The five-year mark

On this day in 2007, I launched Oakland Geology with a photo of what happens when Oaklanders build without geological awareness. Since then I’ve made plenty of posts along the same lines, but geology is not a one-note subject. Most of the time I’ve celebrated what intrigues and tantalizes me about this remarkable city’s natural underpinnings, always with a photo to share.

In 2010 Oakland Geology was named Best Blog about East Bay Rocks. (I really should get myself over there and pick up my plaque.) This week I learned that one of our celebrated local authors, Michael Chabon, cited Oakland Geology in the acknowledgments of his newest book Telegraph Avenue. These are signs of what I always hoped to achieve with the blog: to extend popular awareness of this city’s place in deep time and its deep present, to include our rocks and soils and landforms and geologic forces in the everyday conversation that is constantly weaving our future.

This blog, more than all my other writing, has brought me face to face with interested Oaklanders, who appear to be roused enough by these snippets and snaps to come see me wave my arms, with Powerpoints or vistas behind me, and dump more data on their heads in person. I appreciate your audience and your readership. Long may we continue to wave.

Pershing knob

Pershing Drive sits at the top of the south end of the Piedmont block at nearly its highest point, just under 800 feet. At one end of the road is a long exposure of rugged Franciscan chert that is exquisitely integrated into the landscaping, which includes native oaks as well as the usual plantings. It is a challenge to photograph but a delight to see in person.

This is the high-grade, bleached and sometimes greenish banded chert of the Piedmont block rather than the beautiful red ribbon chert of the Marin Headlands. Its outcrop appears on the Oakland geologic map east of Piedmont’s Tyson Lake. The very steep slope on its bayward side is interpreted as a thrust fault separating the higher Franciscan melange from the lower Franciscan sandstone. Melange is not generally considered a strong stone by geotechnical engineers, but recent research shows that it depends. This melange is the best bedrock in Oakland, and worthy of its gracious and picturesque neighborhood.

Linear scarps

Next Saturday, 8 September, I’ll be leading a short, rugged urban walk for Oakland Urban Paths that among other things will visit these faceted spurs along the Hayward fault. Seen from the north . . .

and from the south:

The downhill side is moving north with respect to the near side. The open land in the foreground is the King Estates Open Space.

This will not be a stairway walk. The off-street passages are steep, weedy dirt paths that have not been maintained. The land along the fault is steep, making for nice residential view lots. I haven’t finished the route yet but it will take no more than 90 minutes, 10 to 11:30—I have a lunch destination I’m anxious to make. So I would like to set a good geologist’s pace. Details and questions as they come to you over at Oakland Urban Paths.

Longridge loess

I was walking up Longridge Road and spied an excavation, where a homeowner was replacing some water lines and renewing a driveway. Naturally, I sidled over and took the rare chance to look beneath the skin of Oakland’s Pleistocene fan. The material was massive—unbedded—and clean. I pried off this little piece . . .

. . . and nibbled on it. It was firm, but crumbled like Necco wafers and turned creamy on the tongue with just a hint of grit. Not sticky or chewy with clay. Not indurated like hardpan. No sand or pebbles to be seen. The more I thought about it, the more peculiar this sediment seemed, until I had a wild surmise.

Alluvial sediment is never very well sorted, because it’s carried short distances and laid down by streams. Longridge Road is, as the name suggests, a ridge road running up the crest of a ridge between parallel stream valleys along Trestle Glen and Mandana roads. The crest of a ridge should not be made of this fine silt. But it’s downwind from downtown, which is Pleistocene sand dunes (the Merritt Sand), cousin to the dunes of San Francisco. Dune sand is very fine sand, and the fraction that blows away from the sand is finer still. So my wild surmise is that the fan, at least this part of it, is dusted with a layer of windblown glacial silt—i.e., loess. It’s remarkable stuff, and something I never expected to see in Oakland.

Lower Piedmont Park walk (#28)

Walk number 28 in Charles Fleming’s Secret Stairs East Bay winds around the fine homes and hills of Piedmont along the valleys of Wildwood and Bushy Dell creeks. Here’s the route, shown on the Google Maps topo base.

The first and last part of the loop is in the watershed of Wildwood Creek while the rest is in the Bushy Dell Creek watershed. (They run down Lakeshore and Grand Avenues respectively, separated by Warfield ridge, and combine down at Lake Merritt where their names are sunk in bronze by the pergola.) Geologically, the walk covers the uppermost part of the big Pleistocene alluvial fan and the edge of the Franciscan bedrock block that underlies most of Piedmont.

