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.

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.

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.

Anomalies of Sausal Creek: The Floodplain

28 October 2019

This is the third of four posts covering the Sausal Creek watershed from hill to Bay. It features the part of the creek between Dimond Canyon and Foothill Boulevard. The early Anglo landowners were quick to establish fruit orchards and berry patches there, giving rise to a settlement and district out east in Brooklyn Township they called Fruit Vale.

That was just the right name for this distinctive locality. Its alluvial land, flat, tractable and naturally well watered, was ideal for planting. I say “planting” rather than farming because within a few decades — the commercial lifespan of an orchard — the farms gave way to grand homes with lush landscaping. That’s the “Fruit” part. The “Vale” part refers, as the word has since the 14th century, to a wide, flat lowland between gentle hills with a stream in it. An 1888 pamphlet described it as “one vast garden,” its roadway “lined with elegant residences and villas surrounded by beautiful lawns and gardens. . . . The soil is easily cultivated and everything planted in it thrives.”

The creek itself is hard to see in this stretch today, where homes and overgrowth block the streambanks. It also runs a good 20 feet below the surrounding land, a typical East Bay arroyo with banks like cliffs. Except for a lovely little fenced-off area on Barry Place, distant glimpses are the closest you can get to the water.

By textbook standards, Sausal Creek has the most well-formed floodplain in Oakland. A floodplain forms wherever a stream runs out of downhill energy and has to drop the sediment it’s carrying. The elevation profile I made for Sausal Creek shows the floodplain portion.

This part of the stream is at grade, like the canyon above it and the delta below. It’s very stable. Neither fully erosive nor fully depositional, a stream in a floodplain has little it can do except rearrange the sediment.

Floodplain streams tend to turn snaky, meandering and migrating like the trickles you’ll see on a rainy windshield. They nip off bits of the bounding hillsides and gradually widen their corridor.


From Phil Stoffer’s geologycafe site

A floodplain is a place where sediment languishes during its slow journey to the delta. A lot of that sediment enters the floodplain from the hills on either side. Here are three different views across the floodplain showing its flat floor and abrupt edges. Fruitvale Avenue at E. 27th Street:

Farther upstream at MacArthur Boulevard:

And still higher at Coloma Street.

For real detail, I always look for a digital elevation model or DEM. The DEM of the upper end of the floodplain shows how flat its floor is and how fragile the west side of the valley is. It also shows how much the stream itself is meandering within the straight floodplain valley.

Unlike the rocky flanks of Dimond Canyon, the hills lining Sausal Creek’s floodplain consist of sediment, ancient gravel beds that I’ve named the Fan. Time to look at the geologic map of the floodplain section.

The material of the Fan, labeled Qpaf on the map (“Quaternary Pleistocene alluvial fan”), is unconsolidated sand and gravel that’s prone to landslides on steep slopes. That scoop out of the hillside just left of center in the DEM image is the notorious McKillop slide, in progress for more than a century. Much of the crumpled land there was smoothed over and preserved as William Wood Park, but the adjoining slope to the south, which failed in 2006, is still a mess.

Two things about this stretch of Sausal Creek strike me as odd. The first is how straight the stream valley is here. Streams tend to curve, so if you find a naturally straight one you look for what’s forcing it to be that way.

My tentative theory, which I alluded to at the end of the previous post, is that the Fan was tilted up, not built up, along with the Piedmont bedrock block thanks to tectonic interactions along the Hayward fault. (In other words, the Fan is not really an alluvial fan but an uplifted relic of an older coastal plain.) I suppose that the stream was incised in a straight line by large flood events early in its history, then widened into the floodplain we see today. It’s easy to imagine big pulses of floodwater coming out of Dimond Canyon like a firehose, most likely after a temporary lake above the canyon, dammed up by an earthquake landslide, gives way. Such floods would rush in a straight line to the sea and set the stage for the floodplain that evolved.

The other odd thing is how closely the stream hugs the west side of the valley. So does Peralta Creek, just east of Sausal Creek. I think this may not be an accident. Glen Echo Creek, on the other side of the Fan, hugs the east side of its valley. Perhaps there is some subtle warping of the crust across central Oakland.

Sausal Creek’s fruited plain may be just a vale, but it’s a vale with some intriguing features.