31 January 2014
You may think of Snake Road as the ridge road that climbs the crest between Thornhill and Shepherd canyons, but it’s really Colton Boulevard. For a quiet hike above Montclair with views, you want Colton. Its lower part runs north of Snake across ground underlain by the Redwood Canyon Formation. Here’s a roadside outcrop.
I didn’t even notice the parallel fractures until I opened the photo at home. They probably represent a bit of extra cementation, thanks to a late pulse of fluids, but they may also be original bedding.
I mentioned views. If you loop around Mendoza and Mazuela Drives you can view some immaculate houses and grounds, but I like the big empty lot on Mazuela that overlooks Pinehaven canyon; click it for the bigger version.
28 January 2014
The Lincoln Square shopping center is a little neighborhood-scale set of shops on Redwood Road next to Route 13. It’s not very natural but it has some interesting natural features. Here’s the topography in Google Maps.
The graded area sits across the small valley of uppermost Lion Creek, running due south from top center. (A second branch of Lion Creek is to the west cutting through Holy Names University.) Its east edge is a cut into the hillside, exposing a bunch of serpentine rock. It’s the little strip of purple on the geologic map of the same area.
You can see the rock next to the parking lot . . .
. . . and in more detail behind the Safeway and the other building full of shops. This exposure is quite spectacular, but I was just doing a reconnaissance and didn’t linger.
I was actually visiting here to look for signs of the aboriginal hematite workings. This is where the local tribes came to dig Oakland ochre. This is as close as I got to that, a boulder rich in iron oxides along the north driveway entrance.
I have only the most preliminary ideas about this area. The map classifies this area as Franciscan sandstone, and this boulder doesn’t contradict that. There are other brief nods to the original landscape studding the parking lot, but on whole it’s pretty sorry-looking.
My idea is that in this part of the world the development of ochre requires serpentinite and a suitable host rock for the oxides to grow, and that the process happens underground at the base of the soil. It takes careful excavation by nature to reveal this fragile material without washing it away, and Lion Creek and the Hayward fault (on the left edge of the map) combined to do that here.
18 January 2014
If you’ve read this blog over the years you’ve seen me talk about the Piedmont block, a big hunk of Franciscan rocks riding north along the western side of the Hayward fault. Its easternment end tails off in a narrow wedge of undifferentiated rock, shown here in the geologic map.
The next three photos are taken from the locations marked with numbers. That’s 35th Avenue there, right at the curve in the road where it becomes Redwood Road. The curve is where the fault crosses the road, too, so it’s an apt place for the change of name. Just below the bend is this roadcut in hard bedrock. It’s mapped mostly as the material labeled KJf, undifferentiated Franciscan, on the geologic map plus some of the volcanic rocks (Jsv) exposed in the Leona quarry.
I don’t know how old the roadcut is. The road hasn’t changed course since the 1800s, but I guess it was widened in the 1960s or so, because the map base shows the split roadway in purple, meaning a recent change of the same vintage as I-580’s construction. Perhaps the road had a hump in it as it crossed the ridge. Above the bedrock ridge is a small valley with Jordan Road in it, shown below. The homes on Victor and Herrier Streets are visible on the Franciscan ridge beyond this swale (especially in the big version if you click on it).
A bit to the north, Peralta Creek runs into this swale (mapped as a sag basin related to the fault) and then cuts through the ridge in Rettig canyon. I can see the swale filling with water and emptying over the millennia, perhaps occasionally down Cortland Creek past the south tip of the Franciscan spike, as earthquakes and landslides rearranged the topography. The fault is mapped right at the intersection of Jordan and 35th on the west side, but I’ve never seen any evidence of creep there.
The roadcut, according to the geologic map, should expose two kinds of bedrock. It’s covered with boulders of basalt or greenstone, presumably quarried from the spot.
Bits of bedrock peek through, so it ought to be possible to trace the contact between the two rock types. That’s on my list of projects.
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.
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.
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.
The block’s surface is festooned with slickensides:
And the sheared matrix is also well exposed on its upper side.
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.
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.
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.
I can’t pronounce this a fault trace, but it’s consistent with one.
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.”
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.
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.
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 slide. The site of my photo is at the “J” on this portion of the Oakland geologic map.
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.