We start at the Lakeshore-Winsor split in the stream valley and make our way toward the divide. On Portsmouth Road the high ground of the bedrock zone stands out ahead.

At the far end is a steep climb to Wildwood Avenue, where we can look back across the stream valley to the ridge topped by Calmar Avenue, on the Oakland side of the city line.

Turning the other way, we look over the valley of Bushy Dell Creek. Once a large formal garden, this part of the valley was filled and leveled for its current use as a sports complex. It appears never to have been a quarry, unlike Dracena Park to the north or Davie Tennis Stadium to the south.

We turn upstream along the creek, where the land is relatively untouched. Just above this spot is the site of what was reputed as a sulfur spring.

The geologic setting doesn’t really give much support for the presence of a proper sulfur spring like the one in Walnut Creek, but after all this time the question is moot. Certainly I didn’t notice any odor. The grotto was very pleasant anyway, and there’s real bedrock all around. It’s mapped as Franciscan sandstone of the Novato Quarry terrane.

The route goes farther up and takes a loop past a pair of boulders.

Take a close look at these: they’re genuine Oakland-style blueschist, globs of old ocean crust that have been carried tens of kilometers down into the earth along a subduction zone, then spat back out, possibly more than once. (The details are at the bleeding edge of California geology.) The one boulder displays good color and mineralogy:

The other has some nice slickensides to show us.

Coming back downstream and past the baseball diamond, we pass the entrance to the football field. The view looks down the valley toward the lake and downtown.

Near here we can see more exposures of the sandstone bedrock, but soon afterward the route returns to the alluvial fan. The two substrates make subtly different topography, but that can be hard to see given the heavily landscaped landscape.

Palm Drive offers a picturesque farewell view of the Bushy Dell Creek valley.

Again we cross the divide between the two watersheds at Wildwood Avenue. The near valley is accentuated by glimpses of the higher hills.

I never get tired of this stuff.

Here’s the route in more detail.

Bella Vista hill

Bella Vista hill is part of the big Pleistocene alluvial fan that sprawls across the middle of Oakland, as shown by the “Qpaf” code on the geologic map.

The “Qmt” code is the marine terrace I’ve discussed before. The fan is dissected by modern streams into several lobes, which take their position in today’s cityscape as distinct topographic hills as seen in Google Maps topography:

Bella Vista hill lies in the polygon defined by Park Boulevard, East 34th Street, MacArthur Boulevard, 14th Avenue, East 18th Street and 8th Avenue. All of those street run in valleys or saddles, except that East 18th is on a break in slope. The numbers refer to the following photos, taken from across or in the valley of 14th Avenue Creek.

The hill is low on its bayward side, as shown here from E. 22nd Street.

It has a corrugated surface, such that 9th, 10th and 12th Avenues run on high ground and 8th, 11th and 13th run up declivities. Higher up on the hill, the ground is steeper and the allee of palms marking the former Francis Smith estate is the defining feature of the hill.

The top of the hill has two eminences. The more dramatic one is on the south where Highland Hospital frowns down upon the stream valley.

Hospitals, like schools, were traditionally sited to take advantage of fresh air. Highland Hospital took its very name from this practice. Here’s a view of the whole hospital complex on the southern boss of the hill.

The northern knob is higher, exceeding 200 feet elevation. At its high end, 10th Avenue becomes Bella Vista Avenue and curves across this peak. Old palm trees from the Smith estate mark it from a distance.

Borax Smith knew what he was doing when he picked this hilltop for his home base. The hills on the fan to the south are a bit higher, but the views to every other direction were unimpaired from that spot. If you want to walk the crest of this hill, go up 10th or 12th Avenue and jog across to 13th and take it all the way up to the I-580 overcrossing.

Knocker 10

Mountain View Cemetery has been clearing its upper reaches aggressively this year—so much so that a new knocker came into my ken the other week. This view is looking down at it from the brow of the maintenance area, sheltered by a clump of trees.

It appears to be the usual sandstone, although I didn’t inspect it closely. There is a good deal of poison oak around it, as there is near its neighbor, the “high knocker“:

Knocker 10 overlooks the new Golden Lotus Mountain section.

If you haven’t been up to the top of the cemetery lately, the view east is phenomenal with the eucalyptus trees gone.