Lake Chabot’s north shore

23 December 2019

Oakland has a lot of ground to cover, and it can take a while for me to return to places I’ve been before. In this case, it’s been four years since I traversed the trails on the north side of Lake Chabot, between the reservoir and the golf course.

The land is much the same (though I’ll point out some differences). It’s my frame of reference that’s changed.

Here’s the Google Earth view of the area. My walk started near the dam and went up the trail (the Bass Cove Trail) along the west edge of the lake, then back down on the unmarked, unsigned trail running just west of all the chaparral.

The latter trail is basically an access road for the power line that runs through here over the hills. The land ownership is mixed, but there are no barriers.

For reference, here’s the bedrock map of the same area. (It also shows the access road snaking along the power line.)

Jsv is the Leona volcanics, KJk the Knoxville Formation, and the slightly darker green on the right is the Joaquin Miller Formation. The blue section labeled KJfm is mapped as Franciscan melange. I’ll show you some of these. What’s changed in my frame of reference is that I’ve learned there are conflicting interpretations of that blue section. I found no smoking-gun evidence in my visit, so you’ll be spared the details in this post, but my eyes were peeled in a way they weren’t before.

I like a lot of things about this time of year. For purposes of geologizing, the footing is firm and quiet even off the trail. Also, the rain has washed the outcrops clean.

Of course, rocks like this are still covered with lichens so you can’t be quite sure what they are. The best-exposed rocks are in the streambeds, like this blueschist boulder.

All three of these are in the melange unit. For purposes of my enlightenment, it’s the matrix between these blocks that’s crucial, but none of that was visible. Just have to keep looking.

The sandstone of the Joaquin Miller Formation is nicely exposed in the rain-washed roadways.

It’s a pretty pure sandstone; there are spots in the trail where rainwater has washed the eroding stone downhill into sandy drifts.

Sand is nearly eternal. It can be recycled time and again in the rock cycle for hundreds of millions of years. But that’s another post.

And off the trail I was pleased to find some excellent examples of the conglomerate at the base of the Knoxville Formation, which is otherwise mostly shale.

The rounded cobbles in this exposure are largely composed of the Leona volcanics, proof of the genetic link between the two bodies of rock and a clue to the geography of ancient California during the Late Jurassic.

Enough bedrock. Other things I like about this time of year are that it’s cool, the air is clean and fragrant, the colors are distinctive and the light is interesting. It’s a primo time, if you ask me, to walk the high hills, and this part of town offers good views of Fairmont Ridge and the lake.

Even a peek at the Hayward hills and Mission Peak beyond.

The parks district has been visibly sprucing things up in the park. And along the power line, it’s obvious that PG&E has been at work too, taking the fire risk seriously by reducing the fuel load.

They’ll probably shred this plant material and leave it on the ground, but I would favor some good old-fashioned controlled burning here in the fire-friendly chaparral. They’re even making a little headway against the eucalyptus, which besides its fire hazard tends to shed limbs. Eucs make fine specimen trees, like the one across the way, but lousy forests when they aren’t well tended — take a look at Australia these days.

Lake Chabot and the surrounding parks are a special part of Oakland. Get yourself out there; let your mind roam free.

Even at the bottom of the year, there’s a lot of good light.

Posted in Oakland blueschist, Oakland conglomerate, Oakland geology puzzles | Leave a Comment »

Drunk on rocks

9 December 2019

Every now and then I come upon sights in the field that transfix me, that make me stop and stare, that suspend time. This is not uncommon with landscapes, of course — that’s why our phones are full of vacation pictures and why I have so many photos to share in these posts. It probably happens a lot less often with rocks, except among geologists and maybe not all of them either.

A few examples. It happened to me this spring at this sandstone roadcut on Bitterwater Valley Road west of Blackwells Corner.

Granite stoned me in the fall of 2006 just east of Donner Pass.

It happened to me in June 2012 a little west of Copperopolis. (I was bringing back a piece of this slate I’d taken two years earlier.)

One day in 2008 I found myself lost in the landscape of this ancient Nevada limestone.

And Oakland rocks can affect me that way too, like this serpentinite in Joaquin Miller Park’s native plant nursery did in 2010.

It can be a little embarrassing when you’re supposed to have your geologist’s eye engaged, and all you can do is stand there stunned. Being drunk on rocks is a subset of an experience I call field intoxication. Professionals need to get over the tendency, and teachers probably see their students fall prey to it, but I can let the high happen even in the presence of experts. That’s a writer’s privilege.

The last time it occurred, and gave me the topic for this post, was in Oakland this summer when John Wakabayashi, a leading figure in California geology and an Oakland native, accepted my invitation to visit some choice localities in his old home town. The finale of the tour was the pod of high-grade blueschist on the grounds of Mills College. I’ve featured it here before.

