Serpentine at Coolidge hook

7 January 2014

At the top of Coolidge Avenue, the road hooks sharply to the left just below the grounds of the LDS Temple. Right there is a nice exposure of serpentine rock.

coolidgeserpcrop

Its green color and scaly appearance are obvious and unlike the brown sedimentary rocks downhill from here. Serpentinite is not mapped here, so make of that what you will. I tend to assume that serpentinite is likely anywhere along the Hayward fault (which is a about a hundred meters uphill from here, running through the temple parking lot). Take a closer look at the rounded blocks amid the scaly matrix.

coolidgeserpblock2

This is the block at upper right in the photo above. It gets its rounded look from being rotated and scrubbed during lateral shear, like a lump of cold butter in piecrust. Here’s the block to its left.

coolidgeserpblock

The block’s surface is festooned with slickensides:

coolidgeserpslick

And the sheared matrix is also well exposed on its upper side.

coolidgeserpfringe

The views in the other direction are great, too. This is pretty much the view you get from the temple grounds, and unlike the temple it’s always open to the public.

The crack by the temple

26 December 2013

The Oakland Urban Paths walk last month took us along a stretch of the Hayward fault that included the Oakland LDS temple. Here’s that segment of the fault on the map.

LDSfaultmap

The temple is just below the dashed line across the map (which marks the 26th kilometer from Point Pinole). The codes refer to geomorphic (G) evidence of the fault, G1 being “strongly pronounced”: a linear valley (lv) and a notch (n). Less obvious are linear scarps (G2, sl) and a scissor point (G3, sc). The circle labeled C2 marks a measured feature that documents creep, a surveyed offset (so) documented in report number L91, which was a 1991 article in the Journal of Geophysical Research.

Knowing all that, I had my eyes peeled as we went through the temple grounds. This is right by the front gate between the grounds and Maiden Lane.

faultLDS

I can’t pronounce this a fault trace, but it’s consistent with one.

Incident at Fontaine

11 December 2013

I want to say at the outset that I am not a licensed geologist, only a writer with a degree in the field. But when I read in today’s paper about a ruptured gas line in East Oakland that started a fire at the intersection of Golf Links Road and Fontaine Street on Tuesday, this was the first thing that came to mind: the Hayward fault. The intersection in question is just to the right of the word “Viejo.”

fontainefault

Perhaps those of you who accompanied me last year on a tour of the Hayward fault in this area thought the same thing.

The paper reported, “An investigation into what caused the fire was underway Tuesday and could take several days to complete, according to PG&E.” Let’s keep an eye out on what they report.

The Kitchener scarp

30 November 2013

The recent walk by the Oakland Urban Paths group took us past a catastrophe I hadn’t seen before: the landslide of 15 January 1970. It removed nearly all the homes on the east side of Kitchener Court, just south of the LDS Temple, and dumped the ground into the valley of upper Peralta Creek. The land is still empty and uninhabitable. Here’s a look south over the scarp from Kitchener.

kitchenerscarp

The slide wiped out the middle of London Road between a tiny stub at the top of Maple Street and the forlorn end trailing off of Maiden Lane. The Hayward fault is mapped right through the foot of the slide. The site of my photo is across the pink strip opposite the “J” on this portion of the Oakland geologic map.

kitchenergeomap

I frankly can’t vouch for any of the bedrock divisions shown here, but the dashed line of the fault is close enough to reality. For orientation, here’s the equivalent area in Google Maps. Rettig canyon, where Peralta Creek cuts through the bedrock ridge of Leona keratophyre (pink) and mixed Franciscan rocks (KJf), is in the patch of green at lower center.

kitchenergoogmap

39th Avenue fault gauge revisited

27 November 2013

2-1/2 years ago, I presented a photo of a cut mark in the curb of 39th Avenue where the Hayward fault is mapped. This month I happened to visit the spot during an Oakland Urban Pathways walk, and I took the opportunity to take a new photo.

39th-faultgauge-2013

It has moved slightly, just a few millimeters, in the intervening time.

