A peek inside the Fan on Piedmont Avenue

19 February 2015

Construction is going on at the lot formerly occupied by a well-behaved motorcycle club, at 4225 Piedmont Avenue by the Kona Club. What caught my eye is that it offered a clean cut into the stuff that constitutes lobe 2 of the Fan.

I’ve referred to the Fan often over the years, but I haven’t formally introduced it. Here it is on the Oakland geologic map.

the-Fan

It’s a former alluvial fan that was last active during the Pleistocene, which has been dissected by several younger streams. There’s nothing else quite like it in the East Bay, and I think of it as the Fan with a capital F. I divide it into eight separate lobes. Lobe 2 has two separate parts, Pill Hill and Montgomery/Thermal hill. Anyway, I keep an eye on it because it’s rarely exposed. Only excavations and a few stream banks display it.

Here’s what it looks like from a distance.

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There’s indistinct bedding that slopes down to the left. The material is gravely clayey sand that’s quite firm and well behaved. Here’s a closeup of a gravely layer; the stones are large pebble size (about 50-60 millimeters) and represent the Franciscan rocks just uphill in the Piedmont block.

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Farther over, the wall of the excavation has been carved with a backhoe, and the clayey matrix is so strong that most of the stones have been cut in two, even the tough black argillite.

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This is alluvium—sediment carried and laid down by streams. The rock clasts are rounded, showing that they’ve been carried in a stream for some distance, although most of the rocks are sandstone that doesn’t endure long. The hardest chert pebbles are still pretty rugged.

Down on the ground was this very typical Franciscan chert boulder, shattered by the builders after enduring for more than a hundred million years.

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The lot will become a nice set of dwellings. The builders are blogging about the job, complete with cool drone shots.

Seminary Creek at Mills College

14 February 2015

The third creek running through Mills College is Seminary Creek. It gets its name not just from Mills, but from the Beulah Heights district that forms its headwaters. On the 1897 USGS topo map it’s the dashed blue line, signifying an intermittent stream, running due south to East Creek (now named Lion Creek).

mills-SemCk-topo

Nowadays it’s almost entirely culverted. Not only that, it’s been kidnapped! The Oakland Museum of California’s creek map shows the creek as being redirected away from Lion Creek. Today it runs just north of Seminary Avenue to a channel called East Creek Slough.

lion-seminarycreekmap

What glory the creek still has today is evident only on the Mills College grounds. It ducks between lobes 6 (Maxwell Park) and 7 (Mills) of the Fan next to MacArthur Boulevard, but a close look at the topography suggests that a landslide or earthquake could easily have made it spill to the east of lobe 7 (past the triangle marked “30” on the stream map). Be all that as it may, the creek daylights just where MacArthur curves west.

mills-SemCk-intro

It then wanders a couple hundred yards through a nice quiet forest, mostly eucalyptus. Other than putting a parking lot over half its course, the college appears to have left Seminary Creek alone.

mills-SemCk

When it hits MacArthur, at 57th Avenue, Seminary Creek disappears. Here’s the last sight of it, from the MacArthur side. Here’s where the stream got kidnapped.

mills-SemCk-outro

I’ve taken photos of the creek farther downstream, but it’s a nondescript ditch and now I feel sorry for it. So I’ll spare you.

Interestingly, the old map showed the creek as intermittent, but I’ve never seen it dry even after our three-years drought. As I mentioned about Chimes Creek, I think undergrounding Seminary Creek has kept it from evaporating.

Chimes Creek and the Hayward fault

7 February 2015

Chimes Creek is the second of the three streams in Mills College. It is said to get its name in reference to the college’s church bells. The sound would have traveled up the creek bed to the meadows behind Millsmont ridge. Today the freeway noise drowns them out. Here’s how it looked to the mapmakers of the U.S. Geological Survey in 1897—it’s represented by the dashed blue line in the middle. Below that is the same patch of land in Google Maps as of today.

chimescreekmap1897
chimescreekmapnow

The land has been changed substantially in the last 118 years, but the creek continues to drain its catchment. Let’s look at the changes from the top down:

  • The headwaters have been filled and paved and are now occupied by Viewcrest Drive.
  • The Leona Quarry removed all the overburden below a short stretch of the upper creek, exposing bare rock.
  • The flats beneath have been leveled and developed, and the creek is culverted.
  • Seminary Avenue has been widened and straightened, putting more of the creek underground.

All of these changes have added to the runoff seeking to enter the creek while constricting its course. A stream will respond by running higher and faster and eroding its banks.

I haven’t yet visited the highest part of the catchment. Here’s a look at it down Altamont Avenue.

chimescreek1

The original creekbed is high above the left edge of the quarry, and the creek ran toward the lowest part of the foreground. Next is the view one block over, at Delmont Avenue and Hillmont Drive looking north. The creek comes out of its culvert behind the houses on the left.

chimescreek2

I should note that the Hayward fault is mapped running right up the valley to this spot. That’s an important detail that no one seems to acknowledge. For my purposes in this post, it means that Chimes Creek is probably cutting downward through fault gouge, the finely ground material that faults make all over California.

Farther downstream, this is looking across the creek valley at Nairobi Place. The sides are quite high here because the stream cuts downward rather strongly.

chimescreek3

The presence of the Hayward fault also explains why the right (opposite) bank of the creek valley is elevated above its surroundings—it’s not a levee, but rather a pressure ridge. Farther downstream along Oakdale Avenue, the valley is at its deepest.

