Archive for the ‘Oakland conglomerate’ Category

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.

Oakland geology ramble 6: Chabot to Leimert via the Oakland Conglomerate reference locality

11 June 2018

In 1914, UC Berkeley professor Andrew Lawson published the first decent geologic map of our area, the San Francisco Folio of the Geologic Atlas of the United States. That’s where the Oakland Conglomerate got its name. But while that fine rock unit lives on in name, the concept it represents has changed. Let’s look at a hundred years of progress in geologic mapping the area just west of Redwood Peak, the heart of the Oakland Conglomerate.

Here’s Lawson’s map from 1914. Orient yourself by finding Redwood Peak. The belt of rocks labeled “Ko” is the Oakland Conglomerate, the road winding along it is called Skyline Boulevard today, and the road heading leftward from it is today’s Castle Drive.

Lawson called Ko the “Oakland conglomerate member of the Chico formation,” a name and classification by which he meant that these rocks were notable but not important enough in the big scheme of things to single out. To be fair, he was dealing with a very large, detailed and cryptic field area at the time.

Fifty years later, along came James Case, who picked this area for his Ph.D. research at UC Berkeley. Lawson, who died in 1952, was no longer around to intimidate his graduate students, so Case was free to argue that Lawson had erred, having lumped too many different rocks in Ko. Some of them were really KJk, the Knoxville Formation, and others Case put in his brand-new Kjm, the Joaquin Miller Formation. (This is similar to what I was saying the other week about the Orinda Formation.) “It is therefore proposed,” he wrote, “that conglomeratic beds exposed along Skyline Boulevard west and northwest of Redwood Peak be considered the reference locality of the restricted Oakland Conglomerate.”

Case’s thesis was approved in 1963. His map, published a few years later in USGS Bulletin 1251-J, is so badly reproduced it’s embarrassing. Instead I’ll show Dorothy Radbruch-Hall’s elegant map of 1969, which incorporated Case’s work.

And for completeness’ sake here’s the current standard map, by Russ Graymer from 2000.

It’s a cool area. I recommend visiting it the way I did: take the 339 bus up to Chabot Observatory and hike down to Montclair or the Leimert Bridge to catch the 33 bus back downtown.

First, go behind the observatory buildings and find the fire road, which is part of the West Ridge Trail in Redwood Regional Park.

The trail exposes the conglomerate beautifully. The cobbles are exceptionally well rounded, a sign that they once tumbled a long way down a steep river into the sea.

You could meander your way to Moon Gate, on Skyline, where you’ll take the trail going left, or you could follow the steep and tempting path up to the water tank. From there the views look north past a wooded hill, with Round Top peeking up behind it, to the Briones Hills . . .

or northwest toward Grizzly and Vollmer Peaks.

A tiny trail on the other side will get you to some proper outcrops of massive sandstone, also part of the Oakland Conglomerate.

Either way, you’ll then be on the Scout Trail heading south, then angling east down to Skyline. Along the way you’ll pass a young and vigorous redwood stand, planted in 1978 thanks to good old Jerry Brown.

Cross Skyline and take the Castle Park Trail west. It’s a lot safer than walking on Skyline, and a nicer walk. When you hit Castle Drive, take the pavement down to the secret fire road called the West Trail.

This is the original century-old road shown on Lawson’s map. Go on, you’ll thank me later.

There’s a particular kind of peace to be found on abandoned roads.

Once the trail ends, back at Castle Drive, you can pick your own best way down, an exercise left to the reader. This was my way — 3.5 miles long, 1100 feet downhill. . .

through the redwoods and across Leimert Bridge.

How useful is the Orinda Formation?

30 April 2018

Walking along the paved trail north of Inspiration Point, I was brought up short by a splendid outcrop of conglomerate.

It’s strongly reminiscent of the Orinda Formation conglomerate exposed to the south in Claremont Canyon, in Sibley Volcanic Preserve and along Route 24 east of the Caldecott Tunnel.

Naturally I fired up the geologic map (I keep USGS map MF-2342 on my tablet) to see how the locality is mapped. It’s the little hill northwest of Inspiration Point, right above the word “Nimitz” where a power line runs.

But instead of Orinda Formation (the orange unit labeled “Tor”), which underlies Inspiration Point, it’s mapped as “Tus,” or “unnamed sedimentary and volcanic rocks (late Miocene).” Turns out there’s a major fault that separates two big blocks of young East Bay rocks — that thick black line with the teeth that represent the upthrown side — and even though the rocks look the same, we can’t say for sure they are.

The area of “Tus” rocks is rather large; in fact it’s the largest single rock unit on the geologic map.

I poked around the literature and found that the Orinda Formation has drastically shrunk over the years. As one example, here’s part of a 1973 map of the Lafayette area (Calif. Div. Mines & Geology Map Sheet 16) that classified a bunch of rock as Orinda Formation, drawn with the exquisite attention that emanates authority.

But the details are quicksand. First, the map is not about bedrock per se, but landslide hazards. Second, the author’s citations are generally very old, not that there’s anything wrong with that, but still. Third, the author’s idea of the Orinda is a unit that mixes lava beds (lumped today in the Moraga Formation) with the coarse sediment that defines the Orinda today. A long footnote explaining his thinking shows that he basically made an arbitrary choice of stratigraphic nomenclature to match the informal usage of local engineering geologists, who tend to talk about “Orinda-type” materials (like I was thinking at my outcrop) without making sure the stuff actually matches all the criteria for the Orinda Formation.

