Oakland Geology

Since my previous post, I’ve gone ahead and hiked the faceted spur next to the Montclair spur. It holds the Glen Highlands and Merriewood neighborhoods, so I’ve named it the Merriewood spur because Merriewood Drive is the nicest part of the walking route.

The 1897 topo map shows the spur in its clean, original state between the narrow mouths of two wineglass valleys, Temescal and Thornhill Canyons. The Hayward fault runs diagonally through Lake Temescal.

As I detailed in my circumambulation of Temescal Canyon, this part of town has undergone a serious amount of human modification since then. But it has appreciably affected the ridge route on the spur only at its northwestern tip, as seen in the digital elevation model.

Looking at this image, I note that the spur is larger and more deeply eroded than the Montclair spur. It’s conceivable, though not at all certain, that the main incision includes a large landslide. Other than that the image reveals no great secrets.

The route around the spur is about four and a half miles. This is one version that starts and ends at the north parking lot of Lake Temescal Regional Park. The hike along the north limb of the spur follows PG&E’s major power line, and the portion on the south limb follows city streets.

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

It’s a climb of about 800 feet. A shorter version could start from Montclair Village and reach the north end of the spur via Pali Court. Or you could try the version I took, going up to Broadway from the Rockridge BART station and ending in Montclair (thus qualifying as a ramble).

The rest of this post will focus on the rocks to be seen along the north limb, because unlike the Montclair spur, the Merriewood spur includes a variety of rock units, as shown on the geologic map. I can’t vouch for the accuracy of the map along the south limb, where precious little bedrock is visible in a casual walk.

^{Jsv, Leona Volcanics; KJfm, Franciscan melange with chert and greenstone blocks; KJk, Knoxville Formation; Kr, Redwood Canyon Formation; Ku, undivided Great Valley Sequence; Tcc, Claremont chert; Tes, Eocene mudstone; Tsm, Sobrante Formation; Tush, unnamed Miocene mudstone.}

Here’s a view of the west end of the ridge from halfway up, just to the left of “Tush” on the geologic map. I was last here in November of 2008.

It shows Lake Temescal and the shutter ridge behind it, the PG&E station in front of it and the power line pad carved into the west end of the spur.

I climbed up from Broadway to the power line because the footing is pretty good at this time of year. The clean-cut slope exposes dark, crumbling shale of the Knoxville Formation. I’m excited to see this perishable stuff.

The western tip of the power line pad exposes the familiar rugged, red-stained rocks of the Leona Volcanics.

The contact between these units is important, but rarely exposed. I want to return and look for it.

Farther up the slope, the rocks change to this sandstone. The boundary between the black shale and the blond sandstone is the enigmatic, long-inactive Chabot fault, and the two rocks differ in age by about 60 million years.

The “Ku” unit lumps together a bunch of rocks that correspond to a whole set of Cretaceous rocks, the Great Valley Sequence, that farther south is divided into several different formations. It might be possible to map those units into this part of the hills, but the last person to try, Jim Case in the early 1960s, gave up.

Turning around, I spotted the same set of stones that puzzled me in 2008.

This time my accrued years of experience, plus the acid bottle I didn’t have with me the last time, told me right off that these are limestone, not native to the area (unless I and all my predecessors are badly mistaken). They look like river rocks that have been etched by exposure to our slightly acidic rainfall over the years.

Presumably PG&E brought in a truckload once upon a time for some reason. I also visited the old, deteriorating Horse Hill dirt jump and saw that it had a fresh tire track.

A little farther uphill, the roadbed displays some bedding features — offshore channel deposits, tilted nearly perpendicular.

This view looks up toward the top of the climb right at the edge of the “Ku” belt, where the soft Sobrante Formation mudstone underlies a long-lived landslide I’ve featured here before (in 2008 and 2018).

The continuing collapse of the hillside has opened a new front. Landslides don’t give up easily. The continued existence of this walking route is not guaranteed.

That’s the north limb of the Merriewood spur. Having walked it, I can now vouch for the whole circumambulation of Temescal Canyon, and maybe some day I’ll actually hike it all in one go.

This view is from the south limb, near the bottom. Thornhill Road, visible at lower left, marks the valley of Temescal (formerly Kohler) Creek.

We’re looking across the narrow mouth of Thornhill Canyon, then across the narrow valley marking the Hayward fault, then across the narrow water gap of Dimond Canyon, then across the wide San Francisco Bay basin to the Santa Cruz Mountains.

This entry was posted on 20 March 2023 at 8:02 am and is filed under 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.