Archive for the ‘Oakland rock types’ Category

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.

The pyrite orebody of Leona Heights

10 May 2021

Through historical accident (or fate), I’ve been a longtime reader of the late Oakland fiction author Jack Vance. As it happens, Vance was exposed to geology by coursework in mining engineering at UC Berkeley, and one of the most charming and memorable features of his Planet of Adventure series, written in the late 1960s, was the mineralogical currency of the planet Tschai, called sequins.

In volume 3 of the four-book series, we learn that sequins grow in a locality controlled by the alien Dirdir species, who amuse, enrich and feed themselves by hunting the sequin hunters. Sequins come in a range of colors, the clear ones being worth the least and the rare purple ones the most. I no longer have the text in front of me, but I remember them growing out of the ground, literally cropping out. Over at Tor.com, reviewer Paul Weimer does have (and loves) the text and reveals the additional detail that sequins consist of “a uranium mineral called chrysospine.”

The name is mineralogical fantasy, and possibly misleading in that “chryso-” refers to a golden or light green color. But come to think of it, uranium impurities often turn minerals brown from radiation damage, and radiation damage to an originally clear or golden mineral might result in a fair purple by analogy with “sun ripened” glass. An analogy with ripening fruit, too, is irresistible.

In populating his planet with this precious crystalline substance that grows in the ground like mushrooms, Vance evokes truly ancient geological notions that are natural among people who know nothing beyond the most basic alchemy. Gold Rush California saw a lot of that pre-industrial thinking among the amateur prospectors who scoured the state, and the Cornish miners who worked in the hard-rock Mother Lode mines brought along their own ancient customs and superstitions.

In Oakland, the people who exploited the pyrite in the Leona Heights mining district didn’t have the advantage of magic. But Fritz Boehmer, the canny Prussian immigrant who spearheaded mining in these hills, was apparently prone to dreams of earthly wealth, a deep California trait. He was not especially well educated, having apprenticed in metalworking. When he learned of the ore underneath his ocher deposit — one story is that he was digging postholes for a cattle fence, another that “a Japanese” was seeking water for a large fish pond — he thought he had an iron and copper mine, but the professionals set him straight. The copper was only a few percent (although later it was by-produced in paying quantities) and the iron was waste; the money from pyrite (FeS2) was in the sulfur. He let the Stauffer Chemical Company run the mines and gave scientists of the time free access to them.


Pyrite on quartz

The mines ran, interrupted by fires, for about 30 years starting in the 1890s. There were at least three of them. Records are confusing and I’m still trying to sort them out.

The best ore in Leona Heights was in pods of hard, dark, solid pyrite yielding 50 percent clean sulfur that sat, like layers of frosting in a chocolate cake, within a zone 12 to 30 feet thick that tilted into the hillside. The people who published papers about this district scratched their heads at the deposits. They all concluded that the Leona volcanics (“Leona rhyolite” as they knew it) was so jammed with pyrite that the upper part weathered into iron oxides (which stayed behind as the ocherous “iron cap” or gossan) and iron sulfate, which leached down in solution and was reduced back to pyrite beneath the water table in the so-called vadose zone, where it was exposed to a lot of carbonaceous material.


Fine-grained pyrite concentrated in the Leona volcanics, Campus Drive

The trouble with the kind of intermittent research these geologists pursued in the operating mines is that each person who visited the workings saw a different set of rocks. The Leona Heights mines were also prone to fires, so parts were off limits for years at a time, or abandoned.

Henry Mulryan, in a 1925 Master’s thesis, summarized the previous work and consulted their authors, but with several parts of the mines closed off by fires he failed to find any of that carbon-rich rock in the areas he had access to. Unable to prove anything one way or the other, he was forced to punt, saying he would rather wait and see what further digging revealed at depth. “If the Leona Orebody is derived under vadose conditions, then it is the only one known to the writer and should take its place in the world’s literature on ore deposits.” (I too am skeptical about this carbonaceous rock, but the Oakland Hills are complicated here so who knows?)

That was a hundred years ago, before geologists made huge strides in understanding this class of “volcanogenic massive sulfide deposit,” not to mention a scientific revolution, in the years between then and now. Meanwhile the mines are long closed and will never be reopened. The samples, if they still exist, are gathering dust in obscure cabinets. I’ve read all the contemporary literature (except for some theses — Leona Heights seemed to be a handy subject for Stanford and Berkeley students at the time), which is an absorbing chore because the records are sketchy by modern standards and the terminology has changed. But there are rewards; Mulryan had some good photos of the Leona sulfur mine circa 1924.


