Merriewood spur

20 March 2023

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

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.

Montclair spur

6 March 2023

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

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.

Lake Chabot Quarry, San Leandro

20 February 2023

You’ve all seen this quarry, looming over Lake Chabot Road and eating its way into Fairmont Ridge. It has a long history, and like all abandoned quarries it has a long afterlife ahead.

Seen from south Dunsmuir Ridge

The ridge was first opened up in 1886 by the Stone brothers, Egbert and Andrew, whose father Lysander made his fortune from the rich soil of the area now known as the Stonehurst neighborhood. Their construction firm, the E.B. and A.L. Stone Company, was a wide-ranging business that operated several quarries in the Leona Heights area.

By 1918 the quarry was owned by Joseph Costello, and in 1929 it came into the hands of the newly formed San Leandro Rock Company, which has owned it ever since although operations ceased before 2000.

Here’s the setting, from Google Earth. The quarry scar is in the upper center south of the dam at Lake Chabot.

The quarry exploited a body of rock mapped as basalt lava, of Jurassic age, that forms part of the Coast Range ophiolite. Here’s the same area of the geologic map. The Hayward fault is the solid black line just west of the quarry site.

Jurassic basalt (Jpb) with San Leandro gabbro to the west, Leona volcanics (pink) and Knoxville Formation mudstone (green) to the east.

And just for context, here’s the digital elevation model giving a closer view of the quarry site and the canyon of San Leandro Creek below the Lake Chabot dam, with the active strand of the fault shown in red and older strands in yellow.

Lidar data from Opentopography; fault traces from USGS. Illumination from the northwest.

The boulders that line the entryway are a good sample of what’s inside. They’re dominated by the dark, largely featureless basalt.

Here and there you can spot flow features and glassy regions that support the interpretation of this rock unit as pillow lava, the kind of blobby flows that form where red-hot lava meets freezing seawater. Hence the map symbol “Jpb” for Jurassic pillow basalt.

The quarry face itself isn’t half as well exposed, but beneath the rubble and grass the same material shows up in spots.

The thin white veins, when you can find a specimen that exposes them, appear to consist of hydrothermal quartz and olive-green chlorite. These rocks went through a few changes after the basalt first froze.

More entertaining is the view over the canyon and the Bay area beyond. You can see possibilities, whether you’re a would-be home developer or a would-be park planner.

The planner is the East Bay Municipal Utility District — the water company — which wants to buy the land and use the pit to dispose of the soil it digs up during trenching. It’s clean dirt, so no problem there; it’s useless land for anything else, so no problem there. And when they’re done, EBMUD wants to make it into a park. The problem emerges when they spell out the nitty-gritty in their project proposal: “The first stage includes using trench soils for fill operations for long-term phased placement and stabilization of approximately 3.4 million cubic yards of trench soil at the Quarry Site over approximately 40 to 80 years.” The problem is all those dump trucks over all those years on Lake Chabot Road.

The road that serves the quarry has always sparked contention. When it was the old back road between San Leandro and Castro Valley, the heavy-duty traffic from the quarry left the dirt-and-gravel road in rough shape. Eventually the route was paved, but the hillside is precarious and parts of it gave way during our latest wet season. All this before the next big earthquake.

The old road was bypassed by Fairmont Drive, a four-lane highway that was pushed over the ridge in the 1970s, but pleasure drivers, commuters and residents still use it. To preserve their quiet byway, the residents have opposed the heavy trucks of quarry traffic for fifty years, and they oppose it this time too. Today I’m fully on their side.

A possible route from the south would reach the quarry via Fairmont Drive. It would run up this valley, which is owned by the East Bay Regional Park District and is otherwise unusable because it runs right along the Hayward fault.

Considering that the park district and the water company are two trunks from the same root, maybe they could get along with a suitable easement through here.