Archive for the ‘oakland rocks’ Category

MapView: Not ready for Oakland (updated)

11 March 2013

The U.S. Geological Survey’s programmers have made a nifty nationwide map server called MapView that lets you play with geologic maps from most of the country. Naturally I zoomed in on Oakland. I expected to see something like this when I looked at the Toler Heights area.

graymerbit

It’s kinda garish, but it’s from the USGS and as authoritative as these things can be. Instead, MapView shows me this:

dibbleebit

It’s crisp, it’s suitable for colorblind readers, but it’s wildly different. It shows simpler divisions and more limited areas of bedrock. It shows the active trace of the Hayward fault running on the opposite side of the hill from where it actually moves. Then there are things I notice: the names of the rock units have an antique feel and very few faults are mapped. And what’s with the ludicrously small landslide (“Qls”) and serpentinite pod (“sp”) in the middle? Why such a mixture of vagueness and precision?

In fact, this is not a USGS map at all, but a map issued by the Dibblee Foundation. Dibblee is the late and distinguished Tom Dibblee (1911–2004), popularly considered “the greatest geologic mapper who ever lived.” I consider him one of the greatest reconnaissance geologic mappers ever because that was his M.O.: to take his Jeep out to various high spots in poorly mapped territory and sketch out the bedrocks in the landscape onto a map base, then do field-checking until it was ready to publish. His skills were more than just fieldwork; he knew the literature and the community too, both scientific and industrial. I don’t have the talent to question his talent.

But. If you download the map and look at its sources, you’ll see that it’s based on Dibblee’s fieldwork in 1963 and a short return visit in 1977, plus three “preliminary maps” issued by the USGS, one of them in 1967 and all of them superseded by the map I use, Russ Graymer’s USGS Map MF-2342 published in 2000. I can only infer that this map was based on Dibblee’s old field notes as edited, posthumously, by John Minch in his role as official map editor for the Dibblee Foundation. I don’t question his talent, either, but it would be a major undertaking to update this map, one that has not been done.

What’s a few years, you might ask; the rocks never change. Well, consider that this map misplaces the Hayward fault. How do I know? It ignores the experience of Jim Lienkaemper’s meticulous mapping, which checks out in the field wherever I’ve looked. This is from his 1992 compilation of the fault trace and the supporting evidence.

lienkaemperbit

The Dibblee Foundation map is beautiful, but in this respect it is simply wrong. It didn’t go through the rigorous USGS review process; in fact I am confident that if it were submitted it would be rejected.

I’m going to ask the MapView people to reconsider using these maps in preference to USGS maps. For now I have to say that MapView is not curated to my standards.

UPDATE: The MapView administrator responded to me promptly and politely; I’ll excerpt his reply: “the point you raise has been a real concern . . . but [we] didn’t know the entire story nor had we been presented with a clear example of the problem, as you did in your blog. . . Our starting assumption is that newer maps supersede older mapping, and so unless there’s a compelling reason to not do so, we show the newer map. . . . The most effective way to improve upon what’s shown in MapView is for local and regional experts to weigh in with their opinion and experience, as you have done. I sincerely thank you for contacting us, and assure you that we’ll remove the Dibblee Foundation maps in all cases where there isn’t an older map of that scale that is ‘better’.” Translation: We pick maps by their release date and we won’t adjust that until someone squawks.

The other attractive feature of the Dibblee maps is that they’re standard 7.5-minute quadrangles, which makes things much easier for the MapView programmers. But ease of programming is not the same as usefulness.

Encroaching slopes

27 November 2012

crumbling slope

This is a block of Swainland Road, at the high end of the Glen Highlands neighborhood hard under the powerline ridge just south of Route 24. The bedrock is mapped as Great Valley Sequence sandstone. It’s a lot harder and stronger than much of the rock in the hills, but it still doesn’t like being exposed and oversteepened. It’s steadily encroaching on the sidewalk, trying to reach a more comfortable angle. Vegetation would probably help.

Rocks of the Caldecott Tunnel

23 November 2012

I mentioned a few posts back that I would be doing a field trip related to the new bore of the Caldecott Tunnel. Here’s how it went.

