Archive for the ‘oakland geology puzzles’ Category

Rettig canyon

5 May 2011

I paid a visit yesterday to part of the Hayward fault in Oakland, but while there I felt the pull of a neighborhood treasure I call Rettig canyon. The name is from Rettig Avenue, which traverses it. Here’s the topography, from Google Maps. This is just south of the LDS temple.

rettig map

The fault runs from top middle down Jordan Road and exits where Victor Avenue leaves the map. The hills to the west are the southernmost part of the Piedmont block of Franciscan rocks. Rettig canyon cuts right through the hills thanks to Peralta Creek, which comes here from Butters Canyon on its way to the bay through Peralta Hacienda and Foothill Meadows Park.

Normally when you see a stream cutting through a bedrock ridge, you explain it as either stream capture or a water gap. That is, either the stream eroded its way headward through the ridge or was running that way already when the ridge rose underneath it. Given the intense tectonic activity here, I’m inclined to call it a water gap, as I do Dimond Canyon (with the addition of tectonic stream capture).

I saw some possible evidence of this in the streambed. But first, a look at the scene.

rettig road

Rettig Road is a single lane through the canyon and is coned off as a landslide zone. It’s been that way for at least six years; I hope a local will say more in the comments. The canyon is steep, dark and thickly wooded. You can scarcely see the stream, but you can hear the water everywhere.

rettig canyon

But there is a place to scramble down to the streambed. It’s well populated with rocks that appear to be local Franciscan melange, pretty jagged and hence not transported far.

rettig streambed

I was looking for bedrock and found some candidates like this scaly schist. I didn’t have my hammer and was reluctant to disturb the scene anyway, so I can’t say much about it. It might be serpentinite.

rettig schist

This is the outcrop that excited me, showing what looks like a thrust contact.

rettig contact

Ignore the green patches; that’s just algae. The rock on the left is fairly soft and foliated parallel to the contact. I picked out a small piece and can’t say much about it, but in the hand lens it looks like a highly altered talcy kind of stone. At the base is a good centimeter-think layer of nice gray clay, then we hit clean tightly packed sediment with highly tilted bedding; indeed it’s tilted steeper than the contact above it. So my best guess is that it may be the contact between the Franciscan and much younger Pleistocene sediments. Due to squeezing along the Hayward fault, the older rock has been thrusted up and over the sediment. This isn’t unheard-of, but I haven’t seen it documented around here so I could easily be wrong. But that would explain the rising ridge, the topography of Jordan Road (which sits in a long trough here that may well be a sag basin) and the course of the stream.

I couldn’t resist bringing home a pocket-sized cobble of beautiful actinolite schist.


4 February 2011

Along the south shore of Chabot Reservoir, in the mudstone of the Joaquin Miller Formation, I spotted this exposure of tiny, enigmatic mineral crystals last year.

joaquin miller formation

They’re sprinkled across what looks like a bedding surface in the mudstone. This shot shows an area just a couple inches across. Because I was just taking a walk, I didn’t have my hand lens with me and I didn’t try to bring some home, so this is all we have to work with until I, or someone else, finds the spot again.

A medical maxim that’s just as useful in geology is, “where you see hoofprints, look for horses and not zebras.”

Most of the time, large grains in a mudstone are quartz. The crystals are elongated and appear to be prisms with points, just like quartz. But the honey color appears to be part of the mineral and is most unusual for quartz. Any quartz in this rock would be detrital, not authigenic—by which I only mean that quartz would not grow here, but would come in with the other sediment that turned into this rock. And if it were detrital, it wouldn’t have this clean sparry shape. Instead it would be ordinary sand grains. So I’ve ruled out quartz.

My working hypothesis is that these are calcite crystals in the typical “dogtooth spar” shape that have grown here, or very near here. I could confirm that with a quick acid test, and one little tiny piece of a small puzzle would be clarified.

More Rockridge Rocks

21 November 2010

Lately I’ve returned to Upper Rockridge in search of the elusive Rockridge Rock. On Bowling Drive I captured this view of Cactus Rock, which perches high above Acacia Avenue. (A reader took the same photo in July 2008.)

cactus rock

Cactus Rock is impressive, but I still have trouble believing that picnic parties would come up there regularly; it’s 1000 feet and almost a 200-foot climb from Broadway Terrace. The climb to “Mount Ararat” would have been shorter; but I still lack evidence that a large rock sits up there.

