Claremont Canyon conglomerate

18 March 2019

I haven’t mentioned it lately, but I’m writing a book on the geology of Oakland. One chapter is about a road trip that takes you to nearly every different rock unit in town — actually it’s pretty much the one I posted in 2017. One problem with that posted route is that it doesn’t pass an exposure of the Orinda Formation with its beautiful conglomerate.

The Orinda underlies the four-way intersection at the top of Claremont Canyon, but you can’t see any of it there. The intersection is in a topographic saddle, and it’s a saddle because the rock erodes more easily than the lava flows and chert beds that flank it on either side. That’s right, conglomerate — that rugged-looking stone studded with cobbles and boulders — is crummy rock.

So my road tour has to detour at this intersection, going down Claremont Boulevard a little ways to a spot where you can study the conglomerate at leisure. You’ll pass excellent exposures of it on the way, but there is literally no space to stand there.

The closeup of that rock that I posted here back in 2008 (still a favorite shot) was acquired at some peril. No, instead of stopping, you go around the hairpin turn and pull over at the entrance to UC Berkeley’s open space reserve. If you’ve followed along over the years, this is a familiar stop, the type locality of the Claremont Shale. Just uphill from that classic spot, the chert beds give way to Orinda conglomerate.

Look back across the road toward the head of Claremont Canyon, and that fire road (the Summit House Trail) is what you’ll take, just to where it enters the trees.

If you hurry, these daffodils will still be there to check your credentials and wave you through.

The uphill slope is dotted with big clean boulders. Some are showoffs . . .

and others are shy.

But all are worth a close look.

I ranged up the slope here, looking for proper outcrops of the conglomerate, but there are none to be found. It seems these boulders rolled down here the last time Claremont Boulevard was upgraded, and it’s pure serendipity they’re so nicely on display for you now.

The nearest thing to an outcrop is a spot where the fire road scrapes down to bare rock. It’s a clean fine-to-medium sandstone, also part of the Orinda Formation.

This finer-grained rock is where fossils are occasionally found, but I have to tell you I’ve never seen one myself. When the Caldecott tunnels were being bored through the hills, paleontologists were hired to collect fossils from the spoil piles. (See some of the Orinda Formation fossils here and here.) The fossils testify to a warm land of woods and year-round moisture, not much like today.

These boulders look like they’ll be around for a while. Pay them a visit some time in the next few decades.

But ultimately, rocks are perishable. Sooner or later they’ll make their way to the sea and be recycled in new rocks. The cobbles preserved in the Orinda conglomerate are all that remain of a whole landscape of rocky uplands that once existed here in Miocene times, some 11 million years ago. Not even mountains last forever.

Geology of King Estate Open Space

4 March 2019

After tramping all over Oakland, I still find its landscape full of uncertainty and mystery. The alluring hills of King Estate Open Space Park have brought me here time and again, sometimes to lead walks, sometimes to just stop and smell the flowers. Last month I came back yet again, this time to look harder at it.

The best resource on the park’s history and vision is on the Oak Knoll Neighborhood Improvement Association site at oknia.org. The aspirational Site Plan has the following concept for the park: “The winds sweep my imagination across the horizon. We move over the hills exploring the wilds and oaks embrace us. Here, in this place for everyday we cultivate community.”

King Estate Park is a grassy ridge at the south end of the Millsmont-Eastmont hills, an apparent pressure ridge that stretches along the Hayward fault from Mills College to the Oakland Zoo area, between Seminary Creek and Arroyo Viejo. What drew me here as a geologist was the geologic map (USGS MF-2342) that depicts the area as a peculiar ancient gravel (Qpoaf on the map), the largest piece of this material in Oakland.

The attractive thing is that according to the map key, these deposits “locally contain freshwater mollusks and extinct Pleistocene vertebrate fossils.” The odd thing is that they don’t match another criterion: that this dense gravel “can be related to modern stream courses.”

I explored the portion of the park north of Fontaine Street. Here’s the street map, marked with the three locations I’ll be showing photos of.

And here’s the aerial view, from Google Earth.

