Archive for the ‘oakland geology walks’ Category

Rocks of the Skyline High neighborhood

6 July 2015

Walking the hills around Skyline High School makes for a nice workout, and there are some rocks, including serpentinite, the Joaquin Miller Formation and the Oakland Conglomerate. Here’s the Google Maps topography with the photo locations.

skyline-redwood-topomap

And the corresponding geology.

Skyline-redwood-geomap

The neighborhood centered on Balmoral Drive has lovely views, but few visible rocks. This is looking west to the Serpentine Prairie and beyond.

Balmoralview

But at the very end of Tartan Way we behold some mixed shale and mudstone of the Joaquin Miller Formation (Kjm).

JMF-tartanway1

JMF-tartanway

The area is mostly underlain by the Oakland Conglomerate (Ko), but the only glimpse of it you’ll see is on the high school grounds behind a fence.

Ko-SkyHigh

I think that the rocks here were examined during the construction of the homes in the late 1960s(?), because the strike-and-dip symbols are all located off the road, presumably in the excavations.

Now we’ll cross Skyline and explore the loop formed by Fernhoff and Bacon Roads. I particularly wanted to see this because it exposes the last bit of Oakland’s serpentinite I hadn’t yet visited. And at the entrance to Fernhoff Court, here it is.

serpFernhoffCt

The westernmost appendix of Bacon Road also dips into it.

serpBacon

But just uphill on Bacon, there’s some nice bits of Joaquin Miller again.

JMF-Bacon

The best place to see the Oakland Conglomerate is on Skyline Boulevard, specifically along the footpath in the median. Some places it’s well-bedded sandstone . . .

Ko-Skyline2

. . . and other places expose the classic deep-brown conglomerate with its well-rounded river cobbles.

Ko-Skyline1

There’s more along the roadside, but that’s probably not safe to visit. People drive fast here.

Ko-Skyline3

Thank goodness for the path in the median! It’s a nice amenity.

Shepherd Canyon: Type localities of Oakland rocks

29 June 2015

Yesterday I led a walk for the group Wild Oakland that took in the rocks of lower Shepherd Canyon, which are the westernmost outcrops of the Great Valley Sequence. These are the same kinds of rocks that make up the monumental set of ranges marching up the western side of the Central Valley from Taft to Redding. The map below shows our planned route. The red dots mark the beginning and end of the route plus mileages. (In fact, for lack of time I cut the walk short where the 3-mile mark is, so we didn’t see the loop on the right side. I leave that as an exercise—and it is exercise—for the reader.)

shepcynwalk-topomap

This is the geology along that route.

shepcynwalk-geomap

The starting point, where we climbed up to the unpaved start of the Montclair Railroad Trail, offers a nice view over the valley of the Hayward fault, here at Montclair Playground . . .

wildoakshep1

. . . and looking northwest up the fault valley.

wildoakshep2

This outcrop, above the curved cut in the railbed, shows the Oakland Conglomerate to advantage.

wildoakshep3

And maybe 100 feet away, the rock abruptly changes to shale of the Shephard Creek Formation.

wildoakshep4

This spot corresponds to the symbol on the geologic map with the number “73” on it, which means that the bedding here is tilted 73 degrees from the horizontal.

Farther up the valley, we examined this outcrop of the Redwood Canyon Formation.

wildoakshep5

I pointed out the thin set of shale beds running up the center of the image and showed how the sandstone beds on the left side had been laid down on top of the shale—that is, the internal evidence shows that this whole set of rocks here is tilted up beyond vertical and is upside-down. This spot corresponds to the symbol on the geologic map labeled “78”.

The last spot is in the Shephard Creek Formation where a large sandstone bed sits amid the shale. The location is just about where the word “Park” is along the walk route. On the underside of that sandstone bed is a splendid set of sole marks. This shot shows how the underlying shale is bent by the pressure of the overflowing sand avalanche that built this sandstone bed.

wildoakshep6

And this shot looks up at the underside. When there are a sufficient number of these marks, the geologist can work out what direction the avalanche flowed.

wildoakshep7

As I said, we cut the walk short at this point and came down through Shepherd Canyon Park along this stream valley, which is filled with a peculiarly flat deposit that I strongly believe is landfill. It forms the higher terrace in this view looking back from the soccer field.

wildoakshep8

Does anyone know the history of this piece of land?

If anyone would like a copy of the handout I prepared, I’ll send you the doc file. Just write to geology at andrew-alden dotcom.


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