Archive for the ‘Oakland rocks’ Category

The St. James Drive roadcut

11 December 2017

Recent work in far east Piedmont has exposed some excellent bedrock worth a close inspection. Because the town government won’t put an interpretive sign there, this post will have to do.

To my knowledge, there are only two sites of powerline towers in Piedmont, one at the mouth of Estates Drive and the other at 298 St. James Drive, near Park Boulevard. Last year the power company replaced the latter pair with shiny new towers, and as part of the work it cleared the roadcut of its cover of acacia trees (Google Maps shows the site as a thicket going back to 2007). Shortly afterward I discovered it and had high hopes, though it wasn’t much to look at in late October.

By January, the exposure had been stripped of loose rock. Already it was clear that it would be a showcase of slickensides.

By August, a strong concrete wall had been put in place and landscape plantings made.

The slickensides turned out to be fabulous. These are the polished marks made as movement along faults grinds rocks against each other.

And here’s a closeup.

Also visible is evidence of brecciation, the geologist’s word for shattering rocks and cementing the pieces together.

The rock here is sandstone of the Franciscan Complex, specifically part of the Novato Quarry nappe. This is a thick slice of fine-grained sandstone that was laid down off the ancient coast of California, then shoved against the continent’s edge and pulled apart into lumps by the San Andreas fault system. A bunch of it underlies Marin County, and more makes up Point Richmond and El Cerrito as well as Piedmont and points south. This tectonic history probably accounts for the wear and tear visible in the roadcut.

When I visited the roadcut again last week, I annotated and recorded the site in the ROCKD smartphone app and announced it on Twitter. I’m trying out the app just for fun as a way to make some of my observations public. I’m looking at other apps for more rigorous mapping purposes.

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The Dunn-Spring Quarry, north Berkeley

27 November 2017

Glendale-La Loma Park, a little ballfield/playground complex in the north Berkeley hills on La Loma Road, is a repurposed quarry that’s had a true Berkeley history. The original quarry, operated by J. J. Dunn, appears to date from 1892. John J. Dunn, a Canadian immigrant born in 1839, was a major contractor in California building roads and sewers starting in the 1870s. This is Dunn portrayed in 1896 in the Oakland Tribune.

In 1900 Dunn advertised the quarry for sale “on account of sickness,” and died of Bright’s disease (kidney failure) in St. Helena that June.

In 1904 the Dunn quarry was reopened by Louis Titus, former head of People’s Water Company, as part of the Spring Construction Company. Its dawn-to-dusk blasting operations infuriated local residents, who obliged it to shut down in 1909 by threatening the business-friendly city council with a recall campaign. The company made the gaslighting claim that they were not operating a quarry, even though the rock from the pit was being used for streets and homes in the Thousand Oaks tract, but were in fact building a reservoir for the People’s Water Company.

After its abandonment the Spring quarry became an attractive nuisance, drawing generations of youngsters to its steep sides and deep swimming hole. Even those who didn’t swim in the cold, murky groundwater must have enjoyed the view.

In 1950 an 11-year-old boy drowned, and the city fenced off the quarry. The city acquired more property around the site in 1957, and eventually it became what you see today.

Unfortunately I have found no details of the site’s geology, although Titus called it a basalt quarry. The geologic map shows the downhill side as basalt of the Moraga Formation and the uphill side as the conglomeratic Orinda Formation.

This is the basalt. The brownish streaks are slickensides — friction marks from faulted fractures.

Other volcanic rocks here include rhyolitic tuff, a minor component of the Moraga Formation. It’s distinguished by its light color and broken (brecciated) texture.

Superficially, it resembles the Northbrae Rhyolite that makes up Grotto Rock Park and its sister parks, but unlike that heroically strong stuff it degrades quickly when exposed at the surface.

At the foot of the cliff you’ll find pieces of this excellent conglomerate from the Orinda Formation.

But the fact that it has crumbled down the cliff and will fall apart in your hands means that the rock face would be treacherous climbing. The city should be more forthright in discouraging climbing here; instead they just say “check out the other Berkeley parks with rock features…” Stay off it, and if an earthquake strikes while you’re standing there, jump back fast.

