Oakland geology ramble 8: Piedmont Ridge

19 August 2019

I don’t always care about rocks; geology is about more than rocks. I don’t even always care about geology; sometimes I just want a vigorous, geographically arbitrary hike. Ramble 8 is one of those — a traverse of the ridge crest above Piedmont, our highest ground west of the Hayward fault. It goes from the Rockridge BART station to the 33 line bus stop at the Leimert Bridge, on the lip of Dimond Canyon, about five miles end to end. Nevertheless, it has geology and rocks along the way. Here’s the route. There’s a map with more detail at the end of the post, where I also have some announcements.

The BART station features the “Rockridge” destination signage, mounted above a selection of Sierran boulders. This time, some ten years after it was installed, I noticed that the lettering design is quirky, contrasting “rock” and “ridge.”

You could climb Keith Avenue and barge up upper Broadway to attain the heights at Contra Costa Road, but it’s more interesting to circle behind the College Preparatory School grounds on Brookside Avenue. The school nestles in a steep little stream valley — unlike many similar places, this is not a former quarry but, apparently, a former turn-of-the-last-century park that had a short existence.

Once you get up to Contra Costa Road via Eustice and Buena Vista avenues, the walking is pretty and level. While you’re here in this remote part of town, check out the blueschist outcrop at 6063, vacant since the 1991 fire. The entire ridge on this hike is in Franciscan melange, a mudstone matrix containing odd lumps of other rock types. You won’t see much of it.

At the very end of the street is Erba Path, a steep set of stairs down to the saddle in the ridge where Broadway Terrace peaks on its way past the south entrance to Lake Temescal and points east. Cross that busy road and head right back up again on Sheridan Road, visible here at upper left.

Turn right off Sheridan at Agnes Street, unless you want to explore the little-trod path that joins Sheridan’s two halves. (Everyone should at least once.) Take high-flying Cochrane Avenue, where views east of Thornhill Canyon and Glen Highland’s settled slopes beckon.

Then jog right again up the saddle of Florence Avenue (where I sent you in the last post) to Proctor Avenue. This is pretty steep, but short. By this point you’re well above 700 feet. You’ll go higher later, but first you clamber a hundred feet down to the next saddle where Moraga Avenue crosses the ridge. Take the well-marked pedestrian crossing, but beware — this is the most dangerous road crossing of the hike.

Proceed on Estates Drive, which climbs nearly to the 800 foot contour. Up here are two curious reservoirs that date from Oakland’s water wars, when private water companies struggled to supply the fast-growing region from local sources while they vied with each other in deadly capitalist strife. (A ruinous series of bankruptcies and mergers ended in the 1920s with the formation of EBMUD.)

The Dingee and Estates reservoirs were constructed, fast and furiously, at the highest point of the Piedmont hills to provide good water pressure. EBMUD is upgrading these old concrete bathtubs to proper steel tanks, built to withstand big earthquakes on the Hayward fault just a few hundred yards east.

From here you head down again to La Salle Avenue, which takes advantage of yet another saddle in the ridge. If you’re ready to quit at this point, go left on La Salle, left again on Bruns Court and cross that high pedestrian bridge over the Warren Freeway to Montclair Park (because you can!) and catch the 33 bus at La Salle and Moraga. Otherwise, cross La Salle and stay on Estates, which is a little to the right.

Estates climbs again, not so far this time. Because this walk hits the highest spots, take Dawes Street up the hill and over, where you simply must visit the south end of Pershing Drive and admire Oakland’s best outcrop of Franciscan chert.

From here on it’s all downhill. Dawes rejoins Estates Drive here, and as you start down Estates you can see across Dimond Canyon.

Geologically and geomorphically speaking, the other side is also part of Piedmont Ridge, but the large water gap of Dimond Canyon is impassable without a long detour. So, down you go to the bus stop at the Leimert Bridge.

Along the way are two more highlights. First, at the Piedmont line the road passes the head of the former Diamond Cañon Quarry, which today houses the Zion Lutheran Church. Recent foundation work here has exposed fresh rock; maybe you’ll see some too.

And second, enjoy this wonderful volcanic breccia used for the landscaping at 170 Estates Drive. There are whole walls of it.

The house itself is something to see, too.

And as promised, here’s the detailed route map (1126 X 1126 pixels), followed by some announcements.

The excellent, out-of-print book Stories in Stone: Travels Through Urban Geology by David B. Williams is being reissued in paperback by the University of Washington Press. If I didn’t have the hardcover already I’d buy this classic. More information on David’s website.

My own book manuscript is making the rounds of a publisher, and while I await a yea or nay I’m trying to get my arms around a whole lot of scientific literature pertaining to Oakland’s rocks. It’s a ridge walk of the intellect, but I want to make Chapter 5 as good as humanly possible. I hope to buttonhole some of the real experts next month at the Geological Society of America’s annual meeting in Phoenix.

But first I’ll be giving a talk to the Friends of Sausal Creek, on 18 September at the Dimond Library, titled “Sausal Creek: The Last Million Years or So.” You read it here first (unless you follow me on Twitter, @aboutgeology), and I’ll repeat the announcement in the Q&A thread soon.