I knew he’d like it, and in fact he was delighted. He got animated. He climbed around, waved his hands and pointed out telltale features. He didn’t seem drunk at all. But while I noted what he was saying, I sat down dazzled into stillness. You see, this time the outcrop was clean after the spring rains and fully illuminated by the high July sun. I’d never seen it that way before. The rock shone blue as brilliant as the sky itself, and at all points it glittered with minute crystals and mineral flakes. All I could do was let this light wash over and into me, a sauna of stars. Knowing the camera was helpless to capture the moment, I didn’t attempt a photo. (Sorry.)

That’s getting drunk on rocks. Keep your eyes open and it might happen to you.

Posted in Oakland chert, Other topics | 2 Comments »

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.

Posted in Oakland geology walks, Old quarries | 5 Comments »

Anomalies of Sausal Creek: The Delta

11 November 2019

This is the last of four posts about Sausal Creek from the hills to the Bay focusing on its odd features, stuff that’s been bugging me like a seed stuck in a tooth. Here I’ll talk about the creek segment below Foothill Boulevard, where the floodplain ends and the delta begins.

A delta is a wedge of sediment, built near a river’s mouth where it deposits most of its muddy load. Streams tend to spread out in their deltas, sending sediment here and there like a Vegas card dealer. While the mouth of a big river like the Mississippi or the Nile fans out in a set of multiple distributaries, little streams like Sausal Creek move their courses every once in a while so that over thousands of years, every part of the delta gets its share of dirt.

Today, Sausal Creek officially meets the Bay in this culvert next to the Fruitvale Bridge . . .

. . . with this monumentation.

But it’s all totally artificial. This body of water is a large canal that was built in the late 1800s as part of the Oakland harbor improvements. Before that, Alameda was not the “Island City” but a town on a wooded peninsula called the Encinal, and the only way to get there on dry land was through here, across the delta of Sausal Creek.

The creek’s delta is unlike the deltas of Oakland’s other major streams. Here’s what I mean. Look at this map of central and east Oakland that shows only young material, whatever is not bedrock.


From USGS map OF 2006-1037

I’ve labeled Sausal Creek, shooting south-southwest straight to the Bay where the Alameda peninsula sits in its way. The dark purple stuff labeled “afem” is all landfill (“artificial fill over estuarine mud”), and the light yellow part marked “Qhf” is young river sediment (“[Quaternary Holocene] alluvial fan deposits”). The three pink blobs labeled “Qds” are areas of old Ice Age sand dunes: one under downtown Oakland, one making up the Encinal, and one under Bay Farm Island.

So before the canal was dug, Sausal Creek, unlike all other Oakland creeks, dumped its load here against a buttress of sand. The willow thickets that gave Sausal Creek its name must have thrived here. The early Anglo settlers were quick to put roads and rail lines through this area, and the brushy marshy creek delta would not stand in their way for long.

Which way did the creek run from here, to the right or the left? The only clues are a few old maps, not all of them trustworthy. The first official map of Alameda County, published in 1857, shows Sausal Creek, at top center, draining to the right into San Antonio Creek, the tidal inlet now known as the Oakland Estuary.

The Haynes map, published in 1878, shows it petering out and not even reaching the Bay. Other Oakland creeks, like Courtland and Upper Elmhurst and Seminary Creeks, were the same before they were diverted into pipes and ditches.

But every other map, of this vintage and later, shows Sausal Creek draining to the left into San Leandro Bay. I have no doubt that the road and railroad builders dug a ditch through the sand to control it (and conveniently mark the boundary between Alameda and Brooklyn Townships). The Thompson map of 1878 is a good example, followed by a map of the same area today. Sausal Creek is at top left.

Is it possible that the creek mouth shifted naturally from the right to the left during those years? Certainly; in fact the notorious winter of 1861-62, the wettest in our recorded history, could have done that by sending a big pulse of mud down the creek as it overspilled its banks. And the 1868 earthquake could have liquefied the ground here. The rarest events make the biggest difference. But in this case I would blame us.

The geologic map of Oakland shows the delta in more detail, outlining especially young wedges of sediment (Qhaf1) that were laid down by the creek in very recent geologic time, probably within the last few thousand years.

It’s even plausible that Peralta Creek, just a few blocks east, joined Sausal Creek off and on over the centuries and contributed to this delta. A prominent example is up in Richmond where Wildcat and San Pablo Creeks form a joint delta, at one point flowing just a stone’s throw from each other. But today Sausal Creek’s mouth is a truncated version of its true self, trapped in culverts for the foreseeable future, a dead delta.

Posted in Oakland geology puzzles, Oakland streams and water, Sausal Creek watershed | Leave a Comment »

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