And just for fun, here’s the digital elevation model, with buildings and trees removed.

OK! Location 1 is on the steep western slope at the north end of the park, which I climbed. Halfway up is a sizable area of rock rubble consisting entirely of Leona volcanics, the same bedrock shown in pink labeled “Jsv” on the geologic map. The near-outcrop is in the lower left corner of this shot.

And the rock looks like this.

Down at location 2, there’s a spiral labyrinth that people have made in the last few years; I don’t remember seeing it before. But on the assumption that it’s made of stones from the immediate surroundings, I infer that it’s Leona volcanics over here too.

The Leona is generally light-colored with some greenish bits and some red-orange coatings where it’s weathered, hard to describe in detail but distinctive once you’re familiar with it. Once a range of underwater volcanoes that subsequently underwent a lot of alteration, it offers up a variety of intriguing bits that lacking a petrographic lab I can only scratch my head at.

Anyway I’m looking all over, and every bit of gravel on this hill is Leona volcanics. Now a river gravel, which is what the map describes and what I expected (indeed, what I actually perceived on previous visits!), consists of rounded clasts and a variety of rock types from the stream’s catchment. Other gravels in this town are just that way, but not this. The whole time I’m there I’m muttering to myself, “this stuff is colluvium” — raw rock rubble, mixed with soil, that hasn’t moved from its birthplace except maybe in landslides.

In location 3, we have proper bedrock. It show up where the soil has been scraped bare . . .

. . . and farther down the slope in genuine outcrops, which I always cherish.

So in sum, the whole north half of the park, far as I can tell, is either bedrock or colluvium of the same stuff: Leona volcanics. How did it get to be mapped as Pleistocene river gravel? The MF-2342 geologic map was published in 2000. There are two previous serious maps of this area. Dorothy Radbruch mapped it in the 1960s for map GQ-769, and there the area looks like this.

She called it “Qg”, “gravel, sand and clay” and noted that it contains pebbles of Leona Rhyolite (what I call Leona volcanics). She also said this: “Contains molluscs of probable early Pleistocene (Irvingtonian) age.” They were at the locality marked by the triangle with “22133” next to it; that number refers to a “report filed at Washington, D.C.” which is probably gathering dust deep in a back room. She also referred to reports on two boreholes, numbered 95 and 96 on the map, which recorded various kinds of gravel down as deep as 45 feet. That could’ve been deeply weathered Leona as easily as anything else.

But you know what? I’m going to go with Andrew Lawson’s original map of the area from 1914 (Folio 193), in which everything is just straight Leona.

Even though he thought the Leona was very young (hence the name “Tln” meaning Tertiary Leona), he could tell what the ground was saying. At least, he and I agree. I’m sure, though, that he scratched his head as much as Dorothy Radbruch and I did. And they must have enjoyed the view as much as me.

I’ll just have to poke around here some more.

A circumambulation of Shepherd Canyon

18 February 2019

I seem to give myself odd assignments. The latest one was to take a hike around the crest of Shephard Creek’s watershed, better known as Shepherd Canyon. Only during the final mile of that 6-mile walk did I realize what it meant: an outing exclusively on ridge roads.

I got the idea from my circumambulation of Claremont Canyon three years ago. That walk features a variety of rock types and a good set of views into Oakland’s best wineglass canyon — a stream valley with a wide upper watershed and a narrow outlet where it crosses an active fault. Most of Oakland’s major streams that cross the Hayward fault have wineglass valleys that are more or less well-formed (and faceted spurs between them), but you may have to scrutinize them pretty hard to see them, let alone walk their rims.

Shepherd Canyon is another of our wineglass canyons where the roads make this exercise feasible — and an exercise it is, with a thousand-foot elevation gain. The trouble is, you don’t get clear views into the canyon itself. Whereas Claremont Canyon is a viewshed as well as a watershed, Shepherd Canyon’s watershed has an inside-out viewshed. Thanks to our high parklands, this ridgetop walk affords good views around the canyon’s outer side.

Enough preamble — let’s amble. The route starts and ends in Montclair on the geologically friendly 33 bus line. Here’s the layout.

