Mine creek at Twitter Court

17 August 2014

I’ve shown you the ugly orange streambed just below the old McDonell pyrite mine. Farther downstream, the creek (which I’ll name Mine creek) emerges from private backyards next to Mountain Boulevard at Twitter Court.

mine-creek-at-twitter

It’s still pretty orange here, from extremely small (colloidal) particles of iron oxide minerals that form as the acid drainage from the mine is neutralized. The creek enters a culvert here and disappears. Somewhere under the Warren Freeway, it joins Lion Creek on its way to the bay. Lion Creek appears next as Lake Aliso on the Mills College campus, and unless the lakebed is all orange too, the pollution has been fully neutralized by that point. As I’ve said before, the pollution looks awful, but without chemical tests we can’t tell if it’s poisonous. Iron oxides by themselves are not a great hazard.

Local boulders in the Calaveras triangle

8 August 2014

Where the Warren Freeway ends in its merge into I-580, most people drive south onto 580 east. The handful of locals or lost drivers who instead take the last exit to get onto 580 west will go through perhaps 580’s most deserted interchange. That’s where the highway builders installed this humble triangle, splitting the freeway exit for the even smaller handful of drivers going east on Calaveras Avenue to Mountain Boulevard and those going west, briefly, on Calaveras and onto the upramp to 580 west. The triangle is paved with river cobbles and populated with natives.

13-to-580triangle

The oak trees are easy enough to see in this view from across Calaveras. But what’s that in their shade? Why it’s one of several large local boulders.

13-to-580trianglerock

I didn’t take notes, but I think these are Leona volcanics, the same stuff that was quarried nearby for decades at the Leona Quarry. Pay them a visit next time you’re walking or biking through that godforsaken area. Sit on them in the shade; they like that.

More local serpentine

1 August 2014

Every now and then I come across a wall or a garden boulder that’s so beautiful I have to take a picture. Such was the case with this serpentinite specimen on or around Huntington Street.

huntington-serp

This is near Oakland’s great serpentine patch, and it likely came from there.

Geranium Place rocks and runoff

27 July 2014

Geranium Place occupies a sloping bit of land just north of Horseshoe Creek below Redwood Road. This map shows the location plus the sites of the photos in this post.

geraniummap

The bedrock here is mapped as Franciscan melange on the west and the Leona volcanics on the east, the same stuff exposed in the huge Leona Quarry just down the Warren Freeway. The rocks I saw were clearly the latter, but you have to take the lines on the geologic map as approximations. If I’d been there fifty years ago when the homes here were being built I might have been able to tell better, while the foundations were exposed. BUT! there is bedrock to look at in any case.

The first thing that caught my eye, though, was the gutters. What’s going on here?

geranium1

Iron-rich runoff like this is not expected from undisturbed land. Perhaps the area was once a borrow pit or small quarry. Across Horseshoe Creek is the big quarrying operation above Laundry Canyon, and just beyond is the notorious McDonell sulfur mine site, so naturally the resemblance to the nasty-looking streambed down there is striking. What etched away the cement in the gutter? Probably not acid drainage.

geranium2

The white crust is another clue. In our deathly dry conditions it might conceivably have been salt, but it had no taste when I nibbled a fragment of it. It’s probably either gypsum or carbonate; without any chemicals handy I couldn’t learn anything more. But a buildup of crystals like this could gradually disintegrate the cement in the gutter. It may also be seepage from the serpentine exposed along and above Redwood Road here. In sum, very hard water here, but probably not nasty water.

The bedrock is heavily iron-stained and chewed up. Being so near the Hayward fault (it grazes the lower left corner of the map area) surely accounts for that.

geranium3

Some of the households here have worked with it to good effect.

geranium4

A closeup is impressive.

geranium5

This is breccia—pervasively shattered rock—that has been abraded by tectonic shearing so the pieces are rounded, as if you took crushed rock and rubbed it between your hands with a giant’s strength. It’s fairly well cemented, not crumbling apart, so this process happened at some depth.

At the northernmost bend of Geranium, up against the highly cantilevered Redwood Road, the ground is empty and there are monitoring wells of some sort. We have deeply disturbed this area.

Courtland Creek cut

19 July 2014

Courtland Creek runs just south of High Street; presumably the valley was a footpath long before High Street was laid out in the 1800s. It has the peculiarity of crossing the old alluvial fan without cutting out a floodplain, as shown here in the geologic map.

courtlandcreekgeo

I visited it a few weeks ago. As you go upstream along Courtland Avenue, this dirt road appears. Dirt roads are always interesting in this city.

courtlandcreekrow

It’s the old right of way for the Key Route line, and it leads to Courtland Creek Park, a cool streamside strip with some understated concrete work meant to evoke the history of the area. At one point there’s some unusually elaborate rockwork leading down to the creek itself.

courtlandcreekrockwork

Farther upsteam, too, is a cut into the side of the Maxwell Park hill; this view is looking back west.

courtlandcreekwall

And at the upper end of the park is one of those excellent mosaic trashcans that make this city so special.

courtlandcreekcan

As I’ve mentioned before, the topography of this part of Oakland, in the Allendale flat, suggests to me that the drainage has switched between streams at various times. It will be fun to poke around here some more.

BTW please see the Q&A page for an announcement.

Casting pond, upper Lion Creek

26 June 2014

One of Oakland’s most beautiful places is tucked in the woods next to the Warren Freeway at Carson Street: the casting pond complex of McCrea Memorial Park, along Lion Creek. Entering the park took my breath away the first time I visited.

castingponds

Lion Creek leaves the grounds of Holy Names University and runs in a steep gorge behind Elinora Avenue, evading the freeway for a short stretch that includes the park. Horseshoe Creek joins it at the south end, and the combined stream enters a culvert beneath the freeway running to the Mills College campus.

This part of the streambed is highly engineered. The 1947 topo map shows an ordinary stream valley here with an intermittent stream indicated, so the wide glade for the ponds was built and the stream shunted aside. Farther downstream are some empty ponds whose purpose I don’t know; perhaps one of you does.

lion-creek-at-mccrea-park

The woods have made the area their own. This was once part of Leona Heights Park, which was cut in half by the freeway, and a pedestrian bridge that may be Oakland’s least-used one connects the two sides.

Hayward fault slickensides

9 June 2014

Here’s an odd curio that I spotted down in Menlo Park at the U.S. Geological Survey campus. During a trenching study of the Hayward fault up at Point Pinole, the active trace was uncovered and this plaster cast was made. It sits by the entrance of the Map Sales Room in Building 3.

HF-slicks

Today we would make a lidar scan of a surface like this to document its slickensides in a precise database. But you have to feel some affection for this low-tech artifact, which presumably has also been scanned to ensure its immortality.


Follow

Get every new post delivered to your Inbox.

Join 4,030 other followers