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.

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Tour of the Fan: Lobe 4

11 September 2017

During the eight years I was surveying every sidewalk in Oakland, taking in the geology and geography as I went, I took thousands of pictures. Here’s a baker’s dozen of my favorite shots from the ancient alluvial fan that dominates central Oakland, specifically the large segment I call lobe 4.

Every part of the Fan makes an interesting walk because of its rolling topography and the charm of the neighborhoods, different from block to block. If you get any message from this love note to lobe 4, I hope it’s an urge to take lots of walks there.

The lobes of the Fan are defined where streams have cut through it all the way across to bedrock. Whereas the four outermost lobes are simple landforms, the inner four have been sculpted within their borders as well as around them. And lobe 4, the biggest one, is full of variety at all scales. Here’s a closeup from the geologic map.

I think of lobe 4 as being divided, by two streams and a freeway, into four segments. (That would make them anthropo-geomorphic entities, I guess.) I’m giving them names for convenience, not to be confused with the names of the neighborhoods.

Lobe 4, like the other lobes, has an outer edge that’s rather steep, although the elevation change hardly reaches 100 feet. Here’s the north edge of the Glenview segment, seen from below in Indian Gulch.

This is looking over the edge of the Haddon segment at Lakeside Park and Adams Point Hill, in lobe 3.

This is the view west across Alameda from the crest of the San Antonio segment, in San Antonio Park.

And this is looking over the valley of Sausal Creek from the east edge of the lobe.

The Haddon segment is separated from the Bella Vista segment by the valley of Park Boulevard Creek. This view from Ivy Hill looks across the valley to the Haddon segment’s highest point, occupied by St. Vartans Church.

From the top of Haddon Hill you can see over Oakland High and the freeway to the Glenview segment, where St. Mary Margaret Church sits. (The chapel in the foreground houses a pre-school today.) On the skyline is Sugarloaf Hill.

The Bella Vista segment is easy to spot from all over town, because of the allee of very tall palm trees on 9th Avenue leading to the former site of the “Borax” Smith estate. This is it as seen from the north on the Haddon segment.

And this is it from the other side, looking north across the valley of 14th Avenue Creek from the San Antonio segment.

The other major highlight of the Bella Vista segment is Highland Hospital, sitting at the top of the stream valley where it was sited to catch the healthful Bay breezes and let in the beneficial sunlight. Today’s hospitals have sealed windows and filtered air.

The weedy foreground is the scar of a landslide. The steepest parts of the Fan are prone to them.

From the high part of the Bella Vista segment, we can look east across the valley of 14th Avenue Creek to the San Antonio segment. The creek forks here, accounting for the bump in the road covered with fresh blacktop.

All three of the western segments are themselves dissected by small valleys, each segment with its own distinct pattern. On the San Antonio segment, among other features, is the gentle valley of 23rd Avenue Creek, occupied at its mouth by Garfield School and the colorful Church of Tonga. This is looking down the valley from its unobtrusive head.

Not all of lobe 4 is sloping. The Lynn and Tuxedo neighborhoods, on the Bella Vista and San Antonio segment respectively, are pretty flat. Tuxedo flat is strikingly flat, so level that I occasionally wonder if it’s a wave-cut terrace. But it’s probably just undissected.

The heart of the Glenview segment is rather flat, too. This is looking east on Excelsior Avenue toward the rise at the back side of the Altenheim, which overlooks the Dimond district in the valley of Sausal Creek.

To help you with all those locations, here’s a key to the 13 photos.

I hope this inspires you to get out on the Fan. Maybe I’ll see you there.

An early look at the Fan

4 September 2017

Lots of people love old photographs of familiar places, or old landmarks when they were new. I love old photos of Oakland because they show the land before it was paved over and/or forested.

The early California photographer Carleton Watkins was instrumental in saving Yosemite as a public park, simply because his large-scale images let people see its beauty and grandeur for themselves. But he also practiced his trade up and down the Pacific coast, including Oakland.

Here’s a stereograph he made here in 1876, looking east from the roof of the Grand Central Hotel at 12th and Webster. Click it to see its full 1000-pixel size.

Stereographs were made to be viewed through a simple apparatus that allows you to see a 3D image. You can do it without a viewer if you cross your eyes carefully. (Don’t worry if you can’t; you’re like most people.) To simplify things, I’ve turned one of the images into black and white and zoomed in a bit (800 px).

This is a pretty good image of Oakland’s high hills, the highest of which is Redwood Peak at the left edge. Lake Merritt sits between downtown and the hills of old Brooklyn. That hook in the shoreline is the cove where Wesley Avenue splits from Lakeshore Avenue today, heading left up a little valley. The right edge of the photo is where East 18th Street meets Lakeshore.

Between Redwood Peak and the shore of Lake Merritt, the top one-third is the high hills and the bottom third is the low hills. The middle third, hard to see, is the rocky hills above Piedmont and their eastern continuation in Oakmore and Redwood Heights.

Note that almost all of this is treeless. That was the state of the whole East Bay when the Spanish and the Americans came here. The exception was the redwood groves, which occupied the highest part of the ridgeline until they were logged out before 1860, and the coastal oak groves nearer the bay.

Anyway, what caught my eye in this photo was its view of the undeveloped lower hills that are almost totally hidden by houses today. Those hills were overridden first by wealthy estates, then by homes and apartments whose selling point was their views. To me, the real view was what those structures wiped out. Today only three bits of that terrain are left: San Antonio Park, Home of Peace Cemetery and Evergreen Cemetery.

Here’s how the Fan fits into that view. We’re looking east across lobe 4, the central segment of this ancient alluvial fan that dominates central Oakland.

Today, you can walk all over those hills and picture how they would have looked before development, but it’s a struggle. The old photographs are indispensible, and precious.