Archive for the ‘oakland geology puzzles’ Category

Dunsmuir Ridge and the Irvingtonian gravels

26 June 2009

Just northwest of Lake Chabot are some tiny areas mapped as “Irvington Gravels,” high above the Sheffield Village neighborhood in the Dunsmuir Ridge Open Space. They caught my eye because Irvington (part of present-day Fremont) is the site of a famous set of Ice Age fossils, from which the Irvingtonian age of North American land mammals was established. Yesterday I checked the area out, in case there were some sabertooth-cat fangs lying around. This entry has a lot of photos.

You get there starting at the access at the end of Covington Road, a dirt fire road that goes straight up a steep hillside. The Hayward fault crosses the road partway up, at a little level spot at the edge of the woods. To the west of the fault, the rocks are mapped as San Leandro gabbro (Jurassic rocks of the Coast Range Ophiolite), but it’s really hard to tell:

dunsmuir ridge

Across the fault the rocks change to Late Jurassic volcanic rocks of the Great Valley Sequence, the same stuff exposed in the big Leona quarry:

dunsmuir ridge

Higher up are three small terraces where the gravel is mapped. This is looking south from the northernmost one:

dunsmuir ridge

It looks like a hopeless task to find rocks here. Luckily for me, the fire roads have recently been graded, so there was a window into the substrate. As I approached the terraces, the roadbed started to display river cobbles, quite unexpected in this setting:

dunsmuir ridge

I made a point of crossing the grassy slope to the other two terraces, looking for stones the whole way. Nada. From the southernmost terrace, here’s the view north. Click on the picture for a stereopair:

stereopair

There’s a house on a knoll at the same height as the terraces. The upper part of the Knowland Park Zoo land also lines up with the terraces. No gravel is mapped at either place, but there might be some.

Now the cobbles in the roadbed start to look interesting:

dunsmuir ridge

Above is another, higher terrace. It’s over 500 feet above the starting point and a bit of a trudge.

dunsmuir ridge

Just below it are scattered outcrops of the volcanic bedrock:

outcrop

The roadbeds on the upper terrace also have interesting cobbles. I took a few home to clean up and photograph. Remind me to bring them back on my next visit.

rocks

Russ Graymer, who prepared the Oakland geologic map, describes the suite of cobbles thus: “Cobbles . . . consist of about 60 percent micaceous sandstone, 35 percent metamorphic and volcanic rocks and chert probably derived from the Franciscan complex, and 5 percent black laminated chert and cherty shale derived from the Claremont Formation.” He holds that these little terraces started out near Fremont and were carried here by the Hayward fault. They started out at a much lower elevation too, I would think; just a sign that fault movements are not straightforward.

View of “Mount Ararat”

31 December 2008

mount ararat oakland

One of the geologic puzzles I worked on during 2008 was the identity of “Rockridge Rock.” One candidate is a crag along Acacia Avenue once known as Cactus Rock; another candidate is an outcrop that supposedly was dynamited away in the early 1900s. I’m reluctant to accept either as the true Rockridge Rock without better evidence. In the meantime I’m intrigued by the hillock called “Mt Ararat” on an old map of the area. It’s topped with a bedrock eminence that’s almost completely hidden by homes and trees, but if you remove those (and there’s good historical reason to do that) I think what’s left is a legitimate candidate. This summer, while exploring the ground north of Mountain View Cemetery, I got a fair shot of the hill. Click the photo for a larger view.

I’ve been too busy to carry my search further lately, but I haven’t given up.

Thrust and fold

21 November 2008

The same day I was up at Redwood looking for the bent trees, I ran across this fine example of a thrust fault right next to the Huckleberry Botanic Reserve.

folds

Today I finally got around to putting it in my gallery at About.com—not as a thrust fault (I already have a good one), but as an example of a drag fold. Looks pretty good there, almost textbook quality. But here’s a secret: look at this view of the fault.

folds

I can’t figure out what the double curvature means! I can’t figure out the relationship of the fault to the folds. I feel like a freshman in his first field course. A real geologist would crawl all over this, including the hillside on top, until everything was clear. But I tell myself, the key to being a good scientist is to admit when you’re mystified because enlightenment comes that way. A certain set of people can visualize things ten times as complicated, and I hope one of them will pipe up.