I’ve read my share of geologic engineering reports; any large construction project has to have one prepared. They’re good for their purpose — ensuring that the work is suited for the ground — but they don’t critically assess all the details of the science. And they probably shouldn’t. Instead, they line up the existing literature, outmoded and current alike, and discuss or dismiss it on the way to reaching their conclusions and advising their client.

Maps like Sheet 16 propagate obsolete or informal nomenclature, and thus stratigraphic concepts that are outmoded or discredited persist in the geotechnical literature like zombies long after research scientists have moved on. But I don’t blame people. The old idea of the Orinda Formation, widespread and simple, was very handy. The current idea of the Orinda, constricted and specific, is less handy because it leaves a large area of bedrock with the mumbly label “unnamed sedimentary and volcanic rocks (late Miocene).”

Geologic maps aren’t written in stone. Only stone is, and we’re still learning to read it.

Oakland geology ramble 5: Grass Valley

19 March 2018

Over the last few weeks I’ve been exploring the remote land just east of Skyline Boulevard, over the city line in Anthony Chabot Regional Park. Time to show you some of the charming features of Grass Valley, seen here from Redwood Ridge near the Parkridge land bridge.

In classic Geology Ramble style, this walk (a set of them actually) starts at one bus stop and ends at another. It starts at the top end of the 39 line and ends at the top of the 46L. The challenge is that you can only do this on weekdays because neither line runs on weekends; moreover both lines only run once an hour. Here are the two routes I’ve done so far, in the Google Maps terrain view just to give a feel for the territory (800 x 100 pixels). There are 12 photos in this post, locations given in a map at the end.

The route on the right runs down Redwood Ridge, and the first part is just like what I showed you last month only more so. Where it meets the other route, at the Bort Meadow staging area, you get this view of the valley floor. You can see it looks completely different in cloudy weather. Without the distant views across the Bay and south into the Diablo Range, it’s intimate and secluded. (The rest of my photos are from the western route.)

Amelia Sue Marshall, in her new book East Bay Hills, a Brief History, says that Grass Valley was never settled by whites or Ohlone because the stream was unreliable. Back in the day, redwood lumber was hauled through the valley to Castro Valley; only a few redwoods actually grew there. Later, cattle were driven through it between Oakland and Moraga. The Grass Valley Ranch raised cattle there for many years until the water companies moved in. The East Bay Water Company and the Contra Costa Water Company both planted huge eucalyptus forests there, and finally East Bay MUD took them all over. When they secured Sierra Nevada water, their East Bay land was transferred to the East Bay Regional Parks District, and that’s the story.

The ramble starts at Skyline High School. The scenic way is up the median path of Skyline Boulevard, charming in all seasons. It also exposes the Oakland Conglomerate, as I’ve posted previously.

The road up to the city stables is marked by a sign. Go past the gate and enter the Goldenrod Trail, an old dirt road popular with horse riders. It’s pretty, and you’ll see outcrops of Oakland Conglomerate along the way.

You knew I’d get to the rocks. The gross terrain is shaped by the rocks beneath it, but not much rock is actually exposed. These rocks began as sediment, and to sediment they quietly return, mostly covering themselves in a forested blanket of soil.

The important map units are Ko, Oakland Conglomerate; Ksc, Shephard Creek Formation (mostly shale); Kr, Redwood Canyon Formation (mostly sandstone). Grass Valley is strongly confined by the shale. The upper slope, though, is conglomerate. You can usually tell by the well-rounded stones embedded in it — samples of ancient mountain ranges. They get as big as this.

And there’s more than rocks to shake a camera at. For instance, banana slugs.

Take the Ranch Trail or the Buckeye Trail down into the valley. Near the bottom the woods open up nicely.

After a while you reach Bort Meadow. A hundred years ago the water company called it the Big Trees area, but the park took down most of the eucalyptus and it’s much better now.

The stream gradient is very gentle; this area looks like it could have been a lake at various times in the recent geologic past, especially if landslides dammed it.

Grass Valley is still a ranchy, horsy, countrified place. Though it’s gone from drivin’ dogies to walkin’ doggies.

The first time I came through the valley, unsure of my pace relative to the bus schedule, I was trotting. After that I knew I could amble instead. The whole hike is less than 6 miles, and the climbs aren’t that strenuous.

Farther down the woods rise up and close in, first oak and then eucalyptus.

By the time you reach the Stone Bridge the woods are thick. I haven’t yet gone farther toward Lake Chabot; the public transit logistics are pretty daunting. Take a minute to look at the streambed above the bridge; the Shephard Creek Formation is well exposed there, but look from afar because the creek bed is fenced off as sensitive habitat.

From the bridge turn up the Jackson Grade, where you’ll meet the bottom end of Skyline Boulevard. (There’s a water faucet at the top of the grade.) From there it’s a quick downhill into the Grass Valley neighborhood. The eucalyptus allee on Grass Valley Road is pretty to look at, harrowing to drive and inadvisable to walk.

Instead, cut over to Scotia and down Shetland and contemplate the classic postwar burbia around you as you head for the bus stop.

As promised, here’s the photo key.

You could take this route the other way, but it’s uphill, from about 500 to about 1100 feet.