We (the members of the Northern California Geological Society) started with a talk at the Caltrans headquarters in Lafayette. They showed us historical photos and engineering diagrams, explained the New Austrian boring technique, and ascertained that we weren’t interested in a “photo opp” near the tunnel entrance. Apparently that last is a prime demand of many tour groups, but we didn’t care.

First stop was at the old tunnel entrance, which I’ve shown here before. The old plaque has been refurbished and looks garish now.

old tunnel

The tour leader unfurled a huge geologic map of the tunnel and talked a lot about all the rocks (click the image to see the 1000-pixel version). I studied it closely, because it’s the best we have for this part of town.

tunnel geologic map

The blue stripes on the west side of the map are the Sobrante Formation, the green, purple and tan stripes are the Clarement Shale, and the buff stripe on the right is the Orinda Formation. These are stacked up in order of age, old on the east, and tilted nearly straight upright.

Next we wound our way around to the extreme top of Broadway, next to the tunnel’s west entrance. Here’s the view from there looking north.

The rocks in the immediate area are in the putative Sobrante Formation, but the Claremont formation takes over just to the east (right). Here’s the Sobrante exposed at that spot.

sobrante formation

It’s pretty crummy rock, and the tunneling was quite slow there. Every couple of meters, the tunnelers drilled a fan of holes over the roof of the upcoming dig segment, each one reinforced and grouted. That reinforcement allowed the tunnel roof to stay up long enough to spray it with shotcrete, which in turn held up the roof until the real concrete tunnel lining could be emplaced. The tunnel’s final shaft is round, as strong as an eggshell in its resistance to the earth’s weight. (To see what I mean, try gripping a raw egg firmly in one hand, fingers on all sides, and attempt to crush it. You can’t do it.)

Next we visited the type locality of the Claremont Shale, where I learned something new about it.

claremont shale

The unit here is mostly thin beds of chert and thinner interbeds of shale, but there are also lumpy beds in it—see one running down the center and another one near the right edge. Those are dolomite. It turns out that carbonate and silicate minerals dislike each other enough that they tend to segregate themselves: in chalks, you get flint lumps, and in cherts (as here) you get limerock lumps.

Our last stop was on Fish Ranch Road just east of the tunnel, where we looked at the Orinda Formation in the surrounding hillsides. The Caltrans people brought along a box of the fossils recovered from the fourth-bore project, so I got to lay my hands on this Miocene horse tooth . . .

horse tooth fossil

. . . and this slab of fossil leaves.

leaf fossils

The Caltrans public-information official was at obvious pains to say how happy the paleontology contractors were and how well the fossil collecting had gone. Obvious to me, anyway. I won’t forget the complaints I heard from the other party, but I will keep in mind that most stories have at least two sides, and that every fossil preserved is a victory over the universal decay of the world.

Tunnel approach

31 October 2012

I’ll be taking a tour of the new Caldecott Tunnel bore on Friday. This is the view of Route 24 from the ridge south of it, across from Hiller Highlands. Everything here is east of the Hayward fault.

great valley

On the right is uppermost Broadway, where a line of locals and advanced commuters chronically hope for a few seconds’ advantage by merging at the last second before the tunnel. I’ve decided that it’s faster to get in the left lane of 24 as soon as possible and stay there. Anyway, all the land from here to the far curve is underlain by rocks of the Great Valley Complex, of Late Cretaceous age. A fault separates it from the Sobrante Formation behind it, which is much younger. The lower part of the skyline ridge is Sobrante, but the high part is chert of the Claremont Shale. The tunnel penetrates both of those units, and I hope for a good look at it.

I was standing by the power line; you get to it by hiking down the road from the sports complex, on Broadway at the overcrossing, or by ducking around the gate at the top of Pali Court. The Great Valley here is a mix of fine-grained sedimentary rocks. Exposures are poor and the fabric is disrupted. Here’s an exposure on Pali Court.

great valley

And here’s a closeup from nearby.

It’s very shaly. The lenses of more siliceous stuff don’t add to its strength.