That leads me to 5920 Broadway Terrace, where the owners have recently finished a major upgrade to the grounds revealing some very impressive rocks. (A commenter mentioned it last year.)

broadway terrace rock

Click the photo for a larger view. You can’t quite see it, but the rock beneath the porch now has a fountain installed that spills in a waterfall over it into a pool. Could this house be sitting on the original Rockridge Rock?

Chabot gorge

24 January 2010

As you walk up to Chabot Dam the valley carved by San Leandro Creek opens up below, and it’s quite impressive (click photo for bigger version):

chabot gorge

At the top, the dam is buttressed against a ridge of the volcanic rock of Leona Quarry. Before the dam was built, there must have been quite a defile here:

chabot dam

The rock is nicely exposed just next to the dam, worth taking a close look at:

chabot rock

The gorge is more than 100 meters deep and only a couple hundred meters wide. Like other Oakland stream valleys, it seems large for the stream running through it today. I think much of the hardest work carving the canyons was done during periods of much wetter climate than today. Or maybe it was drier, with the land being more prone to desert-style flash floods.

What makes the Bay?

28 September 2009

san francisco bay

We all admire the Bay when we visit high places, whether it’s here at Dunsmuir Ridge (click for a 2X version) or elsewhere in the Oakland Hills. But why is there such a big basin here?

One ready answer comes when you contemplate the major faults, the San Andreas and the Hayward, that bound the bay on the west and east sides respectively. They aren’t quite parallel, but fan slightly apart as you progress to the north. As the crust moves along these faults, the part between would sink, just like what happens at the smaller scale when a sag basin forms. Maybe it does. Why, then, are the Oakland Hills still rising from compression across the Hayward fault? Well, maybe things shift direction slightly from time to time and the sense of stress across the fault changes. So today we have compression while other times we have extension. But maybe the analogy of sag basins is the wrong one.

Geologists have the same question when we consider the Great Valley, that vast trough between the Sierra and the Coast Range. Most sedimentary basins are depressed by their load of sediments, but not to the point of lying at sea level like the Great Valley. The current working hypothesis about the Great Valley is that it’s a captured slab of oceanic lithosphere, a dense bottom layer, with thick sediments on top. It naturally, persistently rides lower than the continental rocks around it. Might the San Francisco Bay basin be a chunk of the same slab? That’s one hypothesis; we don’t really know.

The roots of the Bay are pretty deep for active-source seismic imaging, the kind of technique used at the Garrido property in Antioch to study a crime scene. The Bay is also pretty small for the passive-source seismic studies used to study the crust and mantle at the regional and continental scale. If you gave me a few million dollars, I could make a stab at an investigation, but no one can spare that for an idle question. So we’ll keep wondering as we stand on the high places.

Cemetery Creek canyon / Moraga Canyon

18 August 2009

cemetery creek canyon

The valley of Glen Echo Creek, “Mountain View Valley,” is much larger than the current creek has the power to excavate. This is especially apparent at its head end, above Mountain View Cemetery where the creek is also known as Cemetery Creek. In this view (click it for a larger version), the valley is a steep-sided, tree-filled dogleg, almost a gorge, with homes perched high above a landfilled floor and Moraga Avenue running through it. I think of it as Cemetery Creek Canyon. The Google map below gives a feeling of how odd the canyon is, almost a box canyon with no sizeable watershed to feed it.

UPDATE: By the way, a story in the paper referred to this as Moraga Canyon, so I should probably call it that too, even though Moraga itself is way over the mountain. Once upon a time, Moraga Road did go to Moraga.

I wonder if, like Dimond Canyon, there is an explanation that involves a beheaded watershed from movement on the Hayward fault. The rock at the valley head, above the narrowest spot at Maxwellton Road, is Franciscan mélange, which is not an especially rigid rock. A bit of compression across the fault might squeeze it up, like clay in the hand. I don’t have a way to test this hypothesis, with rock exposures so scarce there, but the idea tickles me every time I look here.

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:


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:


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.


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.


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