Archive for the ‘Oakland geology walks’ Category

Oakland geology ramble 8: Piedmont Ridge

19 August 2019

I don’t always care about rocks; geology is about more than rocks. I don’t even always care about geology; sometimes I just want a vigorous, geographically arbitrary hike. Ramble 8 is one of those — a traverse of the ridge crest above Piedmont, our highest ground west of the Hayward fault. It goes from the Rockridge BART station to the 33 line bus stop at the Leimert Bridge, on the lip of Dimond Canyon, about five miles end to end. Nevertheless, it has geology and rocks along the way. Here’s the route. There’s a map with more detail at the end of the post, where I also have some announcements.

The BART station features the “Rockridge” destination signage, mounted above a selection of Sierran boulders. This time, some ten years after it was installed, I noticed that the lettering design is quirky, contrasting “rock” and “ridge.”

You could climb Keith Avenue and barge up upper Broadway to attain the heights at Contra Costa Road, but it’s more interesting to circle behind the College Preparatory School grounds on Brookside Avenue. The school nestles in a steep little stream valley — unlike many similar places, this is not a former quarry but, apparently, a former turn-of-the-last-century park that had a short existence.

Once you get up to Contra Costa Road via Eustice and Buena Vista avenues, the walking is pretty and level. While you’re here in this remote part of town, check out the blueschist outcrop at 6063, vacant since the 1991 fire. The entire ridge on this hike is in Franciscan melange, a mudstone matrix containing odd lumps of other rock types. You won’t see much of it.

At the very end of the street is Erba Path, a steep set of stairs down to the saddle in the ridge where Broadway Terrace peaks on its way past the south entrance to Lake Temescal and points east. Cross that busy road and head right back up again on Sheridan Road, visible here at upper left.

Turn right off Sheridan at Agnes Street, unless you want to explore the little-trod path that joins Sheridan’s two halves. (Everyone should at least once.) Take high-flying Cochrane Avenue, where views east of Thornhill Canyon and Glen Highland’s settled slopes beckon.

Then jog right again up the saddle of Florence Avenue (where I sent you in the last post) to Proctor Avenue. This is pretty steep, but short. By this point you’re well above 700 feet. You’ll go higher later, but first you clamber a hundred feet down to the next saddle where Moraga Avenue crosses the ridge. Take the well-marked pedestrian crossing, but beware — this is the most dangerous road crossing of the hike.

Proceed on Estates Drive, which climbs nearly to the 800 foot contour. Up here are two curious reservoirs that date from Oakland’s water wars, when private water companies struggled to supply the fast-growing region from local sources while they vied with each other in deadly capitalist strife. (A ruinous series of bankruptcies and mergers ended in the 1920s with the formation of EBMUD.)

The Dingee and Estates reservoirs were constructed, fast and furiously, at the highest point of the Piedmont hills to provide good water pressure. EBMUD is upgrading these old concrete bathtubs to proper steel tanks, built to withstand big earthquakes on the Hayward fault just a few hundred yards east.

From here you head down again to La Salle Avenue, which takes advantage of yet another saddle in the ridge. If you’re ready to quit at this point, go left on La Salle, left again on Bruns Court and cross that high pedestrian bridge over the Warren Freeway to Montclair Park (because you can!) and catch the 33 bus at La Salle and Moraga. Otherwise, cross La Salle and stay on Estates, which is a little to the right.

Estates climbs again, not so far this time. Because this walk hits the highest spots, take Dawes Street up the hill and over, where you simply must visit the south end of Pershing Drive and admire Oakland’s best outcrop of Franciscan chert.

From here on it’s all downhill. Dawes rejoins Estates Drive here, and as you start down Estates you can see across Dimond Canyon.

Geologically and geomorphically speaking, the other side is also part of Piedmont Ridge, but the large water gap of Dimond Canyon is impassable without a long detour. So, down you go to the bus stop at the Leimert Bridge.

Along the way are two more highlights. First, at the Piedmont line the road passes the head of the former Diamond Cañon Quarry, which today houses the Zion Lutheran Church. Recent foundation work here has exposed fresh rock; maybe you’ll see some too.

And second, enjoy this wonderful volcanic breccia used for the landscaping at 170 Estates Drive. There are whole walls of it.

The house itself is something to see, too.

And as promised, here’s the detailed route map (1126 X 1126 pixels), followed by some announcements.

