Archive for the ‘Oakland conglomerate’ Category

Arroyo Viejito

6 January 2020

Some of Oakland’s most interesting land is also its most inaccessible; I’m speaking of our streambeds. And on the whole, the largest remaining stretches of wild streambed belong to Arroyo Viejo. Just to orient you, here’s the Arroyo Viejo watershed, as it’s mapped today by the Alameda County Flood Control District. The red stripe, which I added, represents the Hayward fault. (I’ll return to that.)

Here’s a zoom-in to the lower right corner, showing the upper part of Arroyo Viejo and the valley of a defunct little stream that I’m calling Arroyo Viejito.

The peculiar feature that caught my eye several years ago is how Arroyo Viejito runs parallel to Arroyo Viejo, very close to it, with a distinct rocky ridge between the two streams. Today the two valleys are very different, and a century’s worth of maps hints at what happened. Here’s the 1897 topo map showing the two streams, underneath the word “Viejo.”

In 1915, the area was more accurately mapped, and the two streams are shown as extremely close together at one point.

Everything changed after this. The country club was expanded and the adjoining land was subdivided and developed into the very exclusive Sequoyah district starting in the early 1920s. At that time Golf Links Road was pushed through to what would become the Grass Valley district in the 1950s, and Arroyo Viejito was diverted into the large stream at their closest approach and a sewer line inserted into the abandoned valley. It was very handy for the developers. As of 1947, the little stream had vanished and the land lay open for a new wave of luxury homes.

As of 1980 the buildout around Arroyo Viejito was complete.

The sewer line is accompanied by a maintenance road that is now a nice place for the locals to walk, and it connects with the little-visited creek trail at the north edge of the zoo’s property. I featured this area, in passing, three years ago in Ramble 3.

The reason these two streams ran so close together is related to the Hayward fault. It’s been dragging the lower, western half of Arroyo Viejo north, and for the last few hundred thousand years the stream has stretched out along the fault line before turning toward the Bay. Models of landscape evolution suggest that the headwater streams have been getting squeezed, aligning themselves and crowding together.

The combination of an especially large earthquake and a major flood could cause Arroyo Viejito to break through the narrow waist and join Arroyo Viejo farther upstream, abandoning the stretch with the sewer line and leaving the ridge standing there for a few more thousand years until it erodes away. But impatient developers have short-circuited all of that, and now the little stream is defunct, its former catchment part of a sterile golf course.

As I said, it was the ridge between the two streams that caught my eye and dared me to set foot on it. It’s in the middle of this Google Earth view looking west.

Its sides are very steep; it’s like an island. One day I found that it has a tiny trail running along its top, and signs of an old road and excavations. My guess is that the ridge was dug up for fill material when the sewer line was put in. The high-resolution lidar data acquired along the Hayward fault a few years back covers the west half of the ridge, and the resulting digital elevation model (with the trees and buildings stripped away) shows these features plainly.

Lately I’ve visited this ridge and the stream valleys of both Arroyos, in search of access and ultimately in search of rocks. Access beyond what I’ve already mentioned is difficult, and I have paid dearly for it in poison-oak rash. But I shall return.

The bedrock map looks like this, but I am suspicious of all of it given the difficulty of access and the paucity of outcrops. One big goal of mine has been to inspect the stream bed where bedrock might be exposed, for some real ground truth. I suspect that geologists, while doing their best, have resorted to drawing lines based on the topography.

The green zone marked KJk is shale and conglomerate of the Knoxville Formation, and that’s what I’ve always found in the eastern chunk of it. This shale is just west of Golf Links road where it crosses the creek.

And the conglomerate is abundant as loose boulders (not bedrock) downstream. It’s beautiful stuff.

But I have found none of it yet in the western section. Instead, everywhere I’ve looked the rock is either coarse sandstone shot with calcite veins, interpreted as the very oldest part of the Knoxville . . .

. . . or familiar rocks of the Leona volcanics (Jsv).

This includes up on the little ridge and down in the Arroyo Viejo streambed.

I still have a good bit of territory to visit, though. The streambed will have to wait until dry season, when I can poke around this weird-ass lime-cemented breccia.

And there’s more ridge to check out. Outcrops like this are so crusted with lichen that I might need to bring a rock hammer for some very careful, unobtrusive chipping.

There are some other charms in this northernmost stretch of Knowland Park. Every time I’ve visited there are fresh deer bones, indicating a mountain lion’s sphere of influence. And the cries of exotic animals occasionally drift down from the zoo’s hilltop center.

No other place in the world exactly like that.

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.

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.