main case for this being the Bay area’s best landscape. But there’s more — there are rocks. I always make sure to visit this outcrop of red-brown radiolarian chert on the hillside behind the garden mausoleum, plot 3.

Other parts of the cemetery consist of shale, like this bit left behind from an excavation in plot 9.

The road up to the top of the cemetery exposes some of the well-bedded mudstone that underlies much of the grounds, but look in the gutter for the freshest exposure.

And once up there, make your way bayward from the northern tip of plot 77 to this outcrop of green and red chert.

I’m glad to entertain arguments that one place or another might be superior to our cemetery. For sheer viewshed, Mount Diablo is a candidate, as is Tamalpais. Twin Peaks in San Francisco gives excellent views of Oakland. Mount Livermore, on Angel Island, is worth a special mention. The South Bay and North Bay have many more picturesque places, not to mention the Peninsula. Lots of these spots survey a more spacious territory, but Mountain View surveys the most gracious territory, a viewshed of singular integrity that extends from infinity to your feet. In a region full of landscapes, this one offers as much elegance as it does grandeur.

The Idaho connection

13 February 2017

I’ve been getting into the weeds as I work on my book manuscript about Oakland’s geology (tentative title, Deeper Oakland). Where did Oakland’s rocks come from? Specifically, how did they get from where they formed to where they are? This problem is particularly vexing for the older rocks with Mesozoic ages. The western edge of North America has been built, unbuilt, shifted, rebuilt and disassembled for hundreds of millions of years.

Generally the pieces have been carried northward by the vagaries of plate tectonics. Rocks that were once Californian now sit as far north as Alaska, and likewise rocks that live here now come from as far south as Mexico.

The rocks in lower Shepherd Canyon (the Shephard Creek and Redwood Canyon Formations) belong to the huge set of sandstones and related rocks underlying the Central Valley — the Great Valley Group — but are separated from them. How they broke off and how they wandered to where they sit today are, as we say, poorly constrained. One clue may be within the sand itself.

Last year a paper in Geology laid out an intricate case that the sediment making up one part of the Great Valley Group was eroded from an ancient set of rocks in Idaho, the Lemhi subbasin of the Belt Supergroup. Papers dealing with the assembly of the North American Cordillera usually have gnarly figures, because the story is so complex, and this one, the product of an all-California team of geologists led by Stanford’s Trevor Dumitru, was no exception.

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The whole scenario is based on microscopic grains of zircon, which lurk in many sandstones because the mineral is extremely durable. Luckily, we can determine the age of zircon grains because they’re superbly suited for the gold standard of dating techniques, the uranium-lead method. Think of them like pennies with dates on them.

So there’s a big body of rock in the Lemhi subbasin full of a unique combination of zircon ages, including a bunch around 1380 million years old. At one point during Cretaceous time, around 80 million years ago, a mountain-building episode pushed these rocks into a knot of high peaks, which eroded into sand that was carried by rivers in at least four directions. Dumitru and his coauthors duly gave these hypothetical ancient rivers names, because that’s one of the perks of doing historical geology.

Sandstones containing Lemhi zircons, with their telltale 1380 Ma peak, are found in Wyoming, Oregon, Washington and California. In the Bay Area, the paper identifies them in sandstone from Del Puerto Canyon, west of Patterson in the Central Valley. Apparently a huge pulse of Lemhi sand poured down the “Kione River” and filled the whole sedimentary basin for a while. (Basins keep sinking as they fill, because the sediment load depresses the crust.) That sandstone is mapped as the Kione Formation, a portion of the Great Valley Group that’s been considered mysterious because the sand clearly didn’t come from its usual source, the nearby (ancestral) Sierra Nevada and Klamath Range.

The point of all this is that the Oakland rocks I’m talking about date from this same period! If only we could get a zircon researcher to check them out, we might learn a little something. I mean, Dumitru dated rocks from Albany Hill, Stinson Beach, Bolinas Ridge and Sutro Baths (localities H, G, F and E in the inset map) among other places. He dated rocks from the Novato Quarry terrane of the Franciscan complex, the same unit our own Franciscan rocks belong to. He dated rocks from the Sierra de Salinas Schist, down near Monterey.

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I love it; it has its own cool story. So why not visit Oakland too? I guess rocks on this side of the Hayward fault aren’t as important for the bigger story. But you never know until you find out, right?

By the way, I will be speaking at the upcoming East Bay Nerd Nite, Monday the 27th; watch its Facebook page or website for details. The topic is, “Are Rocks People?”

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.