Tour of the Fan: Lobe 6

18 December 2017

The ancient alluvial fan of central Oakland consists of eight lobes. To refresh your memory since the tour of Lobe 4, here they are labeled on the geologic map. This week I want to show you Lobe 6.

The dominant feature of this lobe is the hill on which the Maxwell Park neighborhood was developed nearly a century ago, but the hills has lesser rises around it defined by stream valleys and a freeway. I’ve named these divisions for convenience as follows.

The southern edge of the lobe is marked by changes in slope above Foothill Boulevard. The changes are definite, but the slopes are gentle. Here it is seen looking up 38th Avenue from Foothill. (All of the photo locations appear on a map at the end of this post.)

And looking down 47th Avenue past Melrose Street toward the bay.

Looking down Trask Street from Cole Street. That’s the Bay-O-Vista hillside neighborhood of San Leandro left of center in the back, and the hills above Hayward in the back center.

And looking down Camden Street from Madera Avenue onto MacArthur, with the trees of Mills College in the back left.

The higher, eastern end of the lobe is not as well defined except on its north side, shown here looking from High Street onto Bayo Street.

The other edges of the lobe are quite a bit more rugged. This is the north side of the Jefferson segment, seen from Gray Street. Just to the left of this frame is the landslide scar of Jungle Hill, which I featured in my post about peculiar Harrington Ridge.

Brookdale Park on High Street, where the Jefferson and Maxwell segments meet, offers a rare glimpse at what the Fan is made of: ancient, well-compacted clay and sand with some gravel here and there.

The Maxwell segment dominates Lobe 6 in height and area. Its steep northern and eastern faces were shaped by stream erosion from below. Here, on MacArthur just south of I-580, it may have been steepened further by development-related excavations.

Seen from across the freeway on Green Acre Road, near the top of the St. Lawrence segment, the spine of Maxwell Hill, about 260 feet high, walls off views of the bay. The dip in the foreground is one of many gulches that punctuate the lobe’s rim.

Lobe 6 is nearly severed in two places. One is where the freeway punches through, having enlarged a pre-existing saddle between the Maxwell and St. Lawrence segments. The other is where Courtland Creek cuts between the Jefferson and Maxwell segments. High Street took advantage of that gap in East Oakland’s earliest days, probably following an older footpath. The gap carved by the creek is visible in this view west down Maybelle Avenue. The church at the left edge is St. Lawrence O’Toole, perched on its namesake segment above the Laurel district.

This view south from High Street shows San Carlos Avenue crossing the valley of Courtland Creek. Courtland Street runs up the creek now, built on the old Key Route right of way.

Other small streams have carved valleys into Lobe 6, but today the valleys only carry culverts and add charm to the topography. The valley of Vicksburg Avenue, separating the Fairfax and Maxwell segments, is the headwater catchment of little 54th Avenue Creek.

More sizeable is the valley of Kingsland Creek, now culverted beneath Kingsland Avenue. This view across the valley is looking north at the north end of Walnut Street. (Did you know Oakland has two entirely separate Walnut Streets?)

Birdsall Avenue, just to the east, also runs up a nice creek valley. These “low roads” running along the stream valleys are your best bets for a pleasure walk, since the streets of Lobe 6 were otherwise laid out in a grid without ridge roads. The nearest thing to a ridge road is 47th Avenue, in the small Fairfax segment. At its top it dips into Vicksburg Valley and offers this view of Home of Peace Cemetery on the Maxwell segment, a pleasant place to visit and an easy landmark to spot from the BART train.

The high streets are a bit awkward to get around on, but they have lots of charming spots. This is where Storer Avenue sweeps around the northernmost rampart of Maxwell Hill, seen from the top of the Herriott Avenue steps.

And of course there’s the postcard view of Meldon Avenue, worth seeing even when Redwood Peak and the high hills aren’t visible.

I always enjoy tramping around here. Photo locations and a hint of the terrain below.

In other news, I will be taking the rest of the month off. Come January, I’ll throttle back my time on this blog, posting biweekly instead of weekly. More details in the Announcements/Q&A page.

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The St. James Drive roadcut

11 December 2017

Recent work in far east Piedmont has exposed some excellent bedrock worth a close inspection. Because the town government won’t put an interpretive sign there, this post will have to do.

To my knowledge, there are only two sites of powerline towers in Piedmont, one at the mouth of Estates Drive and the other at 298 St. James Drive, near Park Boulevard. Last year the power company replaced the latter pair with shiny new towers, and as part of the work it cleared the roadcut of its cover of acacia trees (Google Maps shows the site as a thicket going back to 2007). Shortly afterward I discovered it and had high hopes, though it wasn’t much to look at in late October.

By January, the exposure had been stripped of loose rock. Already it was clear that it would be a showcase of slickensides.

By August, a strong concrete wall had been put in place and landscape plantings made.

The slickensides turned out to be fabulous. These are the polished marks made as movement along faults grinds rocks against each other.

And here’s a closeup.

Also visible is evidence of brecciation, the geologist’s word for shattering rocks and cementing the pieces together.