Pershing knob

9 September 2012

Pershing Drive sits at the top of the south end of the Piedmont block at nearly its highest point, just under 800 feet. At one end of the road is a long exposure of rugged Franciscan chert that is exquisitely integrated into the landscaping, which includes native oaks as well as the usual plantings. It is a challenge to photograph but a delight to see in person.

chert

This is the high-grade, bleached and sometimes greenish banded chert of the Piedmont block rather than the beautiful red ribbon chert of the Marin Headlands. Its outcrop appears on the Oakland geologic map east of Piedmont’s Tyson Lake. The very steep slope on its bayward side is interpreted as a thrust fault separating the higher Franciscan melange from the lower Franciscan sandstone. Melange is not generally considered a strong stone by geotechnical engineers, but recent research shows that it depends. This melange is the best bedrock in Oakland, and worthy of its gracious and picturesque neighborhood.

Collecting the Orinda

4 July 2012

Lately I’ve been putting together a rock collection for the Chabot Space & Science Center. The last rock I needed was conglomerate, and I slapped my head and said “Duh! Use the Orinda Formation.” So it came to pass that I was on the Gudde Ridge roadcut, east of the Caldecott Tunnel, admiring that distinctive body of rock. But its exposures were off limits due to the construction.

orinda formation

There were two problems. Finding exposures of the conglomerate was difficult. And once I got close to the rock itself . . .

conglomerate

. . . this magnificent stone turned out to be rotten. You may think of conglomerate as a rugged rock, with all that grit and gravel in it. Indeed it can be. But the young, minimally processed conglomerate of the Orinda Formation doesn’t hold up to sun and weather very well. The pebbles work loose and the matrix turns crumbly in a matter of decades. The stuff in that impressive set of stairstep cuts is actually ready to give way, thudding dull under the hammer and totally useless for my purpose.

In connection with the Caldecott Tunnel work, the Fish Ranch Road offramp looks like it may have some fresh exposures. But it’s fenced off.

orinda formation

In the end, I had to find my conglomerate elsewhere. The fresh stone is beautiful, though, and being able to finish the collection made my day.

orinda formation specimen

Searching for the Sobrante

8 June 2012

A few weeks ago I went out looking for the Sobrante Formation, because it’s such a prominent rock unit in the Caldecott Tunnel. On the Oakland geologic map, it’s the tan unit labeled Tsm. This post features photos from the four numbered locations.

sobrante formation

The Sobrante is mudstone, laid down in a shallow marine setting during Miocene time. It varies between shale and fine-grained sandstone. The west half of the new Caldecott Tunnel bore goes through it, and paleo work there has found abundant fossils of fish scales. It’s not a competent (strong) rock, so you won’t see it outside of roadcuts. That’s what I went looking for.

I started in Claremont Canyon because the map shows the Sobrante just touching the road there. But between the Claremont chert . . .

claremont

and this unnamed coarse sandstone . . .

claremont

I couldn’t see anything beneath the vegetation. So I struck out there. When I checked out the winding part of Tunnel Road, though, I had better luck. There were several roadcuts that displayed the Sobrante, though not especially well.

sobrante

This is the mudstone close up, nicely laminated and fairly coherent.

sobrante mudstone

But the unit is prone to collapse, as those of you who bike here may remember. A big washout a few years ago has since been repaired.

sobrante slide

The third locality is on Thorndale Drive, a steep and narrow road heading down from north Elverton Drive. The rock is quite crumbly; indeed, the hillside may be moving. Here’s a closeup.

sobrante thorndale

And just a little lower is a large exposure where the rock spalls off so often that vegetation can’t get a foothold.

sobrante thorndale

If you can step out of the way of the shortcutting local drivers, this is a nice sheltered spot to study the unit.

But probably the most accessible roadcut is the fourth locality, along Skyline Boulevard between Snake and Shepherd Canyon roads.

sobrante skyline

This is the stuff I described a few years ago as punk shale.

The Sobrante continues southeast beyond the city line, a narrowing ribbon down Redwood Canyon. At the village of Canyon, the stream bed intersects the Sobrante and stays in it for a mile or so. It peters out in the Upper San Leandro watershed, and some day I’ll look for it down there.


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