The U.S. Geological Survey monitors the fault closely through Oakland. They don’t measure this mark, or if they do it’s not definitive. The definitive survey is along a longer line across the fault, because the fault movement isn’t limited to a perfectly thin geometric plane. Their measurements show that this part of Oakland is creeping approximately 4 millimeters per year. Heck, here’s a good source, from a 2000 paper by the USGS guys that was published in Geophysical Research Letters:

HFcreeptable

The authors note that Oakland has a relatively slow rate of creep, and they interpret that as a sign that the fault here is more extensively locked than it is elsewhere. The area and degree of locking bears directly on the energy the fault is capable of releasing. Mind you, we have over a decade of new data and new thinking since that paper was published, but the data is sound.

Huckleberry saddle

18 November 2013

The entrance to Huckleberry Botanic Regional Preserve is at a low point in the spine of the Oakland Hills, where the steep eastern arm of Thornhill canyon has cut headward toward the equally steep canyon of San Leandro Creek. Both canyons are steep because the rock between them is the tough Claremont chert. The saddle between the two canyons provides good views to the west and east, and a little path (labeled Huckleberry Path on the map) leads north from the Huckleberry entrance, a private entrance to the preserve for the residents of Elverton Drive. That’s where I got these pictures, first looking southwest toward downtown:

elvrtn-lakemerit

and due west, over the opposite wall of Thornhill canyon, toward the Golden Gate.

elvrtn-ggate

Turn around, and it’s just a few steps to find these views of the Las Trampas Ridge area—

elvrtn-trampas

—and Mount Diablo, which never ceases to surprise me when I see it from Oakland. I think of Diablo as a whole different domain, reachable only by a drive through the tunnel and not visible west of Lafayette. But here it is, looking downright neighborly.

elvrtn-diablo

Only a few Oaklanders get to see the view east; we’re a westward-leaning city.

The Oakmore block

7 November 2013

The Oakmore district is quite a distinct part of town. Here’s how its geology makes it so. I’ll call it the Oakmore block, although that’s a bit of a misnomer—it’s the eastern end of the Piedmont block.

oakmore-geomap

The neighborhood is defined by the light blue of Franciscan sandstone of the Novato Quarry terrane, bounded by three canyons. The canyon on the northeast side is occupied by Route 13, of course, which corresponds to the Hayward fault. Here’s the view across it, looking east from the easternmost corner of the Oakmore block (the end of Braemar Road) to Joaquin Miller Park. The top of the grassy slope is the overlook at Lookout Point.

joaquinview

Dimond Canyon on the west is the most dramatic boundary, but the bedrock is the same on both sides; there is some accident of geologic history that has maintained this deep streamcut. Leimert Boulevard defines that side of the neighborhood. Roadcuts about midway up Leimert expose a lot of strong sandstone like this.

leimertcrop

And on the southeast is an unsung stream valley cut along the edge of the Piedmont block. Whittle creek, I guess I’d call it, because Whittle Avenue runs up it, and the Head-Royce School is nestled in it. The valley grows into a nice amphitheater at its head. This is the view from there—the end of Melvin Road—across to the Greek cathedral and Mormon temple. Lincoln Avenue is the ridge road on the other side of this valley.

oakmore-temples

Over here I see more shaly bedrock, like this stuff exposed on uppermost Fruitvale Avenue . . .

oakmore-shale

. . . and bit more structure here where Wrenn Street meets Hoover Avenue.

oakmore-wrenn-hoover

All of this is expected in the Franciscan. The odd bit I haven’t figured out is near the end of Melvin, which looks to the naked eye like a volcaniclastic rock. Perhaps the boundary on the geologic map is a little off.

oakmore-volc

All I know is that when you take your eyes off the ground and look off toward the Bay, Oakmore is a mighty fine place.

oakmore-view

I don’t know exactly where the realtors put the southern line, but the dotted-line contact running from Sausal Creek along Whittle, marking the hidden thrust fault at the edge of the hills and the bedrock alike, works for me.


Follow

Get every new post delivered to your Inbox.

Join 4,040 other followers