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The lots along Hillmont Drive, across the creek, are being undermined as the invigorated stream does its work.

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I’ve love a good look at this material, but I’ll probably never get the chance. The geologic map shows this area as the northernmost splinter of the San Leandro Gabbro.

The creek enters a culvert under Seminary Avenue here . . .

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. . . and emerges here on the grounds of Mills College for a couple hundred feet. Then it enters its last culvert and joins Lion Creek underground.

chimescreek-mills

The Chimes Creek Neighbors site has thorough documentation of the human squabbling over this much put-upon watercourse. The neighbors know it as a permanent creek, although the 1897 map showed it as intermittent except for its lower reach on the Mills College campus. I suspect that the land-use changes of the last century have turned it into a permanent and more powerful stream.

Some East Oakland stones

31 January 2015

Oakland’s rocks aren’t all in the ground. They’re in our yards and homes, too. Here are a few presented in the order I found them lately.

There’s a house on 60th Avenue that stopped me in my tracks, its walls studded with stones. A neighbor down the block told me “Oh yes, those are wonderful! We looked at that house when we were buying in this neighborhood. The owner’s daughter runs a preschool across the way.” Click that one for an 1100-pixel version.

morse-at-60th-450

A few weeks later I visited Best Avenue, high in Maxwell Park, and was arrested by the front yard here. Sometimes rocks, like people, look best with painted faces.

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A block away is another property treated by the same decorator.

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And just yesterday this basket of painted stones seized me by the eyes. It’s at the Free Oakland UP gallery and workshop, in the Loard’s plaza at Coolidge and MacArthur.

oakland-up-rox

Lion Creek in Mills College

29 January 2015

Lion Creek was once an Oakland landmark, named for the mountain lions and their habitat high in its catchment. Today it sneaks its way, mostly underground in culverts, to the bay. Didn’t use to be like that. Nowadays, the only place the creek is cherished and well cared for is on the Mills College campus. Here’s a brief tour from top to bottom.

At the top (west) end of the campus, Lion Creek feeds Lake Aliso, as I mentioned a few posts back. I was astonished on my last visit to find that the lake has been drained! This is the view from the dam. It’s a temporary calamity for the lake, but now the creek is visible.

aliso-drained1

I’ve been trying to reach someone at Mills to tell me the story without success. Maybe one of you knows. In this close-up, you can see that the water is yellow, I assume from the rehabilitation work going on upstream at the former sulfur mine.

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And maybe the lake is drained just to let that crap wash out to sea.

The creek is, to my mind, the aorta of the campus, the way Strawberry Creek is at UC Berkeley. The old oaks and bay trees are still in place as the creek meanders graciously west.

mills-lioncreek

Just before the old west entrance, the creek is culverted, curving left past a small lobe of the Fan on its way out.

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And that’s it for Lion Creek, except for a little exposure along 64th Avenue by Bond Street, until it hits the old Coliseum Gardens property in its remade bed.

Pine Top, Mills College

20 January 2015

At the back of Mills College is a steep little hill called Pine Top. It looms especially high over Lake Aliso. The geologic map shows it as consisting of Jurassic basalt.

mills-college-geomap

As I walked up Pine Top Road to reach it, I saw only what looked like nondescript sandstone, but I didn’t look closely. Rocks may be altered, and the Hayward fault running through here is notorious for swapping splinters of rock from one side to the other, so who knows.

As you round the hill, the view opens across Seminary Avenue, which runs up the valley of Chimes Creek, to Millsmont hill.

pinetop1

At the top is this big old stone stage, where it’s easy to imagine the young women of Mills assembling for all kinds of ceremonial occasions.

pinetop2

What’s harder to imagine is the view that this place once commanded. All the trees are so mature, both pines and eucalyptus, that it’s frustrating.

pinetop3

On the other hand, people live on all sides now, so privacy is more important. And the freeway behind the hill is so noisy that any ceremony you could think of would be spoiled by the din.

As I said, there was no obvious evidence of basalt exposed along the road to Pine Top. So what was this lump of conglomerate doing there behind the stage?

pinetop-cgl

Just another reason to come back and poke around the flanks of this hill.

The Mills College high-grade blueschist block

14 January 2015

Longtime commenter artisancrafts reminded me, in a comment to my last post, that there’s a nice exposure of blueschist at the north edge of Mills College. Yesterday I easily located it by following his/her directions. First there’s the old railbed that once ran to Laundry Canyon. This stretch of it, which once continued down through the Fan parallel to High Street, used to be part of Courtland Street.

mills-railbed

At the spot in the distance where the sun shines across the roadbed is this lovely exposure, about 2 by 3 meters.

mills-bluschist1

It’s clearly worn by water, and a little concrete-lined ditch running along the uphill side of the roadbed feeds it. Following it upstream takes you right up to the 580 freeway abutment, where it veers north in a culvert. The stream is a mystery that I won’t try to solve today.

Back to the rock. Getting closer to it, you may not believe your camera. Clear blue skylight can do that.

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This closeup, showing tightly folded lamination in the cleft on the right edge of the first shot, is a truer indication of its color thanks to my camera’s flash.

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It’s classic blueschist, the largest outcrop of it I’ve seen in Oakland. Let’s call it blueschist-grade melange, what’s usually referred to in Franciscan circles as a high-grade block, and I’m very pleased to know that we have one in town.


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