The Skyline chert exposure

1 August 2022

For several miles starting near the top of Marlborough Terrace, Skyline Boulevard runs through a belt of blond chert that makes for striking roadsides and some of Oakland’s highest homes and best views. Near the south end of this stretch, north of Elverton Drive, the road runs east-west for about 700 feet. It’s just you, the view, and these rocks. In my book I describe them as “pale, flinty chert in layers a few inches thick alternating with thinner layers of soft brown shale. The layers, thousands of them, stand on end like a storm-swept forest of golden bamboo.”

Note that the nice wide verge is now blocked by a line of huge logs. These extend for miles along Skyline and Grizzly Peak Boulevard. There are still small places to park, though.

Here’s the location on the map. The land is an outlier of the Sibley Volcanic Regional Preserve, and a little trail runs along the ridgetop between the two boulevards.

The rock in this roadcut is chert: a hard, waxy-looking stone made from accumulations of microscopic diatom skeletons. It ranges from white to brown to black, simple to complex. It dates from the middle of the Miocene epoch, probably the Tortonian age around 12 million years ago.

The complexity extends to the larger, outcrop scale. Notice how the straight, even bedding on the left gives way to distorted layers on the right. The boundary between them is a fault. There are several more along the roadcut.

In addition to the faulting, this even-bedded, slowly accumulating sediment was periodically disrupted while it was still soft, presumbly by large undersea landslides triggered by earthquakes, just like today. Here’s a more extreme example; notice the wisp of chert entrained in the mixture on the left.

Here’s a foot-thick bed of clean white sandstone, preserving an ancient submarine landslide. By the way, it appears that nearly all of these rocks are overturned, their upper surfaces now facing to the right.

In the middle part of the roadcut, it really gets hairy. (This is the biggest image of the set.)

Other parts of the roadcut look tidy and regular, but erosion has cut into softer parts of the formation in the century since Skyline was built to attract developers.

Here’s a closeup of the groove in the middle. What caught my eye was the dark zone in it with orange veins, which shed pieces onto the roadside slope below.

The material (on the right) is a lightweight siltstone shot with films of black.

It’s likely that the black films are the remains of crude oil — not the only possible explanation, but it’s how to bet. Chert is a source rock for petroleum because the diatoms that compose the rock manufacture oil to help their opal shells float. As the diatom ooze becomes rock, the oil is released and migrates elsewhere until it’s trapped in an underground structure, eaten by microbes or oxidized in the atmosphere. Our Miocene chert is a close relative of the widespread Monterey Shale, which is responsible for California’s richest oil deposits. So the zone of orange veins might be a former escape avenue for the local oil — an injectite.

But above the roadcut, a little off the ridgetop trail, I encountered an intriguing alternative in this heavily etched material impregnated with silica.

Here’s a side view of another specimen showing the veins of hydrothermal quartz (chalcedony). The matrix between them appears to be the same stuff of the orange veins.

Now the formation of chert from plain diatom ooze, or diatomite, takes place at low temperatures and pressures. It’s very gentle. Hydrothermal silica is a sign of higher temperatures. Something more active than trickling crude produced this. Again, various explanations are possible, and these scattered blocks of rocks aren’t firmly connected to the putative injectite in the roadcut. I’ll just have to poke around here some more. The complexities multiply.

By the way, maybe some of you remember when this stretch of road was a cool, shady run through a high eucalyptus forest. That was how it was when I first explored the area. Google Earth stacks imagery of this area going back to aerial photos from 1939, and there was no eucalyptus here at all until the 1980s. Yet the trees were a hundred feet high in late 2003.

The Park District took them out around 2005, and in the fall of 2009 just a little fringe along Grizzly Peak was left.

Twelve years later, chaparral and oak-bay-madrone woods had made quite some headway.

But if you ask me, the site would be an excellent one to try restoring the traditional fire-groomed grassland that once covered all the hills north of the redwood groves. The Park District owns it, and California still has experts in the old ways.

PS: I’ve started an occasional newsletter. You can sign up and read the inaugural issue here.

PPS: The book’s fundraising drive has met its $5000 goal, but contributions still come in. More on the Deep Oakland page.