The excellent, out-of-print book Stories in Stone: Travels Through Urban Geology by David B. Williams is being reissued in paperback by the University of Washington Press. If I didn’t have the hardcover already I’d buy this classic. More information on David’s website.

My own book manuscript is making the rounds of a publisher, and while I await a yea or nay I’m trying to get my arms around a whole lot of scientific literature pertaining to Oakland’s rocks. It’s a ridge walk of the intellect, but I want to make Chapter 5 as good as humanly possible. I hope to buttonhole some of the real experts next month at the Geological Society of America’s annual meeting in Phoenix.

But first I’ll be giving a talk to the Friends of Sausal Creek, on 18 September at the Dimond Library, titled “Sausal Creek: The Last Million Years or So.” You read it here first (unless you follow me on Twitter, @aboutgeology), and I’ll repeat the announcement in the Q&A thread soon.

Oakland’s wild rail path

5 August 2019

The seasons are changing now, if you follow the pagan calendar. This weekend marks the turning point between astronomical pagan summer (6 May to 6 August) and pagan autumn (6 August to 6 November), or as I think of them, High Season and Waning. They are offset exactly half a season from the conventional astronomical seasons. High Season consists of long days, and Waning consists of shortening days. (Likewise, Low Season consists of short days, and Quickening consists of lengthening days.)

Nature is acutely aware of these seasons. The belladonna lily (Amaryllis belladonna) sends up its naked-lady flowers at this time. The strawberry tree (Arbutus unedo) ripens its rich little fruits (I can understand why Pliny the Elder named them “eat-only-one” because they’re so satisfying).

And of course the blackberries are in full swing.

I returned last week after eight years to the “secret street” at the south end of Florence Avenue, where it meets the old railbed of the Sacramento Northern Railway (also known as the Oakland, Antioch, and Eastern Railway). Unlike that first visit, when I was busy and could only gaze up the path, this time I had the leisure to walk its whole length, up to Broadway Terrace where it’s fenced off.

The path has geology up at its north end, but it’s worthy just as woods. Even right next to the Warren Freeway, it’s as secluded as any place in Oakland.

It’s shown as the dashed route on this map. You can see that Florence Avenue, heading over a saddle in the ridge above Piedmont, used to connect with Florence Terrace once upon a time. That’s the Lake Temescal park at the top.

There are lots of blackberries growing here, so don’t wait. The first ones are the best. Near the north end is a landslide scar that was repaired with much labor to protect some homes on Sheridan Road. The work was finished with dark shotcrete, but it doesn’t really blend in.

If you look close you’ll see little splotches of white. Those mark cracks where lime-bearing groundwater has seeped through and deposited calcite as it evaporates.

I can foresee these growing into falls of travertine in a few years. Beyond the landslide is a high cut into the hillside, made decades ago when the rail line was first pushed through. And the bedrock here is mapped as classic Franciscan melange, the big blue field on the geologic map — the edge of which happens to correspond to the Hayward fault.

I half expected the rock exposed here to be fault gouge, the fine-ground, mealy stuff that fills many of California’s active faults (for instance at the London Road slide). It’s real close to it: highly weathered mudstone that’s likely to come down hard in our next big quake. Whether the railbed will be cleared again afterward can only be conjectured. I’ll look at this cut again more thoroughly next time I’m here, whenever that might be.

On your way back, look again for blackberries. I know I didn’t get them all.

Oakland geology ramble 7: South Orinda to Montclair

22 July 2019

It’s been a while since I brought you a geology ramble — a no-car-needed hike that starts and ends at different places with public transit. Ramble number 7 connects Montclair and Orinda over the spine of the Oakland Hills. Three years ago, when I presented ramble #2 from Rockridge to Orinda, I said “I have a vague scheme for a southern route,” and this is that scheme fleshed out and walked both ways.

Here’s the 6.4-mile route superimposed on Google Maps. It climbs from about 600 feet elevation at either end to 1600 feet, in Sibley Regional Volcanic Preserve.

The route starts in Orinda, for reasons I’ll get to later. From the Orinda BART station, take the County Connection number 6 bus and get off at Woodland Road, across from St. John Orthodox Church. The first mile is a gentle downhill into the valley of Moraga Creek, on Woodland and then Valley View Drive. You’ll pass this hillside, which exposes lava of the Moraga Formation.