By the way, my spread about Oakland’s geology is in the new Oakbook, the printed one. Go get one for free.

Fossil hunting

25 September 2008

There aren’t many fossils to be found in Oakland. Maybe microfossils—there must be lots of those in the young hilltop rock units. But on the geologic map is a little body of rock up in Montclair that is supposed to have “well-preserved coral fossils” of Paleocene age (that’s 65 to 55 million years old). Recently I checked it out.

outcrop

The rock unit has no formal name; it’s mapped simply as “unnamed glauconitic sandstone” and extends in a belt from Shepherd Canyon north over the ridge to Snake Road. This outcrop is at the intersection of Paso Robles Road and Shepherd Canyon Road. I examined it carefully and found nothing but massive fine sandstone, with almost no bedding and no sign of fossils of any kind.

outcrop

I traversed all the roads in the area where this rock unit was mapped, and the same sandstone was everywhere I looked. At this outcrop on Paso Robles Road, weathering and lighting combined to bring out some subtle bedding planes, but again no sign of fossils.

outcrop

At the north end of the belt, across Snake Road, is Armour Drive. Here is a large landslide scar that has broken Armour Drive in two, and there was plenty of loose rock for me to apply my rock hammer and take home a hand sample. This kind of rock is all I could find, period. But the unit is said to be “coarse-grained, green, glauconite-rich, lithic sandstone” interbedded with “hard, fine-grained, mica-bearing quartz sandstone.” That’s this stuff.

Somewhere in this belt is hiding a bunch of green, gravelly rock with coral fossils in it, and I haven’t looked hard enough to see it. On the other hand, geologic mapping is an imperfect art, and much of the hills has been mapped on the basis of aerial photos with limited work on the ground. Because I work exclusively on the ground, I’m in a position to do better. Maybe the fossils are only along one edge; maybe they belong to a separate subunit that needs to be mapped more precisely. Maybe someone made a mistake. So far, it’s a puzzle.

‘Mount Ararat’, another Rockridge rock

27 August 2008

In my search for Rockridge Rock I’ve found some nice places. I consider the hillock embraced by Wilding Lane to be one, possibly even the Rock itself. One piece of lore that guides me is the fact that in the late 1800s, the Rock was easily reached from town and picnickers enjoyed an expansive view of the Bay and hills. The 1912 Oakland street map I rely on in my research labels the Wilding Lane hill “Mt Ararat.” Here’s the view from Wilding and Canyon View Lanes (click full size). If you strip away the trees and homes, the appeal of the place is obvious.

wilding lane view

Mount Ararat, unlike Cactus Rock near Acacia Avenue, is right next to the main drag of Broadway Terrace. Here’s a map to help you find the place.

Landvale Road

23 July 2008

landvale road

Civilization moves on, and history gets obliterated, especially in the Route 24 corridor. If you walk up Broadway from the park at Lake Temescal, first you pass the back entrance with its spikes, then you see a bit of wasteland and a broken viaduct. Just beyond is a precious Deco structure built in 1922 (that’s it in the trees) that still serves as a PG&E electrical substation. This driveway with its curious inscription leads onto the viaduct, which turns out to be the last remnant of Landvale Road. A 1955 photo at the Oakland Museum shows it intact. The structure looks just like the Golden Gate Avenue undercrossing on Broadway, which is dated 1934. No road is shown there in a 1912 street map, so until someone comes up with more information, those are the constraints we have on its lifespan. This reminds me of certain problems in geology: a feature was absent at date 1, present at date 2 and gone by date 3. It is like talking about some poorly attested ancient Greek thinker: “Landvale Road, fl. 1930s-1950s.”

Cactus Rock

18 July 2008

cactus rock

Walking along Acacia Avenue, you may feast your eyes on the homes and grounds of the core street of the Upper Rockridge neighborhood, but one exception stands out at 6240 Acacia: this rock peeking over the scene. (click it for full size) This appears to be Cactus Rock, attested to in old postcards about the ongoing development of this streetcar suburb (still served by the improbable bus route 59A). I can’t get a good look at it, but it appears to be a standard Franciscan knocker that is much smaller than the mysterious Rockridge Rock. To judge from the prospect at Alpine Terrace, the next street uphill from here, the views from the rock are fantastic.


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