Archive for the ‘Oakland rocks’ Category

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.

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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.

Twenty Oakland rock types in a 30-mile drive

2 October 2017

As far as I can tell, Oakland has more rock types within its boundaries than any other city in America. When I added them up for a talk I gave at East Bay Nerd Nite, I counted more than 25, from limestone to blueschist. This 30-mile road trip will take you to most of them, with the Hayward fault as a bonus. It starts above the UC Berkeley campus and runs south the length of the high hills, then goes back north through Montclair and Piedmont to end at Mountain View Cemetery.

That’s 1200 pixels wide if you click on it, but don’t worry, I’ll show it in pieces below for more detail. In fact, let’s go to part 1 right now. It goes from Grizzly Peak through Joaquin Miller Park on Grizzly Peak and Skyline Boulevards.

The numbered segments correspond to the formations on the geologic map, as follows:

  1. Moraga Formation (basalt, andesite, tuff)
  2. Orinda Formation (conglomerate)
  3. Claremont Shale (chert, shale, dolomite limestone)
  4. Sobrante Formation (mudstone, shale)
  5. Unnamed mudstone/sandstone
  6. Redwood Canyon Formation (sandstone, siltstone)
  7. Shephard Creek Formation (sandstone, mudstone, siltstone, shale)
  8. Oakland Conglomerate* (conglomerate, sandstone)
  9. Joaquin Miller Formation (sandstone, shale)

Then there’s part 2, from Joaquin Miller Park to the edge of Montclair on Skyline, Grass Valley Road, Golf Links Road, Keller Avenue, Campus Drive and Redwood Road.

  1. Serpentine (serpentinite, blueschist)
  2. Oakland Conglomerate* (conglomerate, sandstone)
  3. Knoxville Formation (conglomerate, shale)
  4. Leona volcanics (metatuff, metabasalt)

Part 3 takes the freeway to Montclair Village, where you won’t see any rocks, then goes down Moraga Road to Piedmont’s Dracena Park and over to Mountain View Cemetery, where rocks are abundant.

  1. No rocks to be seen, but do stop on Medau Place and spot the offset curbs where the Hayward fault crosses it
  2. Franciscan melange (argillite, metachert, greenstone)
  3. Franciscan sandstone (sandstone, siltstone)
  4. Franciscan melange

The cemetery’s melange has many bodies of hard rock (knockers) that stand above the ground. They have their own blog category. Search this site, or check the category list on the right, for posts I’ve written about these rock units.

*The cobbles embedded in the Oakland Conglomerate offer more rock types, including granite, quartzite, gneiss and schist. That’s how I get up to 20.

Sibley sights: Lapilli tuff

18 September 2017

Sibley Volcanic Regional Preserve is the site of a small volcanic center that was last active about 10 million years ago. After it fizzled out, the whole thing was gradually buried in younger sediment. Within the last few million years, the action of the Hayward fault squeezed, folded and uplifted this sequence of rocks and tilted it almost to perpendicular. Then erosion of the uplifted hills — and quarrying by a Kaiser company — exposed a good cross section to view.

Stop number 10 on the self-guided geology tour is an out-of-the-way spot where a rare and striking example of lapilli tuff is exposed. Each time I pass by — three times so far — I can’t resist photographing it. There was May 2005:

There was June 2009:

And there was just last month.

I need to unpack the name “lapilli tuff.” Tuff is a rock type consisting of ash — volcanic material that’s been explosively erupted and then lithified. It’s formally called volcaniclastic material: pulverized rather than solid lava. Lapilli is the name for ash particles of the same size range as gravel, or 2 to 64 millimeters across.

These lapilli (a single particle is called a lapillus) are very consistent in size and texture. They suggest that a spray of red-hot lava was erupted from a volcanic vent nearby and fell together in a neat pile. Perhaps there were strong winds at the time that sorted the droplets by size. Whether the lapilli were still so hot that they fused together before they fully cooled — an agglutinate — or fused together later when cold — an agglomerate — is not clear to me.

Whatever the circumstances were, they were unusual enough that only wide-ranging geologists and professional volcanologists are likely to have seen more than one example of rock quite like this. It merits the specialized name lapillistone, because it appears to contain very little material other than lapilli.

Oddly, it seems I never photographed the same rock twice during my visits to this spot, although it’s possible the rocks eroded beyond recognition over that 12-year span. Will have to keep coming back.