Bedrock in the headwaters of Trestle Glen Creek

13 November 2017

Surprisingly, the town of Piedmont has its share of woodland trails — well they’re paved sidewalks, but they’re unused, covered with duff and overhung with untended shrubbery. On a weekday afternoon, you can walk quietly on miles of these soft paths and encounter only a handful of property workers. It’s in that spirit that I recommend a geologizing stroll around the highest part of Trestle Glen Creek’s eastern watershed.

The shaded-relief map below shows the creek valley in eastern Piedmont, with Dimond Canyon on the right edge. The area I’m featuring is the triangle just right of center bounded by Crest Road, Pershing and Estates Drives and Hampton Road.

And here’s a closeup just to display the street names. During weekdays, the 33 bus stops at Lexford and Hampton, where the two valleys in this little watershed join.

The geologic map shows that this area is solidly within the block of Franciscan sandstone (Kfn) that underlies most of Piedmont. The hilltop above it consists of Franciscan melange (KJfm) that includes bodies of chert (fc), notably the one on Pershing that I’ve called the best bedrock in Oakland.

The neighborhood is gracious. This view looks up Huntleigh Road, which runs on the valley floor. As I traversed the streets, I used sidewalks that almost never feel a human foot. At times it was easy to imagine being in a Tolkien novel.

Lexford Road, in its own valley, is more secluded and more whimsical architecturally.

For the geologist, these streets are valuable because they aren’t as tightly landscaped as in most of Piedmont, and the Franciscan bedrock can be seen and studied at leisure in several places where the road builders exposed it. That’s unusual for this town.

Plenty of hand specimens are available too, if that rings your chimes.

There are even a few empty lots here. Unlike the existing homesites, these are especially challenging due to the steepness of the terrain and the strength of the rock — not just on the surface, where the weathered sandstone has fractured into rubble, but also deeper down where foundations would need to be dug into the hard, unweathered bedrock. When this lot was cleared recently, it had shed enough rubble to nearly cover the sidewalk.

A large house has been proposed here for many years, and the record of the intricate wrangling needed to invite and address everyone’s concerns is mind-numbing. However, the record does include geotechnical reports that give us a glimpse underground.

What’s on the site now is this set of what are called story poles, which serve to outline the planned building.

Geologists acquire a certain ability to see the ground through everything growing or sitting on it. It’s an ability to visualize the landscape as if it were covered with story poles instead of vegetation and structures. This bit of watershed is a good field site to practice.

Grotto Rock Park

6 November 2017

In preparation for the 4-mile walk I’m leading on 18 November for the Berkeley Path Wanderers, I’ve been visiting some of the unique and wonderful rock parks in north Berkeley. Grotto Rock Park will not be on the route, so I’ll feature it here.

It’s a little park on Santa Barbara Road at Indian Rock Avenue, just the size of a large lot, that preserves a nice outcrop of the remarkable Northbrae Rhyolite. The first thing you’ll notice about it is its very light color.

This is volcanic lava. Unlike the black, low-silica basaltic lava we know from the Hawaiian volcanoes, rhyolite is light colored and very high in silica. That makes it very stiff, even at the highest temperatures. Rhyolite lavas tend to form domes, like the Inyo Domes just south of Mono Lake or the central peaks of the Sutter Buttes.

After the Northbrae Rhyolite erupted, about 11.5 million years ago, the silica in it permeated the rock and turned it exceptionally hard and solid. It’s just about the best rock there is for climbing. Even the littlest toeholds will bear your weight.

Grotto Rock displays a typical texture of rhyolite — flow banding — that arises as the viscous lava flows like taffy. The name “rhyolite” in fact means “flowing stone” in scientific Greek. It’s also very bare. There’s only a little lichen growing on it because it has few nutrients, being mostly quartz.

Most of Berkeley’s rock parks feature the Northbrae Rhyolite. Yes, the rock is beautiful, but the developers gave the land to the city because the rock is so indestructible the lots couldn’t be built upon. Nevertheless, nature was powerful enough to round the corners off these bodies of lava.

Landslides could have done that, but surely the nearby Hayward fault did its part in rubbing these rocks smooth.

In non-geological news, Grotto Rock is said to offer better views than Indian Rock.

Generations of California geologists, including some quite eminent ones, couldn’t tell that the Northbrae Rhyolite is utterly different from the Leona volcanics of Oakland. Going through the literature on these rocks will teach you humility. It was a re-entry grad student at Cal State Hayward, a climber named Lin Murphy, who straightened everyone out about 15 years ago.

I’m started to get excited about the walk.