Oakland Geology

One of my little geological fetishes is a geographic one: circumambulations. Thanks to the Hayward fault, Oakland has acquired several wineglass valleys, with very narrow mouths and wide headwaters. I’ve pioneered hikes that circle three of them: Claremont Canyon, Temescal Canyon and Shepherd Canyon.

These are strenuous outings, and I’m getting less and less young. This year I hope to resume them, but it will take some working up to. But I had a brainstorm: what about the faceted spurs between the wineglass valleys? I’ve written about faceted spurs before, but it felt kind of obscure so let’s try again.

Here on the 1897 topo map I’ve outlined the faceted spur that overlooks Montclair, between the narrow mouths of Thornhill and Shepherd Canyons. The fault runs from the upper left corner to the middle of the bottom edge. The spur is about 700 feet high and a loop on it is about 2.5 miles, as opposed to a further gain of 300-plus feet and 4 more miles for the circumambulation.

Visualizing this topography with the digital elevation map and picturing the hills covered with grassland, the way they used to be, I’m thinking this would’ve been a picturesque hike, on ridge routes the whole way.

You can see that the spur doesn’t have the ideal flat facet — it’s been dissected somewhat into small valleys — but the ridges that make up the rim are still nice and strong. And a lot of the streets run along the rim to offer a fair approximation of that 1897 hike. Here’s the route, starting from the end of the 33 bus line; I’ve taken it both ways and I feel fitter already. If you try this, be very cautious as you walk along Colton Boulevard.

^{Zoom in on the route at gmap-pedometer.com}

The triangle of streets inside this loop isn’t part of either Thornhill Canyon on the north or Shepherd Canyon on the south; you might call it pure Montclair. It faces southwest, and as you climb you begin to peek over the Piedmont crustal block toward the Golden Gate. This view is from the north end, where the ridge runs east-west . . .

and this one is from the south end, where the ridge runs north-south.

On this ridge too, Asilomar and Drake Drives offer open views over the mouth of Shepherd Canyon toward the South Bay.

Note a couple of things in this view. The notch behind the tree in the middle is where the San Andreas fault runs as well as Route 17 to Santa Cruz. The LDS temple to its right is where the Hayward fault runs, continuing right through Montclair along the freeway. The little valley that holds Montclair owes its existence to the fault, which grinds the rocks to an easily erodible state. Elsewhere along the route, you can look north along this valley, as here at the northernmost end of the Montclair Railroad Trail.

The two ridges meet just above the Forestland Reservoir, which is a nice quiet place to have a sit before starting down — or heading farther up the main ridge to the wonders of Skyline.

The rocks along the way are all pretty much the same: medium-grained sandstone of the Redwood Canyon Formation. The south end of the loop is mapped as the Shephard Creek Formation — sandstone plus shale — but you won’t see any of it.

I have no great insights or cool things to note about these rocks. Like I said, walks like this are a geographic fetish.

This entry was posted on 6 March 2023 at 8:01 am and is filed under Hayward fault, Oakland geology walks. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.