Archive for the ‘Oakland geology walks’ Category

Sibley South

25 November 2019

The wonderful Sibley Regional Volcanic Preserve has a southern extension, still being developed, that opened to the public recently. It’s at the head of San Leandro Creek upstream from the village of Canyon on the west side of Gudde Ridge, a part of the hills that’s always intrigued me. The land is in the foreground of this photo of Mount Diablo I took from Huckleberry Botanical Preserve in April 2009.

The property was homesteaded in the 1860s by Patrick and Catherine McCosker and remained in the family for the rest of that century and all of the next. It was a cattle ranch for most of that time, with a small kitchen orchard. The Oakland, Antioch & Eastern railroad exited the old Redwood Peak Tunnel eastbound near the ranch entrance and stopped there at a station named Eastport, still a locality on the map.

About the time when plans were floated to punch a new state highway up Shepherd Canyon and through the hills to the Lamorinda area, the McCoskers started a rock quarry in hopes of profit during the highway construction. (Transport distance is a very important factor in the crushed-stone business.) It operated from 1958 to 1971, then continued as a rock crushing plant serving various construction projects in the area.

After that business petered out and the land languished for a while, the McCosker descendants sold a 250-acre chunk known as the Texas parcel to the developers of Orinda’s Wilder Ranch community, and as part of a deal the land was donated to the East Bay Regional Parks District. The “McCosker Sub-Area” opened to the public in December 2016.

The parcel occupies most of the valley north of Eastport, as seen here on the 1959 topo map. I’ll show the bedrock map of the same area later.

It’s precious property for native plants and a good wildlife corridor, tucked between the Huckleberry preserve and steep, remote grazing land in the Sibley preserve. The District classifies it as a Natural Unit, managed for its “unique or fragile habitat values with public access primarily limited to trails.” Its permanent stream, a major tributary of upper San Leandro Creek given the name Alder Creek, will have some 1300 feet of decaying culvert removed and be restored to good riparian habitat suitable for native rainbow trout.

To protect this wild country, access for dogs and horses will be tightly controlled. (At present, dogs are not allowed at all.) Plans include a small campground suitable for backpackers on the major Bay trails.

On a misty weekday afternoon, it feels very far from the rest of the Bay area.

On sunny days, though, the views are good. I went back yesterday to capture some.

The rocks here are mainly from three map units: the Claremont Shale (the gold strip along the valley floor), the Orinda Formation (Tor, conglomerate) and Moraga Formation (Tmb, lava flows, and Tms, associated sediments).

The Claremont peeks out of the woods near the entrance with its characteristic, steeply dipping stripes.

The Orinda pokes out of the hillside with its characteristic beds of coarse gravel and cobbles — but only in roadcuts.

Elsewhere, out on the bare hillsides, the ground is nearly pure clay soil with only rare pebbles exposed, tokens of a very different terrain dating from some 10 million years ago.

The Moraga Formation lava is harder stuff, and there are a few actual outcrops in the high hills.

Near the entrance is a flat platform with a large shed, evidently built from quarry waste. It will become a parking lot and recreation area named Fiddleneck Field. For now it’s a parking lot for boulders from other parts of Sibley. Check them out. They represent the Orinda and Moraga formations, plus a few white limestone chunks from the Moraga sediment unit or the slightly younger Siesta Formation.

Some of the lava features the distinctive amygdales, former voids filled with hydrothermal minerals, found at Sibley.

Another thing to notice here and there is small landslides, like this slump at the edge of the Fiddleneck Field platform. Others occur along the ranch roads, which were built long ago by ranchers without the advice of geotechnical consultants.

As far as I can tell, the quarrying took place in scattered pits of no great size. That area is screened by oak/bay forest and cordoned off with barbed-wire fence, so it’s hard to tell. I look forward to seeing this land opened up as the District does more work.

In the meantime, there are splendid views to be had.

Give it a visit some time.

Details of the plans for this area are in the Sibley Land Use Plan, available if you search for it from the East Bay Regional Parks District website.

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.