Avalon Hill

18 July 2022

There’s a little hill south of the UC Berkeley campus, near the Claremont Resort and the Hayward fault, that cried out for a visit. It stands out on Google Maps (with the “terrain” setting, naturally).

On the geologic map, it appears as a blob of Franciscan sandstone.

I thought I’d give it the same treatment I gave Easter Hill in Richmond: explore the rock and comb through old maps and documents.

The road running by the south side of the hill is Avalon Avenue, so I’ll call it Avalon Hill. It was never exactly a landmark, but it has a past and retains a certain presence.

The hill was owned by John Kelsey in the late 1870s, and Kelsey Street commemorates his name. Today’s Claremont Avenue was called Telegraph Avenue at the time, as shown in the 1878 Thompson map.

In 1884 the Berkeley Water Works excavated a large reservoir on the hill, and the following year the Alameda Water Company took it over. The Garber Reservoir held a million gallons and was lined with concrete. Here it is on the 1894 Wagner map.

It is not to be confused with today’s Garber Reservoir, the flat-roofed structure 1500 feet south on the north side of Claremont Avenue.

Development began to surround the hill in the 1890s and 1900s, especially after the 1906 earthquake. From then until the 1930s, the struggling private water companies of the northern East Bay merged, the East Bay Municipal Utility District swallowed them whole, and the water delivery system began a slow and expensive transition to the rational and robust setup we enjoy today. The 1912 street map shows the property still in the hands of the Peoples Water Company, the next-to-last of the private firms.

I don’t know when the reservoir was decommissioned and removed, but the homes adjoining it were built starting in the 1910s. The large home on the hilltop was apparently built in 1960.

The best picture of the hill itself is the digital elevation model made from a special survey of the Hayward fault. Its sides are too steep for streets, which has helped keep it quiet and isolated.

On the north side, Garber Street has a rustic interlude where a narrow road sashays down the hillside like a mini-Lombard Street. Avalon Avenue, on the south side, is blessed with three stairways, one at the end and two going down to Russell Street.

All those old houses with their mature landscaping cover up the rocks very effectively, I can testify. But there’s an excellent exposure at the end of Avalon Avenue underneath a private driveway.

What few flat surfaces there are on it appear to reflect fracture planes, imposed by the tectonic stresses on the rocks over the years, rather than any original bedding.

Up close, the rock is a hard siltstone much like the rocks in the quarries of the Piedmont crustal block: the Bilger, Blair and Davie tennis stadium quarries.

But just looking at it isn’t definitive. The geologists who’ve mapped it have left it unclassified (KJfs) rather than lumping it with the sandstone of the Novato Quarry Terrane (Kfn) exposed in the Piedmont block to its south. Here’s a larger piece of the geologic map showing what I mean: it might be the northern tip of the block or it might be the first of a string of rock bodies to its north. The thrust fault leading up to it, the dotted line with the teeth on the upthrown side, is reasonable but conjectural.

Avalon Hill is a cool little bit of Berkeley. Stick your nose in and poke around some time.

Yes, “Avalon Hill” is a private joke. My siblings and I warped our personalities in the 1960s by moving armies and fighting over the rules of the board-based wargames produced by the Avalon Hill company, starting with Tactics II and Gettysburg.

Skyline panorama from Lake Merritt

4 July 2022

Here’s a project I’ve wanted to do for some time: an annotated panorama of the Oakland skyline. Of course, we have many skylines, as seen from different places, but the first one has to be the view from the mouth of Lake Merritt.

It’s a 4000 X 1200 image weighing 2 MB; for convenience in printing, should that be desirable, I’ve also split it into left and right halves.

The top row of labels is for things on the skyline, and the bottom row, in the water, is for things along the shore. The labels in between are positioned according to their distance. As it happens, I can refer you to previous posts about most of these features.

Top row:

Grizzly Peak
Vollmer Peak
Barberry Peak
Skyline Boulevard roadcut
Round Top
Old Thorn Road Pass
Manzanita Ridge
Pinehurst Pass
Redwood Peak
Redwood Pass
Sugarloaf Hill
Caballo Hills

Middle space:

Claremont Canyon
Route 24 roadcuts
Temescal Canyon
Mountain View Cemetery
Piedmont block
Shepherd Canyon
Pershing Drive
Dimond Canyon
Lookout Point
LDS Temple

The three lower hills belong to the Fan, the crescent of Pleistocene gravel that is Oakland’s most distinctive geologic feature:

Adams Point Hill
Haddon Hill
Bella Vista Hill

Finally, right on the lake itself are these notable features:

Lakeside Park marine terrace
Our Lady of Lourdes
Pine Knoll Park marine terrace

I can see there are a few more things I should write about.