If you have the time and energy, this is an interesting exposure, but there’s a long walk ahead.

The road turns into Lost Valley Road as it turns right. Follow it to Edgewood Road. At the end of Edgewood is this gate leading to the open space of the Orinda Wilder development.

Go on in. The road straight ahead leads to the new homes of Orinda Wilder, but you’ll turn left, up the hill.

Two miles farther and 800 vertical feet up, there’s another gate at the boundary of Sibley. On the far side of the gate is a sign saying “private property.” That’s the sum total of the access restrictions along this ramble.

The full route takes you through 12 different geological map units, by my count.

They are the Mulholland Formation, Moraga Formation basalt (Tmb) and interbedded sedimentary rocks (Tms), the Siesta Formation (Tst), Orinda Formation (Tor), Claremont Shale (the golden stripe from corner to corner), Sobrante Formation (Tsm), the unnamed Eocene mudstone (Tes), the teeny, cryptic Paleocene sandstone (Ta), Redwood Canyon Formation (Kr), Shephard Creek Formation (Ksc), and the Oakland Conglomerate (green).

What particularly interested me about this hike was the stretch through the Siesta Formation and the Moraga Formation sediments, which aren’t exposed in Oakland proper.

The Siesta consists of fine-grained stuff: sand, silt, clay and some limestone. The limestone is what stands out, because it’s white.

Outcrops of the limestone aren’t obvious, but my little acid bottle always reveals it by the telltale fizz.

This large limestone boulder reveals a lot of broken-up structure, including some shale chunks mixed in. Landslides will do this to unlithified sediments — and around here, even 9-10 million years ago, that means earthquakes.

The Siesta also includes a little conglomerate, as seen in this roadside exposure, but in general the rock isn’t highly visible. It likes to turn back into dirt.

Farther up the road, the Moraga sediments show themselves as coarse sandstone, rough stuff that gathered around whenever the lavas weren’t erupting.

That little magenta bit brings me to the other scenery: it’s always more than rocks that brings me to these heights. In early summer it’s wildflowers.

And views.

And views of rocks.

By the time you’re on the ridgetop, you’re well into the volcanic rocks that Sibley is famous for. Look carefully and you may spot the mineral-filled lava bubbles called amygdules. (I left this one for you to find.)

The high point in Sibley is the midpoint of this ramble. It’s all downhill from here to Montclair (where you catch the “geologist’s express” 33 bus), heading south on Skyline and then following the route of my Shepherd Canyon circumambulation. Be sure to look back at that lovely ridge as you start down Skyline.

I’ll end this post with a big fat map showing the topographic contours, more road details, and mile markers.

A circumambulation of Shepherd Canyon

18 February 2019

I seem to give myself odd assignments. The latest one was to take a hike around the crest of Shephard Creek’s watershed, better known as Shepherd Canyon. Only during the final mile of that 6-mile walk did I realize what it meant: an outing exclusively on ridge roads.

I got the idea from my circumambulation of Claremont Canyon three years ago. That walk features a variety of rock types and a good set of views into Oakland’s best wineglass canyon — a stream valley with a wide upper watershed and a narrow outlet where it crosses an active fault. Most of Oakland’s major streams that cross the Hayward fault have wineglass valleys that are more or less well-formed (and faceted spurs between them), but you may have to scrutinize them pretty hard to see them, let alone walk their rims.

Shepherd Canyon is another of our wineglass canyons where the roads make this exercise feasible — and an exercise it is, with a thousand-foot elevation gain. The trouble is, you don’t get clear views into the canyon itself. Whereas Claremont Canyon is a viewshed as well as a watershed, Shepherd Canyon’s watershed has an inside-out viewshed. Thanks to our high parklands, this ridgetop walk affords good views around the canyon’s outer side.

Enough preamble — let’s amble. The route starts and ends in Montclair on the geologically friendly 33 bus line. Here’s the layout.

Those little red blips are mile markers. I took this counterclockwise because I prefer steep uphills to steep downhills, and the south end is steep. A bicyclist might prefer the other way, but parts of this route are footpaths.

You could start and end this walk at the end of the 33 line in front of the Safeway, but I started from the Snake Road stop and finished at the LaSalle Avenue stop. Most of the first mile is a bit of a chore along busy Mountain Boulevard, but these days the rain has made the creek noisy, and you’ll hear it down in the woods as you cross the lowest point near the Park Avenue junction.