Posted in Franciscan rocks, Oakland geology walks, Other topics | 2 Comments »

Oakland’s wild rail path

5 August 2019

The seasons are changing now, if you follow the pagan calendar. This weekend marks the turning point between astronomical pagan summer (6 May to 6 August) and pagan autumn (6 August to 6 November), or as I think of them, High Season and Waning. They are offset exactly half a season from the conventional astronomical seasons. High Season consists of long days, and Waning consists of shortening days. (Likewise, Low Season consists of short days, and Quickening consists of lengthening days.)

Nature is acutely aware of these seasons. The belladonna lily (Amaryllis belladonna) sends up its naked-lady flowers at this time. The strawberry tree (Arbutus unedo) ripens its rich little fruits (I can understand why Pliny the Elder named them “eat-only-one” because they’re so satisfying).

And of course the blackberries are in full swing.

I returned last week after eight years to the “secret street” at the south end of Florence Avenue, where it meets the old railbed of the Sacramento Northern Railway (also known as the Oakland, Antioch, and Eastern Railway). Unlike that first visit, when I was busy and could only gaze up the path, this time I had the leisure to walk its whole length, up to Broadway Terrace where it’s fenced off.

The path has geology up at its north end, but it’s worthy just as woods. Even right next to the Warren Freeway, it’s as secluded as any place in Oakland.

It’s shown as the dashed route on this map. You can see that Florence Avenue, heading over a saddle in the ridge above Piedmont, used to connect with Florence Terrace once upon a time. That’s the Lake Temescal park at the top.

There are lots of blackberries growing here, so don’t wait. The first ones are the best. Near the north end is a landslide scar that was repaired with much labor to protect some homes on Sheridan Road. The work was finished with dark shotcrete, but it doesn’t really blend in.

If you look close you’ll see little splotches of white. Those mark cracks where lime-bearing groundwater has seeped through and deposited calcite as it evaporates.

I can foresee these growing into falls of travertine in a few years. Beyond the landslide is a high cut into the hillside, made decades ago when the rail line was first pushed through. And the bedrock here is mapped as classic Franciscan melange, the big blue field on the geologic map — the edge of which happens to correspond to the Hayward fault.

I half expected the rock exposed here to be fault gouge, the fine-ground, mealy stuff that fills many of California’s active faults (for instance at the London Road slide). It’s real close to it: highly weathered mudstone that’s likely to come down hard in our next big quake. Whether the railbed will be cleared again afterward can only be conjectured. I’ll look at this cut again more thoroughly next time I’m here, whenever that might be.

On your way back, look again for blackberries. I know I didn’t get them all.

Posted in Franciscan rocks, Oakland geology walks, The Hayward fault | 2 Comments »

Oakland geology ramble 7: South Orinda to Montclair

22 July 2019

It’s been a while since I brought you a geology ramble — a no-car-needed hike that starts and ends at different places with public transit. Ramble number 7 connects Montclair and Orinda over the spine of the Oakland Hills. Three years ago, when I presented ramble #2 from Rockridge to Orinda, I said “I have a vague scheme for a southern route,” and this is that scheme fleshed out and walked both ways.

Here’s the 6.4-mile route superimposed on Google Maps. It climbs from about 600 feet elevation at either end to 1600 feet, in Sibley Regional Volcanic Preserve.

The route starts in Orinda, for reasons I’ll get to later. From the Orinda BART station, take the County Connection number 6 bus and get off at Woodland Road, across from St. John Orthodox Church. The first mile is a gentle downhill into the valley of Moraga Creek, on Woodland and then Valley View Drive. You’ll pass this hillside, which exposes lava of the Moraga Formation.

If you have the time and energy, this is an interesting exposure, but there’s a long walk ahead.

The road turns into Lost Valley Road as it turns right. Follow it to Edgewood Road. At the end of Edgewood is this gate leading to the open space of the Orinda Wilder development.

Go on in. The road straight ahead leads to the new homes of Orinda Wilder, but you’ll turn left, up the hill.

Two miles farther and 800 vertical feet up, there’s another gate at the boundary of Sibley. On the far side of the gate is a sign saying “private property.” That’s the sum total of the access restrictions along this ramble.

The full route takes you through 12 different geological map units, by my count.

They are the Mulholland Formation, Moraga Formation basalt (Tmb) and interbedded sedimentary rocks (Tms), the Siesta Formation (Tst), Orinda Formation (Tor), Claremont Shale (the golden stripe from corner to corner), Sobrante Formation (Tsm), the unnamed Eocene mudstone (Tes), the teeny, cryptic Paleocene sandstone (Ta), Redwood Canyon Formation (Kr), Shephard Creek Formation (Ksc), and the Oakland Conglomerate (green).

What particularly interested me about this hike was the stretch through the Siesta Formation and the Moraga Formation sediments, which aren’t exposed in Oakland proper.

The Siesta consists of fine-grained stuff: sand, silt, clay and some limestone. The limestone is what stands out, because it’s white.

Outcrops of the limestone aren’t obvious, but my little acid bottle always reveals it by the telltale fizz.