The rock here is sandstone of the Franciscan Complex, specifically part of the Novato Quarry nappe. This is a thick slice of fine-grained sandstone that was laid down off the ancient coast of California, then shoved against the continent’s edge and pulled apart into lumps by the San Andreas fault system. A bunch of it underlies Marin County, and more makes up Point Richmond and El Cerrito as well as Piedmont and points south. This tectonic history probably accounts for the wear and tear visible in the roadcut.

When I visited the roadcut again last week, I annotated and recorded the site in the ROCKD smartphone app and announced it on Twitter. I’m trying out the app just for fun as a way to make some of my observations public. I’m looking at other apps for more rigorous mapping purposes.

Geologizing on the 33 bus line

4 December 2017

My geologizing habits are unusual: being self-employed and semiretired, I go out on weekdays, when everyone else is busy and I have the landscape to myself. Moreover, most of my outings around Oakland are on foot, with the help of the Citymapper phone app and all manner of public transit, which I’ve praised before. In fact, I’ve written a whole compendium of places to see in Oakland this way.

So I’ve gotten to know the bus system pretty well. And for accessing lots of interesting, walkable terrain, the 33 line ranks right up there. Assembled from parts of the old 11 and 18 lines, the 33 runs from the bedrock hills and uplands of Piedmont, past Lake Merritt, to the bedrock hills and uplands of Montclair, taking in lobes 3 and 4 of the Fan as well.

The northern leg takes you to the well-appointed sidewalks of Piedmont, saving you a tedious climb. But there’s scenery on the way. First you pass the north arm of Lake Merritt, then cross lobe 3 of the Fan and reach bedrock right after crossing upper Grand Avenue.

Destinations in Piedmont include the former quarries of Dracena Park and Davie Tennis Stadium, the canyon and creek of Piedmont Park, and the headwaters of Trestle Glen Creek, plus smaller attractions like the slickensided roadcut of St. James Drive.

The southern leg of the 33 takes you to central Montclair, a jumping-off point for hikes in all directions. But first you pass the mouth of Lake Merritt, then enjoy a scenic ride up the valley of Park Avenue Creek and past the views over the glens (Trestle Glen and Dimond Canyon) in Glenview.

Above Glenview you could get off at Leimert Boulevard and trek through Oakmore, or go the other direction into easternmost Piedmont; your call. But then you’d miss the passage through Dimond Canyon.

From Montclair you can head west back downtown through Piedmont; north to Thornhill Canyon or past Lake Temescal to the Rockridge BART station; south to the trails of Joaquin Miller Park; or east to the high hills through Shepherd Canyon, up the Colton spine, and even all the way through Sibley Preserve, shown below, to Orinda.

None of those possibilities appear on AC Transit’s map of the 33 line, just a lot of human destinations and transit connections.

So climb aboard the 33 some time. Maybe I’ll see you there.

The Dunn-Spring Quarry, north Berkeley

27 November 2017

Glendale-La Loma Park, a little ballfield/playground complex in the north Berkeley hills on La Loma Road, is a repurposed quarry that’s had a true Berkeley history. The original quarry, operated by J. J. Dunn, appears to date from 1892. John J. Dunn, a Canadian immigrant born in 1839, was a major contractor in California building roads and sewers starting in the 1870s. This is Dunn portrayed in 1896 in the Oakland Tribune.

In 1900 Dunn advertised the quarry for sale “on account of sickness,” and died of Bright’s disease (kidney failure) in St. Helena that June.

In 1904 the Dunn quarry was reopened by Louis Titus, former head of People’s Water Company, as part of the Spring Construction Company. Its dawn-to-dusk blasting operations infuriated local residents, who obliged it to shut down in 1909 by threatening the business-friendly city council with a recall campaign. The company made the gaslighting claim that they were not operating a quarry, even though the rock from the pit was being used for streets and homes in the Thousand Oaks tract, but were in fact building a reservoir for the People’s Water Company.

After its abandonment the Spring quarry became an attractive nuisance, drawing generations of youngsters to its steep sides and deep swimming hole. Even those who didn’t swim in the cold, murky groundwater must have enjoyed the view.

In 1950 an 11-year-old boy drowned, and the city fenced off the quarry. The city acquired more property around the site in 1957, and eventually it became what you see today.

Unfortunately I have found no details of the site’s geology, although Titus called it a basalt quarry. The geologic map shows the downhill side as basalt of the Moraga Formation and the uphill side as the conglomeratic Orinda Formation.

This is the basalt. The brownish streaks are slickensides — friction marks from faulted fractures.

Other volcanic rocks here include rhyolitic tuff, a minor component of the Moraga Formation. It’s distinguished by its light color and broken (brecciated) texture.

Superficially, it resembles the Northbrae Rhyolite that makes up Grotto Rock Park and its sister parks, but unlike that heroically strong stuff it degrades quickly when exposed at the surface.

At the foot of the cliff you’ll find pieces of this excellent conglomerate from the Orinda Formation.

But the fact that it has crumbled down the cliff and will fall apart in your hands means that the rock face would be treacherous climbing. The city should be more forthright in discouraging climbing here; instead they just say “check out the other Berkeley parks with rock features…” Stay off it, and if an earthquake strikes while you’re standing there, jump back fast.