Signs of the old Alma Mine

20 June 2022

I devote a chapter of my upcoming book to Leona Heights, where I review the human history of this much-disturbed area and introduce the geologic history of the much-disturbed rocks.

There were three waves of digging in these hills. The Ohlones started it thousands of years ago, harvesting ocher for a thriving regional trade. In 1891, Fritz Boehmer resumed ocher mining in earnest to supply his paint factory. Then the Realty Syndicate, trying to make a few bucks from its vast land holdings, opened a rock quarry where Merritt College sits today. That was in 1896.

Finally Boehmer, whose ocher operation had ended when his factory burned down, found amazing deposits of pyrite ore next to Redwood Road and got into the pyrite business in 1900, opening a mine that he named after his daughter. The Alma Mine produced ore for the next 21 years, one of several pyrite mines in these hills.

Records of those mines are scant and confusing; some appear to have changed names with new owners, and all the shafts and adits (well, almost all) have been sealed. But I’m pretty sure about the Alma Mine: where it was and what it left behind.

Supposedly the first tunnel was dug near Fritz Boehmer’s roadhouse on Redwood Road, which appears to have been where the Lincoln Square Shopping Center sits today. I’m going to show a bunch of maps now. The shopping center’s next to the Warren Freeway, route 13, at the upper left corner of the next two images, a 2006 aerial photo and a blend of the digital elevation model with streets half-superimposed.

The 1915 topo map shows Redwood Road as it used to be, with a symbol where the mine was. It’s where the gas station is today, or under Terrabella Way.

An old-timer told the Oakland Tribune in 1950 that “friction set fire to the ore about 1908. The shaft was blocked off and the company moved operations further around Redwood Road, digging another tunnel to hit the same deposit at right angles.” Plagued by repeated fires from pyrite dust and the mining collapse that followed World War I, the Alma Mine ceased business in 1921.

The next edition of the topo map was issued in 1947, after Redwood Road had been rebuilt, and there was no sign of the mine, or any other structure, from the earlier map.

But aerial photos from the intervening years show what happened. This image from 1939 shows the old course of Redwood Road and the wasteland of tailings left below the first mine site. The second mine entrance was to the east, at the point of the sharp bend in the road. There appears to have been an ore transport line going from there straight southwest down to the old train tracks that used to run beside Lion Creek.

A year later, the area was being rebuilt as a Key Line streetcar route was pushed up Redwood Road, driving residential development past the upper Laurel toward the future Crestmont neighborhood.

This 1947 airphoto shows the new configuration of Redwood Road along with the ghost of the old road. The former mine pit was being mitigated and the new road exposed a lot of fresh rock; at the same time, land was being cleared along the big curve for new structures. (A ground-level photo from 1949 shows the rugged walls of the old pit.)

The 1958 topo map, with updates to 1980 in purple, shows how thoroughly the area was transformed after that.

Which brings us to today. Here’s a closeup of the 2006 imagery with the locations of the following photos I took last week.

The pavement entering Terrabella Way is deeply eroded by acid runoff from the exposed rocks, which are still full of pyrite.

And the iron oxides left behind continue to form fresh ocher.

The 1940s roadcut below Terrabella Place is strongly colored by iron oxides. It’s why this area was rich enough to support a mine.

It will take a long time for vegetation to cover the scene of the old mine.

The pit is really rather deep here. Now it holds the Redwood Reservoir, a big steel tank. I could definitely come back and spend an hour with these rocks.

And then there’s down below, on Geranium Place, where the old mine tailings plus the road construction have left bad ground. That caught my eye a few years ago, and it’s just as bad as ever.

When it comes to mines, especially pyrite mines, the past is never really past.

It’s a bit frustrating reaching the limits of the available information. There are surely other records out there I could consult, but not during a pandemic.

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