Oakland Geology

The most remote part of Oakland will be opened to public access fairly soon, when the East Bay Regional Parks District finally gets around to constructing a trail between Dunsmuir Ridge and Lake Chabot. I made my way into part of the route a few weeks back. It’s interesting and inviting territory, set above an untouched oak-filled stream valley with Fairmont Ridge beyond.

The land has divided ownership, with parcels belonging to the city, EBMUD and the East Bay Regional Parks District. They’re all public agencies, so the bureaucracy must have been difficult. Here’s the setting, as laid out in a 2009 EBRPD map.

The solid blue line is a trail in Anthony Chabot Regional Park that’s been closed for years. The dashed blue line is the proposed Dunsmuir Heights to Chabot Regional Trail.

This summer the EBRPD board was shown this map of the approximate route. Again, the solid line is an existing (deteriorated) roadway and the dashed part will be built from scratch. The photo at the top of this post is the view from the EBMUD water tank at top center. The part I’ll be showing is between there and the “P” mark at the city golf course.

A couple more maps to help you see what’s here. First is the bare land as shown in Google Maps terrain view.

The centerpiece of the trail’s route is the valley in the lower right quadrant. Note its depth and steepness. The permanent stream in that valley has no formal name, so I will hereby dub it Chabot Creek. And finally here’s the same area in the 1947 topographic map, which shows the old roads that will become the trail.

If you look at the upper part of Chabot Creek valley, you’ll see it turns sharply from southeast to southwest as you go downhill. On the Google map, though, the streamcourse is interrupted by a flat area. That’s landfill made of waste from the Cypress Viaduct, which collapsed in the 1989 earthquake. It’s sterile and weedy, but the view is nice.

At its edge is a curious structure, visible in Google Earth, that turns out to be a spillway, made for the event of a large rainstorm during a very wet winter. Presumably the landfill was capped with a layer of clay to stop any contaminants from leaching into Chabot Creek, and thus the site would fill with rainwater quickly and have a risk of spillover.

If you looked closely at the second map, you saw the intriguing pointers to an “old foundation” and a “1936 WPA rock chimney.” The chimney is a massive stone fireplace, suitable for a hunting lodge. But the building it once occupied is gone. Foundations around it show that it was a group facility of some kind. I’m hoping that local historians can say more about it in the comments.

A stone in the entryway is carved with the date 1935, so the map is slightly in error.

In any case, the stonework is indeed classic WPA masonry, of the same vintage, material and durability as the Woodminster Cascades in Joaquin Miller Park.

And speaking of stone, what are the rocks like around here, you ask. Here’s what’s mapped in the area.

“Jsv” is the Leona volcanics, “KJk” is the sedimentary Knoxville Formation, and the blue field is Franciscan melange, the same body of rock underlying Knowland Park. When I visited, I checked out a roadcut right where the Franciscan and Knoxville meet and found an assortment of rocks.

All of these are appropriate for the Franciscan, but the brown sandstone could just as easily be from the Knoxville. A return visit is in order during the upcoming wet season, when the ground is firmer. I hope to see more signs of the Franciscan “knockers” so well exposed in Knowland Park — and on the golf course, like this blueschist knocker cropping out in the rough.

The plans for the Dunsmuir Heights to Chabot Trail are supposed to come up for public comment this fall, with the work to be completed by 2021.

This entry was posted on 1 October 2018 at 8:02 am and is filed under Franciscan rocks, Oakland blueschist, 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.