Geology of the Biff’s site

22 May 2017

Because I walk through the area regularly, I’ve kept an envious eye on the excavation at the southeast corner of 27th Street and Broadway. Since the building slated for the site doesn’t have a name yet, I’ll call it the Biff’s site after the much-loved but long-departed Biff’s Coffee Shop that once sat there. It’s at the center of the maps below — first the Google terrain map.

Then the historic map. This is from an 1888 map compiled by state surveyor Julius Henkenius, served up by the David Rumsey Map Collection. What I like about it is that it shows the creeks. The main stream is Glen Echo Creek, with the Broadway Branch joining it near the top of the map.

The 27th and Broadway site appears to include the southern half of the Cogswell tract, presumably the remarkable Henry Cogswell whose great monument is a highlight of Mountain View Cemetery.

Anyway, what’s under the ground here? For that, we want the geologic map. This site is at an interesting intersection.

The site is flanked by two lobes of the ancient alluvial fan (Qpaf) that covers central Oakland. The one on the left is Pill Hill — which was known as Academy Hill in the 1880s and the site of Anthony Chabot’s first municipal reservoir — and the lobe on the right is Adams Point Hill. To the south, the ground is mapped as Pleistocene marine terrace deposits (Qmt), and the site itself is mapped as ordinary alluvial sediment.

Knowing all that, it would have been fun to poke around as the excavation proceeded from 24 March . . .

. . . to 1 April . . .

. . . to 19 April . . .

. . . to 7 May, when the digging was complete and the foundation prep was under way.

The chances were that no mammoth skulls or other cool megafossils were present, but you never know. It all looked like well-sorted fine sand from my distant viewpoint, what you’d expect. If this had been a Caltrans project, they might have retained a paleo firm to watch the digging and grab any fossils the dozers turned up. But as of last week the exposure is all over.

Most of the time, science is just an indulgence. But as Oakland enters a downtown building boom, it would be nice if the experts got a chance to document and sample some of these big holes.

The Dimond Canyon water gap

15 May 2017

In a city full of geologic features, Dimond Canyon stands out as a classic example of a water gap. But it can be hard to see, even from the prime viewpoint of Leimert Bridge.

Let’s abstract ourselves by studying the overhead views shown in maps. Google Maps, with the terrain view turned on, is where I like to start.

Compare Dimond Canyon, cutting straight through the bedrock ridge of the Piedmont block, with Indian Gulch (Trestle Glen) on the left, a conventional stream-cut valley that fans out against the ridge.

For a starker view of the topography, I like to consult old USGS maps like the 1897 Concord quadrangle, made before most of the area was built up and dug into.

Here we can see that the ridge reaches the same elevation on either side of the canyon — without the canyon cutting through it, this would be a continuous crestline.

There is no sign, either, that Dimond (“Diamond”) Creek cut its way through by headward erosion. That would have left tributaries fingering off on either side, like those visible in the contours above Indian Gulch. Indeed, the single little tributary in the canyon is actually a hanging valley that has to descend steeply as it meets Dimond Creek — not as spectacular as those in Yosemite Valley, but with the same basic configuration.

Finally, we can look at the bedrock evidence in the geologic map.

The whole area around the canyon is mapped as Franciscan sandstone (Kfn), with no hint of faulting or other structure that might have favored the formation of a canyon here. Consider the well-developed valleys above the canyon, guided into existence by the rock-crushing Hayward fault, or the more subtle topographic features where the southern edge of the Franciscan bedrock meets old alluvium.

What we have here, then, is a genuine water gap — a deep pass in a mountain ridge with a stream flowing through it. Geology textbooks will tell you there are two ways to make one. One is for a river to uncover an ancient ridge as it strips the countryside of its sediment cover. The classic case is the Delaware Water Gap in Pennsylvania. The other is for a river to sit there, doing its thing, as the land rises up around it. The Central Valley has good examples at the foot of Del Puerto Canyon . . .

. . . and the Berryessa water gap west of Winters.

What’s odd about the Dimond Canyon water gap is that it’s being carried along the Hayward fault. Every few hundred thousand years, then, it gets itself a new headwater catchment. Today its catchment is Shephard and Palo Seco Creeks. Once upon a time, though, it must have carried the waters of San Leandro Creek. Coming up: Temescal Creek.

Stonehurst Creek

8 May 2017

Stonehurst Creek isn’t really a creek, just a stormwater channel. But there it is on the watershed map, with a name and everything. Of the 13 named tributaries that feed San Leandro Creek, it’s the last one before the Bay. And it’s got potential.

I only discovered Stonehurst Creek because one day last June I set out to follow the Union Pacific track under the 880 freeway. (And pioneer a scenic route to the Cleophus Quealy tasting room.)

The tracks parallel 105th Avenue, and of course it’s private property. I advise you not to go there. If you choose to ignore my advice, however, the easiest access in Oakland is from Knight Street. You walk to the left of the tracks, near the drainage ditch that is Stonehurst Creek.

A maintenance road runs along the far side. This photo, from June 2016 at the height of the drought, shows there’s always water here.

This spring the area’s pretty lush in comparison. And if you look back, it’s not devoid of scenery.

On the other side of the freeway, the tracks cross San Leandro Creek. From there you can spot the mouth of Stonehurst Creek, looking almost creeklike.

I’m telling you all this because the route could become more of a destination under the newly released San Leandro Creek Trail Master Plan Study, a long-term vision of how we can get people back to Oakland’s largest and most important stream.

Because San Leandro Creek is very hard to access upstream from here, there would need to be a detour, and Stonehurst Creek would be handy for that. The Study mentions the “potential restoration of Stonehurst Creek,” which sounds funny because old maps don’t show any creek at all here to “restore.” Apparently the technical meaning of restoration includes building a natural creek from scratch.

You can continue down the tracks past Davis Street and turn right to get to Doolittle Drive, then go north to make your way to Cleophus Quealy — I mean, *I* can. The point is to arrive thirsty. But while we’re on the bridge, let’s look at San Leandro Creek.

Downstream, there’s a maintenance road on the left bank that could be opened up nicely to foot and bike traffic, and on the right bank a tidy, bare open space at the end of Empire Road that could become more of a park.

Upstream is more forbidding: there’s a maintenance road there too, but it’s down in the streambed, below the right bank, and couldn’t easily be opened to the public. Hence the need for a detour up Stonehurst Creek. Conceivably you could walk it in the dry season, but I would advise against that.

The San Leandro Creek Trail Master Plan Study says, “There is a large open space below I-880 that could be used creatively.”

And there are some visionaries hard at work doing that already.

Perhaps you would find their efforts as arresting as I do. Perhaps you would be dumbstruck, as I was. Perhaps you would exclaim, as people who explore Oakland often do, “There’s a there here!”

No matter where you go in Oakland, this town can excite some form of wordless delight.