Looking west on the Leona Mine. The hook in the road is at the end of McDonell Avenue. The rail line carried ore cars to the crusher, then to a 1600-foot aerial tram that carried the ore to the train in Laundry Farm canyon. Chabot Observatory in the background.

I’m still scratching my head about the Leona Heights pyrite, and I find myself envying Jack Vance’s freedom of imagination. Reality can be tough; you can’t just make up something wonderful.

The twilight of California oil

26 April 2021

Last week the governor ordered a state agency to stop issuing fracking permits to oil drillers, starting as of 2024. This is less of a big deal than it seems. Hydraulic fracturing is rarely used in California because the permitting process was tightened in 2014 and because our earthquake-shaken rocks are already well fractured, and only three oil districts do it at all, accounting for about two percent of the state’s production. One place they still do it is in the Lost Hills area, which is fun to drive through if you like taking pictures like this:

This change won’t affect the California oil industry much, but it sounds great and is worth doing.

The governer also ordered another agency to start plans to shut down all oil production in California as of 2045. This is a big deal. Oil is as much a part of California as gold, Shasta and the redwoods. But our oil production has dropped by half since 1985, and now’s the right time to set a deadline. According to a pair of fresh studies, it won’t even start to hurt business for another decade.

Time to start saying goodbye to our old friend.

Natural seeps of oil and asphalt occur all over the state. The one at McKittrick is famous among geologists.

The tar glaciers at Carpenteria State Beach, near Santa Barbara, are a real spectacle.

These materials were used by the native tribes for things like sealing baskets, waterproofing boats and medicine. I’ll bet they made torches with them too.

Americans mined the deposits at first and distilled kerosene from them. That was a dirty business. Starting in 1860, enterprising men tried drilling wells like the first successful ones in western Pennsylvania. The first California oil well to make a profit was drilled in 1876 near Newhall, and we were off to the races.

Petroleum, oil from the ground, was a huge advance. It meant we could stop leveling forests for firewood. It meant we could stop the deadly, wasteful business of hunting whales to make liquid fuels or roasting coal to make gas. No one knew it at the time, but we could invent plastic. The petroleum-based energy and chemical system was eagerly adopted, popular and universal. But today we know how to do even better without it.

As always with this blog, there’s an Oakland angle. The Bay area is oil country.

There are oil seeps in Wildcat Canyon, and the first oil well in the Bay area was drilled nearby, east of San Pablo, in 1862. A short-lived oil field in Orinda, at the Minor ranch on Lauterwasser Creek, pumped greenish crude in the late 1890s. Oakland boosters like H. A. Aldritch, in 1897, were sanguine: “For many years oil has been oozing out of the shale and sandstone formations, and in every instance this oil has been strongly impregnated with gas. That the near future will produce this most promising industry, affording cheaper fuel for manufacturing purposes, is a settled fact. My prediction is that within the next few years Oakland and other cities and towns of this county will be in the full enjoyment of this, one of nature’s greatest blessings.” He was right, but the profitable wells were in the Central Valley.

A large portion of California’s oil originates in the Monterey Formation, a body of ribbon chert found up and down the coast. Oakland has a thick stripe of its close sibling, the Claremont Shale, running through the high hills.

Wherever you see it, it’s generally bleached-looking like this, but underground it’s black with organic matter, from the diatoms whose microscopic silica shells are what make up chert. Diatoms manufacture and store drops of oil inside their shells to help them float, and that oil is what becomes crude oil after cooking underground for geological periods of time.

When the Caldecott Tunnel bores were being dug, oil and gas wafted off this chert and caught fire more than once. During excavation of the fourth bore a few years ago, nothing that could spark was allowed inside. So let it be known: Oakland’s hills are full of oil. I have yet to find an oil or gas seep here, but it’s on my list. I have a theory that one may have had something to do with the great fire of 1991, which burst out in an area where the Claremont Shale is deeply exposed.

Here or wherever, petroleum will always be something to reckon with in California. But we have to start leaving it in the ground at all costs and return it to being a geological curiosity.