Archive for the ‘Oakland rock types’ Category

Open thread

8 November 2021

I’m so busy finalizing the manuscript of my book that I can’t find the bandwidth for my accustomed fortnightly post today. There won’t be one in two weeks, either. Next post will be on 6 December. In the meantime, the comments are open so we can talk to ourselves for the rest of the month. (“Did you even know you have selves you can talk to?” Bob Weir reputedly told the audience at a Grateful Dead show as they took a technical break.)

The photo is a fine specimen of mariposite rock from the Carson Hill quarry in Angels Camp. This popular landscaping stone is shot with a green chromian variant of the metamorphic mica mineral phengite. Portions of the deposit are worked for gold as part of California’s continuing gold crawl. The specimen is a recent addition to the rock garden at Lake Merritt.

What’s up with you?

Load casts in the Shephard Creek Formation

11 October 2021

I see that I haven’t given my photos of this interesting feature their own page. They show some dramatic load casts on the south side of Shepherd Canyon, near the east end of Escher Drive. A load cast is made when heavy material, like a mudflow, crosses soft sediment and sinks its feet into it.

My most memorable lesson about features of this kind happened in 2008 during a visit to Point Reyes. The path leading to the lighthouse passes a chaotic scene that most visitors ignore.

The scene was an offshore basin, probably like the offshore Monterey Canyon today, where every now and then an undersea debris flow, full of gravel and sand, fell rudely upon nice quiet beds of deep-sea clay or soft mud. The results included scour marks . . .

and rip-up clasts, hunks of (easily eroded) clay swept into the flow . . .

and downward-pushing load casts accompanied by upward-pointing flame structures.

It was a great pleasure to come upon high-quality load casts in the Oakland Hills, in the mudstones of the Shephard Creek Formation, and lead group walks past them. Here’s the overall scene, photographed in February 2016. A thick layer of massive (i.e., unbedded) sandstone overlies thin-bedded shale and mudstone.

Near the base of the sandstone, on the lower right side, are these well-exposed load casts.

The previous June, I took a closeup of the underside.

Unfortunately, a Google Maps image from January 2021 appears to show that this feature has crumbled off the roadcut. That’s how geology goes in the Oakland Hills, and that’s one reason I constantly take photos. I also tell myself, in consolation, that new examples could appear on any given day.

The classic 1995 text Sedimentographica has good photos of these and many more features of sedimentary rocks. I treasure my hard copy, but maybe the publisher’s online version will outlast it. That one, YOU can enjoy.

Upper Castle Canyon

30 August 2021

Sausal Creek is formed by the junction of three streams, two of which are well known: Shephard Creek, which drains Shepherd Canyon, and Palo Seco Creek, which drains the bulk of Joaquin Miller Park. In between them is Cobbledick Creek and its steep-walled watershed, hidden green heart of the Piedmont Pines neighborhood.


Source: Alameda County Flood Control and Water Conservation District

The creek has two branches: the northern one, which I’ll call Beaconsfield Creek after Beaconsfield Canyon at its head, and the eastern one, Castle Creek, which drains Castle Canyon. The easternmost portion of Castle Canyon, a gorge running from the hairpin turn on Mastlands Road, is an 8-acre preserve that’s formally part of Joaquin Miller Park. Over the weekend, that parcel was renamed Dick Spees Canyon, with a bench and plaque, to honor the longtime politician and activist who helped keep the land undeveloped.

The interesting thing about Dick Spees Canyon, and the valley of Cobbledick Creek below it, is that it coincides with the inactive Chabot fault, a deep feature of the East Bay that runs roughly parallel to the Hayward fault. It runs diagonally across this digital elevation model of the area; Dick Spees Canyon is right in the center.


From nationalmap.gov

And the bedrock map of the same area is here. The Chabot fault is the dashed line with the pairs of tick marks on the right-hand (downthrown) side.

The Chabot fault juxtaposes two very different rock units directly across the canyon from each other — the serpentine rock of the Coast Range ophiolite (sp) on the west and the Joaquin Miller Formation (Kjm) of the Great Valley Sequence on the east. Dick Spees Canyon aligns very nicely with the upper part of Palo Seco Creek, forming an unusually good topographic expression of this obscure fault. Leona Canyon is another place it stands out; also in upper Knowland Park. The fault has been traced past Hayward. While it appears to have a long history, it’s very much inactive.

The fault is why I made a point of visiting here in 2019 and returned last Thursday. The parcel is almost completely undeveloped; only a rough footpath runs up the narrow valley floor from a short formerly graded stretch, then zigs up to Castle Drive. (I must advise all visitors not to try walking it downhill until the rainy season firms up the soil there.) The following photos are from both visits.

Here’s the lower end of the property. It’s a steady climb.

Soon the rocks make themselves evident, serpentinite on the right . . .

. . . and mudstone on the left.

Whoever built this fire ring used stones from both sides of the fault.

The valley floor is littered with dead cedar and eucalyptus trunks that need clearing out. And all sides of the canyon are very steep. I don’t expect anyone to cut any trails up them for a long time.

But if they do, visitors might glimpse the views enjoyed by the ridgetop residents who surround this neglected gulch with its interesting geology.

Mountains and other awesome things

16 August 2021

As you may know, there is no spot in California that’s out of sight of mountains. I took a long train trip over the weekend, when I wrote this post. Passed mountains the whole way until Nebraska. Now Nebraska is full of geological interest, but it is . . . subdued. It may be the largest state without mountains — no, Kansas is a little larger.

In the interest of taking a break and to practice working in a new image-editing application (Photoshop Elements, now that Paint Shop Pro 9 no longer works with Windows), I’m going to wander off the range and feature some mountains and other awesome features, starting in California — actually, starting with two of the photos I keep on my phone. I don’t believe I’ve shown them on this blog before.

Here’s Gudde Ridge and Round Top, just over the hills from Oakland. They’re honorary mountains, using 600 meters/2000 feet as the cutoff.

And here’s Las Trampas Ridge on the left and Rocky Ridge on the right, west of Danville/San Ramon. Rocky Ridge is just over 2000 feet.

And now let’s go for awesome.

I’ve tried several times over the years to capture this sight on camera: the Kern River canyon exiting the Sierra Nevada east of Bakersfield. In my opinion it’s California’s most dramatic water gap, made as a strong mountain river cut through a rising range. The Golden Gate might outdo it in geographic importance, but that’s a drowned water gap at the moment, with its lower hundred meters covered by the sea.

And here’s another awesome thing: the White Mountains, as seen in the bristlecone pine preserve.

The White Mountains are white in this area because they consist of dolomite marble. How that happens is still imperfectly understood. But what matters here is that dolomite, which resists rainwater much more than its more common cousin calcite, creates a very stable setting for the extremely old bristlecone pines, some of which are approaching five thousand years of age. It’s remarkable stuff to pick up and stare at, just as much as the trees.

And finally here are two photos from Colorado, which I rode through on Saturday. First is an image from five years ago in the mountains north of Red Rocks, showing the classic sandstone of the Fountain Formation that gives the area its name.

And here’s an image from Saturday, taken from the California Zephyr as it approached upper Rube Canyon.

What an audacious feat it was to push a railroad through here, and what an experience it was to ride through it.