This large limestone boulder reveals a lot of broken-up structure, including some shale chunks mixed in. Landslides will do this to unlithified sediments — and around here, even 9-10 million years ago, that means earthquakes.

The Siesta also includes a little conglomerate, as seen in this roadside exposure, but in general the rock isn’t highly visible. It likes to turn back into dirt.

Farther up the road, the Moraga sediments show themselves as coarse sandstone, rough stuff that gathered around whenever the lavas weren’t erupting.

That little magenta bit brings me to the other scenery: it’s always more than rocks that brings me to these heights. In early summer it’s wildflowers.

And views.

And views of rocks.

By the time you’re on the ridgetop, you’re well into the volcanic rocks that Sibley is famous for. Look carefully and you may spot the mineral-filled lava bubbles called amygdules. (I left this one for you to find.)

The high point in Sibley is the midpoint of this ramble. It’s all downhill from here to Montclair (where you catch the “geologist’s express” 33 bus), heading south on Skyline and then following the route of my Shepherd Canyon circumambulation. Be sure to look back at that lovely ridge as you start down Skyline.

I’ll end this post with a big fat map showing the topographic contours, more road details, and mile markers.

Posted in Oakland geology walks | 2 Comments »

Stones of the Broadway Valdez District

8 July 2019

The Broadway Valdez District is a swath of territory, named by city planners, centered on Broadway and extending from 23rd Street up to I-580. The area is undergoing redevelopment at a furious pace. For my purposes, the area will give birth to a showcase in the use of building stone in current architecture. Here’s the first chapter in that story.

Gone are the days when stone was a load-bearing structural material in Oakland buildings. (A few of our older churches survive from that time.) Gone are the days, even, when stone was fashionable for large-scale cladding (see: Kaiser Center’s dolomite and Lake Merritt Plaza’s granite). Today, at this moment, stone is a color and textural accent framing the offset planes of metal and matte components by which today’s designers react to the bland slabs and faceless facings of yesteryear.

First out the gate in the race to develop the Valdez Triangle (framed by Broadway, 27th and 23rd Streets) is the Hanover Company’s two big residential properties, Hanover Northgate and Hanover Broadway.

Hanover Northgate stretches the length of Valdez between 24th and 26th, and its exterior has two different treatments. Here’s the south end.

The part on street level is faced with stone. The blond stuff, which echoes the top level, is an interesting fossiliferous marlstone. And I mean, fossiliferous.

Marlstone is not your traditional white or gray limestone; it owes its hues to clay minerals mixed with the carbonate minerals that define lime rocks. The variegated texture and color is integral to its charm. Notice among the cross-sections of fossil bivalves is a little brown gastropod shell.

Beneath this, at foot level, is a dark gray marlstone.

No obvious fossils here, but there are veins of calcite to lend visual interest. Carbon, not carbonate, provides the black color. Such a rock forms in anaerobic settings, where organic carbon is reduced to elemental carbon rather than being oxidized to form carbonates or CO2 gas.

The other end of the Hanover Northgate has a different palette: a warm buff-gray over a crystalline black footing.

In between the two parts, the carport features this rugged paving. However, it’s just dark-tinted concrete — cheaper than stone, just as tough and more controllable.

On to the real stone: a nicely polished laminated limestone. This panel displays two little faults cutting vertically through it; the middle part has been displaced upward. There’s also a thin vein, the result of a later fracture.

Interestingly, all the panels I took a close look at appeared to be placed stratigraphically upside-down.

One of the first things a geologist looks for is evidence of stratigraphic up and down. After seeing a few hundred outcrops, doing that becomes automatic, especially in the Oakland hills where many of the steeply tilted rocks are overturned past vertical. Sometimes the evidence can be quite subtle. The most obvious sign in this example is at bottom center, where the thin beds sag into the gaps in the thick bed. This rock — in this panel anyway — accumulated in the downward direction.

If I’m right, then the stone supplier made sure to mark this stock with arrows so it would be set in a consistent direction. Although the result might confuse geologists, it would avoid giving observers of the walls, even ordinary folks, a vague sense of disorder.

The dark rock at the base of the wall is “black granite,” better known to geologists as something like amphibolite or gabbro. It has a sandblasted finish for a more matte appearance.

Between the two Hanover buildings is a lovely plaza, open to the public and graced with stainless-steel sculptures by HYBYCOZO: a stylized oak tree and eight California poppies.

Not much stone is in evidence — the plaza is mostly tile and concrete and metal. But the benches are topped with solid black basalt.

Which leads us to the Hanover Broadway building, soon to have a Target store on the ground floor.

There’s not much stone on this building either. Like “Whistler’s Mother,” it’s a study in black and gray. Here’s an odd irregular panel on the side facing the plaza: brecciated limestone.

Here’s a closer look at this intricate material. (All images click to 800 pixels, as is my usual practice.)

And around the side in an entryway are some panels of gneiss, complete with quartz veins. The squashed-and-squeezed veins feature what are called ptygmatic folds, because geologists have come up with the damnedest words for things.

These stones, all of them, are things no human artist or artisan can duplicate. Though it may go in and out of fashion, stone will never go out of style.

Posted in Oakland stone | 3 Comments »

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