Archive for the ‘oakland geology puzzles’ Category

Crestmont hill

20 January 2013

The Crestmont neighborhood is centered around this hill, the one on the left. We’re looking north at it from King Estates Open Space. It’s the highest bit of land in Oakland west of the freeway.

crestmonthill

Crestmont hill stands just over 500 feet high, with Crest Avenue running along its crown. The hill is mapped as the same stone as in the old Leona Quarry, which you see on the right in the background: volcanic rocks of the uppermost part of the Coast Range ophiolite. I say “mapped as” because I haven’t seen a bit of bedrock on it. Perhaps building excavations uncovered it.

The ridge runs south, beneath my feet and beyond past Fontaine Street, where Crest Avenue picks up again. But everything south of Crestmont hill is mapped as sediment instead, an older unit of alluvial-fan roughage. The gravel I’ve seen on its upper surface looks like chert of the Claremont Shale. I guess I’m rambling. This area puzzles me. How much of its shape is due to warpage by the Hayward fault, which runs parallel to this ridge just west of the photo? How old is the alluvium, and what stream delivered it here? How much of the map is real and how much is extrapolation? It is likely that my questions are unanswerable.

Longridge loess

21 August 2012

I was walking up Longridge Road and spied an excavation, where a homeowner was replacing some water lines and renewing a driveway. Naturally, I sidled over and took the rare chance to look beneath the skin of Oakland’s Pleistocene fan. The material was massive—unbedded—and clean. I pried off this little piece . . .

longridge

. . . and nibbled on it. It was firm, but crumbled like Necco wafers and turned creamy on the tongue with just a hint of grit. Not sticky or chewy with clay. Not indurated like hardpan. No sand or pebbles to be seen. The more I thought about it, the more peculiar this sediment seemed, until I had a wild surmise.

Alluvial sediment is never very well sorted, because it’s carried short distances and laid down by streams. Longridge Road is, as the name suggests, a ridge road running up the crest of a ridge between parallel stream valleys along Trestle Glen and Mandana roads. The crest of a ridge should not be made of this fine silt. But it’s downwind from downtown, which is Pleistocene sand dunes (the Merritt Sand), cousin to the dunes of San Francisco. Dune sand is very fine sand, and the fraction that blows away from the sand is finer still. So my wild surmise is that the fan, at least this part of it, is dusted with a layer of windblown glacial silt—i.e., loess. It’s remarkable stuff, and something I never expected to see in Oakland.

The Holy Names hematite workings

29 November 2011

On the grounds of Holy Names College is a locality where the locals, before the Spanish moved in, would find and process the red mineral pigment of hematite.

hematite site

Today it’s the setting of a toddlers’ playground, without a sign of its former prestige. Nearby is Oakland historical landmark 51, the George McCrea House and Indian Campground. McCrea was the prominent architect who designed the house, and I find nothing online about the Indians. I don’t know if these boulders are part of the historical landmark, but they aren’t being treated like one.

I visited the site a couple weeks ago accompanied by a rockhound and a geologist. The boulders have numerous pits, much like the ubiquitous mortars where the natives once ground acorns.

hematite

The material making up the boulders appears to be ancient colluvium cemented by abundant iron oxides. This cementation would not happen at the land surface. They sit on a shoulder of land near a deep ravine of the Lion Creek drainage, evidently exposed by erosion.

The site is not far from the former sulfur mine in the hills above Laundry Canyon, and I was told that other ironstone boulders occur in the neighborhood.

Hematite is an excellent orange-red stain, useful for face paint and similar decoration. There isn’t much around here.

Secret streets

10 June 2011

You’d think that Oakland has erased all of its dirt roads, but one afternoon I found this one, running north after a hairpin turn past the end of Florence Avenue.

florence landvale

Florence ends near a power substation that’s labeled, if I recall it correctly, with the word “Landvale.” Naturally I thought of Landvale Road, the nearly-lost street that has been wiped out by Route 13. Is this the lost south end of Landvale? Or is it a leg of the old Oakland & Antioch Railroad bed that runs through Montclair? If my calf ever heals, I’m heading back here.

UPDATE: I visited the Cal Library’s historic topo maps site and found the road mapped as the railroad grade in 1947, before the Warren Freeway was put in, and shown as the “old railroad grade” in 1959. For now, I guess it’s a decidedly informal walking trail for the neighborhood—certainly a long walk to get there from anywhere else.

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.

Crystals

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?


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