Those little red blips are mile markers. I took this counterclockwise because I prefer steep uphills to steep downhills, and the south end is steep. A bicyclist might prefer the other way, but parts of this route are footpaths.

You could start and end this walk at the end of the 33 line in front of the Safeway, but I started from the Snake Road stop and finished at the LaSalle Avenue stop. Most of the first mile is a bit of a chore along busy Mountain Boulevard, but these days the rain has made the creek noisy, and you’ll hear it down in the woods as you cross the lowest point near the Park Avenue junction.

Be sure and get off the road on this nice path.

It goes around the flattened hilltop housing the Joaquin Miller Elementary School and Montera Middle School.

There once was a rocky hill here, occupied by a Scout camp (which is how Scout Road got its name), but it was vacated and leveled as a handy source of stone and used (I believe) as fill for the Warren Freeway nearby. In any case, the path takes you to the foot of Shepherd Canyon’s perimeter ridge, where you’ll turn right up Mountaingate Way. This view past the foot of Mountaingate looks into the valley of Cobbledick Creek, a tributary of Shephard Creek that runs down along Scout Road. In the next mile you’ll climb 800 feet, pretty steadily.

Soon enough you join Castle Drive and start seeing rocks. Now it’s time for the bedrock map.

The ridge is made up of serpentine rock (sp on the map) plus its typical blocks of blueschist. One of the city’s best blueschist boulders sits where Castle Drive starts, and some of the homes along Castle use the blueschist in their landscaping.

The serpentinite is a sloppier, greener stone that you’ll see along the roadside. Let me warn you that hiking on Castle is a challenge — the road is narrow, there’s no sidewalk, and nobody expects pedestrians. Keep your ears open and step off the road as cars approach.

Where Castle starts to turn north, veer off the road and take the little-traveled West Trail, part of Joaquin Miller Park. You’ll thank me, as I said when I took you down it a few months ago. It’s here that your first views open up.

And the rocks change as you start on the trail. From here on out you’ll be mostly on sandstone. Just for the record, you’ll pass through the Joaquin Miller Formation (Kjm), Oakland Conglomerate (not labeled), Shephard Creek Formation (Ksc), Redwood Canyon Formation (Kr), unnamed Eocene mudstone (Tes), Sobrante Formation mudstone (Tsm) and just into the edge of the Claremont Shale before descending through the same units on the way down.

Castle Road ends at Skyline Boulevard. After a few steps on Skyline, duck into the woods and take the Scout Trail parallel to Skyline, where you’ll meet these guys at the top of the ridge — Oakland Conglomerate.

Where the trail ends at Moon Gate, take Skyline for a few hundred yards, passing the Waterloo Staging Area (unless you’re up for a stroll on the West Ridge Trail, which rejoins this route at Skyline Gate), and take Wilton Drive up onto the ridgetop. It skirts the edge of Redwood Regional Park and has one of Oakland’s best views.

Move on and take Burton Drive, then Shirley Drive. At the end of Shirley is an unmapped trail through Redwood Park that starts with this bench, nestled among boulders of Redwood Canyon Formation sandstone.

Not quite the halfway mark, but just the place to take a rest and a good look around.

The trail descends to the Skyline Gate Staging Area. From there, take Skyline to its confusing junction with Shepherd Canyon/Pinehurst Roads. Don’t take any of those roads — go up Manzanita Drive instead, along the ridgetop. Where the power line crosses the ridge is a fine exposure of the Sobrante Formation’s shale.

This is an unusually good exposure of the Sobrante, which is very sloppy stuff along Skyline, because up here it’s grading into the harder chert of the Claremont Shale.

The two units are stratigraphically continuous, with an arbitrary boundary between them. You’ll see a lot of this rock along the road as you make your way north, then down to the junction of Skyline and Snake Road.

At this spot you overlook Shepherd Canyon on the left, Thornhill Canyon on the right, and the mighty Bay in front as you start down. Do stop at the fire station here and top up your water supply.