Be sure and get off the road on this nice path.

It goes around the flattened hilltop housing the Joaquin Miller Elementary School and Montera Middle School.

There once was a rocky hill here, occupied by a Scout camp (which is how Scout Road got its name), but it was vacated and leveled as a handy source of stone and used (I believe) as fill for the Warren Freeway nearby. In any case, the path takes you to the foot of Shepherd Canyon’s perimeter ridge, where you’ll turn right up Mountaingate Way. This view past the foot of Mountaingate looks into the valley of Cobbledick Creek, a tributary of Shephard Creek that runs down along Scout Road. In the next mile you’ll climb 800 feet, pretty steadily.

Soon enough you join Castle Drive and start seeing rocks. Now it’s time for the bedrock map.

The ridge is made up of serpentine rock (sp on the map) plus its typical blocks of blueschist. One of the city’s best blueschist boulders sits where Castle Drive starts, and some of the homes along Castle use the blueschist in their landscaping.

The serpentinite is a sloppier, greener stone that you’ll see along the roadside. Let me warn you that hiking on Castle is a challenge — the road is narrow, there’s no sidewalk, and nobody expects pedestrians. Keep your ears open and step off the road as cars approach.

Where Castle starts to turn north, veer off the road and take the little-traveled West Trail, part of Joaquin Miller Park. You’ll thank me, as I said when I took you down it a few months ago. It’s here that your first views open up.

And the rocks change as you start on the trail. From here on out you’ll be mostly on sandstone. Just for the record, you’ll pass through the Joaquin Miller Formation (Kjm), Oakland Conglomerate (not labeled), Shephard Creek Formation (Ksc), Redwood Canyon Formation (Kr), unnamed Eocene mudstone (Tes), Sobrante Formation mudstone (Tsm) and just into the edge of the Claremont Shale before descending through the same units on the way down.

Castle Road ends at Skyline Boulevard. After a few steps on Skyline, duck into the woods and take the Scout Trail parallel to Skyline, where you’ll meet these guys at the top of the ridge — Oakland Conglomerate.

Where the trail ends at Moon Gate, take Skyline for a few hundred yards, passing the Waterloo Staging Area (unless you’re up for a stroll on the West Ridge Trail, which rejoins this route at Skyline Gate), and take Wilton Drive up onto the ridgetop. It skirts the edge of Redwood Regional Park and has one of Oakland’s best views.

Move on and take Burton Drive, then Shirley Drive. At the end of Shirley is an unmapped trail through Redwood Park that starts with this bench, nestled among boulders of Redwood Canyon Formation sandstone.

Not quite the halfway mark, but just the place to take a rest and a good look around.

The trail descends to the Skyline Gate Staging Area. From there, take Skyline to its confusing junction with Shepherd Canyon/Pinehurst Roads. Don’t take any of those roads — go up Manzanita Drive instead, along the ridgetop. Where the power line crosses the ridge is a fine exposure of the Sobrante Formation’s shale.

This is an unusually good exposure of the Sobrante, which is very sloppy stuff along Skyline, because up here it’s grading into the harder chert of the Claremont Shale.

The two units are stratigraphically continuous, with an arbitrary boundary between them. You’ll see a lot of this rock along the road as you make your way north, then down to the junction of Skyline and Snake Road.

At this spot you overlook Shepherd Canyon on the left, Thornhill Canyon on the right, and the mighty Bay in front as you start down. Do stop at the fire station here and top up your water supply.

The descent along the lip of Shepherd Canyon is straightforward: Take Colton Boulevard to Asilomar Drive to Drake Drive. Again, pedestrians are not expected, but the roads are generally wide enough to accommodate everyone without jostling or stir. Here and there you can see into the canyon. Watch its walls grow close as you come near the narrow part of the wineglass.

When Drake meets Magellan (I know, those two explorers never actually met), the ridge road is finished. It only remains to take the unobtrusive path leading through the trees from 2133 Magellan down to the Railroad Trail — when else will you ever have the chance? — and on into Montclair to catch the bus.

The nice thing about this time of year is that many of the trees are leafless, so you can see more of the surroundings.

That and the weather is cool. I always seem to assign myself weird walks in the high hills during summer, which can be brutal. Better to get out there now.