Archive for the ‘Oakland geology walks’ Category

Rocks of the Chabot Reservoir northside

30 November 2015

The hike on the Goldenrod Trail from the Grass Valley staging area, where Grass Valley Road meets Skyline Boulevard, down to Chabot Reservoir is a lovely walk. On the geologic map below, it’s the dirt road between the two O’s on the right side.

GrassVly-Chabotgeomap

When I walked here the first time, a few weeks ago, the idea was to check out the Franciscan Complex — shown as blue in the geologic map — where it crops out along the lake. On the east side is the Joaquin Miller Formation, which is a straightforward sandstone here.

GrsVly-JM-Fm

And on the west side is the Knoxville Formation, which is a straightforward shale here.

GrsVly-Knx-Fm

Nice rocks: brown, crumbly. Trees like the soils they make. They don’t stand out. In between is something completely different: blue and green metamorphic stuff. You’ll see it in boulder piles.

GrsVly-KJfrox

You’ll see it in knockers.

GrsVly-KJfknocker

And you’ll see it along the lakeshore. The other two formations leave plain old sand, which the birds seem to prefer, but the Franciscan gravel is worth looking at close up.

GrsVly-KJfbeach

The cool weather is a great time to explore this part of town, even if you don’t care about rocks.

GrsVly-Chabotres

The hill here is Fairmont Ridge — its forested back side. If you’re used to seeing it from anywhere else in Oakland, you won’t recognize it. And that’s what makes this a getaway.

Knowland Park: On geologic maps

21 September 2015

I enjoyed yesterday’s Wild Oakland outing in Knowland Park. It was hot, but a gradually freshening breeze made it a dry heat, and we spent a good bit of time in the shade for discussions.

My goal was to explore the relationship between the ground of the actual Earth, in its strange and opaque reality, and the maps that geologists make of it. Here’s the set of places we visited, in Google Earth — we started at the end of Snowdown Avenue.

Knowlandgeomapwalk

There were obvious things, like big outcrops, and subtle things, like the texture of the dirt in the roads, that yielded information to us about the rocks underlying various places as we walked. But that collection of observations wasn’t always easy to reconcile with the bold, definitive-looking patterns on the geologic map (800 pixels; see the same five localities on it).

Knowland-MF-2342

I tried to explain that a geologic map, especially one of Oakland, is as much an act of imagination as it is of observation. The rocks aren’t very well exposed, the different rock units are hard to describe and each one includes a lot of variety.

The Leona “rhyolite” (Jsv) and the Knoxville Formation (KJk), to the extent we could see them, were easy to distinguish after a bit of exposure to them. But the major rock unit we encountered, the melange of the Franciscan Complex (KJfm), is really a meta-rock unit, a mixture of blocks (“knockers” in the local geo-parlance) of very different lithology. It’s a meta-rock unit in the same way you might call a package of frozen mixed vegetables a meta-vegetable. So that’s not an easy concept to grasp, but I think the group enjoyed getting a taste of the subject.

Notice that almost all of the contacts between different rock units on the map are shown as bold, dashed lines. These all mark faults — fractures where the rocks on either side have been displaced — and none of them are visible on the ground. They are inferred. We’re sure they’re there because our knowledge of rocks in general, and these rocks in the Bay area, leads us to that conclusion.

That may seem like arm-waving, and it is. Geologists have a joke that the way to make us shut up is to tie our arms down. Geology, more than most branches of science, is a tentative discipline. Geologists hold that tentativeness close. Consider how this area was first mapped 100 years ago by Andrew Lawson, professor at Berkeley and highly regarded then and now. The excerpt below is from USGS Geologic Atlas Folio 193, published in 1914. (800 px)

Knowland-Lawson1914

It’s barely recognizable. We’ve learned a lot since then, but there’s no guarantee we know it all, and geologists of 2115 may have a sympathetic chuckle at our mixture of certainty and puzzlement today. Someone asked me what has changed in California geology since John McPhee wrote about it in Assembling California forty years ago, and I said the basics are still sound, but in some important topics our ideas have changed greatly. In the progress of geology there is no prospect of an end.

The Skyline median trail

3 August 2015

My latest outing took me to what I’ll call the middle part of Skyline Boulevard, the part where the road is divided. Starting at Skyline High School and going south for about 2 miles, a footpath runs in the median. It’s a hidden gem. This is the nearest thing Oakland has to a long walk on a country lane.

mid-skyline-trail

Nobody online talks about this trail. I don’t know who made it or who maintains it. Cyclists laud the paved road, of course, which deserves the praise. The MBTR mountain bikers site reviews it under the name Skyline Boulevard Singletrack and gives it high marks, but you’d have to be as singleminded as me AND a mountain biker to seek it out. But the path is a nice way to enjoy this part of the hills.

This end of Skyline is exclusively on the Oakland Conglomerate, which holds up the spine of the ridge. Here’s the terrain:

mid-Skyline-topo

And here’s the geology, with “Ko” representing the Oakland Conglomerate (this is the same sequence seen in Shepherd Canyon):

mid-Skyline-geo

Some parts of the bedrock are straight sandstone.

mid-skyline1

But almost everywhere along the trail you’ll see the rounded cobbles that are this rock unit’s most distinctive feature, either embedded in stone or weathered out like this.

mid-skyline3

A slight detour up Brandy Rock Way will bring you to this fine exposure of the vertically tilted bedding, with a thick sandstone bed and conglomerate on either side (1000 pixels).

mid-skyline2

The road is slightly off the actual crest of the ridge, so the views it offers are mainly over the Bay. Here’s a spot near Cathy Lane overlooking a vineyard, the row homes across Leona Canyon on Campus Drive, the airport tower, the Bay and the San Mateo Peninsula.

mid-skyline4

I’m thinking that this would make part of a nice ramble. Farther south, Skyline merges into a two-way road and the path continues alongside it. Later this summer I’ll get to that part.