The descent along the lip of Shepherd Canyon is straightforward: Take Colton Boulevard to Asilomar Drive to Drake Drive. Again, pedestrians are not expected, but the roads are generally wide enough to accommodate everyone without jostling or stir. Here and there you can see into the canyon. Watch its walls grow close as you come near the narrow part of the wineglass.

When Drake meets Magellan (I know, those two explorers never actually met), the ridge road is finished. It only remains to take the unobtrusive path leading through the trees from 2133 Magellan down to the Railroad Trail — when else will you ever have the chance? — and on into Montclair to catch the bus.

The nice thing about this time of year is that many of the trees are leafless, so you can see more of the surroundings.

That and the weather is cool. I always seem to assign myself weird walks in the high hills during summer, which can be brutal. Better to get out there now.

Oakland buildings clad in stones

4 February 2019

One of the cool things about Oakland is its huge variety of buildings, dating from midcentury — mid-19th century, that is — to brand-new. There’s a particular variety of midcentury-20th-century architecture that’s piqued my geological interest: boxy tilt-up structures that I think of as Old Period Safeway, with a cladding of natural rock. In 2017 I showed an example of the type clad in beautiful serpentinite, so I set out to find others around town.

The sharp-eyed folks in the Oakland History Facebook group pointed me to several candidates, and I took an afternoon to visit them and also cruise much of the main roads by Zipcar. Yesterday afternoon I took one more little outing to capture the distinctive building of KP Market on Telegraph Avenue, the best pan-Asian grocery around.

The stone cladding covers only part of the front, and besides it’s painted over. To see it pristine you have to go around the side and look along the base of the wall, where taggers never venture (and thus paintovers aren’t needed).

This is the same coarse-grained dolomite rock used on the iconic Kaiser Center building.

Now for some genuine Old Period Safeways, but first a lament for rocks that get painted over, which permanently spoils them. The former Safeway on San Pablo at 27th Street, for instance, has lost its original appearance beyond retrieval. Without the colors and lusters of natural rock, the walls are unappealing.

But the former Safeway on upper Golf Links Road, in the Grass Valley neighborhood, is unspoilt.

Close up, it resolves as a mixed metamorphic assemblage of river rock, perhaps from the northern Coast Range but the presence of granite amid the sandstone says Salinian instead; that is, from the Santa Lucia or Gabilan ranges around Salinas. I don’t know for sure, it’s just fun to guess.

Another former Safeway, on 14th Avenue at E. 17th Street, still has its stones out.

This wide-ranging river-rock assemblage is darker and more rounded, and granite is absent while lava is present. I peg it as coming from the Diablo Range, but I warn you I could be totally wrong.

There’s one more former Safeway, a dialysis center on Claremont at Clifton, whose cladding still looks good in Google Streetmap but I haven’t visited it yet. The rest of our old Safeways, and there were about 20 listed in the 1969 phone book, are either gone or have been renovated or didn’t use the rock cladding.

One more supermarket and I’ll move on: the Cardenas grocery on High Street.

The cladding on the front side is painted, but on the side you can see it’s an excellent quarry-crushed golden quartzite mixed with lava and what might be travertine or sinter. I’m thinking the northern Sierra here, but that’s a long way to ship crushed rock, even fancy stuff.

Couple more notable buildings, non-Safeways, to show you. Next to the Post Office building off Hegenberger is a low-slung office building, 8475 Pardee Drive.

It’s clad with a selection of river rock I don’t hesitate to call Franciscan sandstone, maybe from Sonoma County.

Finally there’s the Community School for Creative Education building, on International at 21st Avenue. Something about the whimsical accents and the warm, even-textured cladding makes just the right impression.

This stuff is straight crushed quartzite that’s gotta be from some special place in the Sierra foothills, if not farther afield.

Material of this quality can last for centuries, if well maintained.

One cladding type I didn’t seek out for this post is pretty common: big flat hunks of undressed sandstone — that is, flagstone. I have no idea where the stone might be quarried. Some well-known Oakland buildings use it:

But it can be overdone, bringing to mind another fashion, a late-century one: shag carpet.

My knowledge of architecture and the building trades is pretty scant, so if you know the correct terminology for these features, please add an informative comment.