Archive for the ‘Oakland geology puzzles’ Category

EarthCaches in Oakland

14 March 2022

Maybe you’ve heard of geocaching, the wayfinding sport that’s swept America since 2000. The basic game involves hiding and finding secret packages in public spaces, all of which — GPS coordinates of the hides and logging of the finds — is administered via the geocaching.com website. My first exposure to geocaching was during a 2004 outing to remote Idria, California, where my friend Jef made a stop to visit and log this ammo-box cache in a roadside boulder pile. All he knew was the latitude and longitude.

The geocache visitor may find a paper logbook inside along with various trinkets.

As with a Little Free Library, visitors may take a trinket and leave one for a future geocacher.

I did not take up this hobby until last month, when I decided to take part in the Geological Society of America’s EarthCache variant of geocaching. (I’ve been a GSA member since the 1980s.) An EarthCache is a nonintrusive, virtual geocache without a container or trinkets, just a set of coordinates attached to a little free geology lesson about that location. You find your way there, observe the scene with the help of the lesson, and log the cache by submitting the answers to a set of questions posed by the cache’s creator.

Naturally I wanted to set my EarthCaches in Oakland. My first two EarthCaches have been approved, and as of this writing the third is under review by the GSA team. [Late update — here’s the third.] The first is up in the high hills, and the second is near Montclair. If you’re a regular geocacher, go for it! (There are a half-dozen other Oakland EarthCaches besides my two.)

If you aren’t a geocacher yet, you’ll find when you follow those links that the locations are censored, but the descriptions are visible. That means you can play a reverse version of the game if you like: from the descriptions, figure out the location and post it in a comment.

If that sounds fun, here are hints for each EarthCache. These are photos that visitors have sent me to document their visit. This one is from the first EarthCache.

And this one is from the second.

For Galaxy Brain status, tell me the locations without visiting the EarthCache pages.

This is a side project that I’ve spun off the book project, which is coming along nicely.

In search of McAdam’s quarry

3 August 2020

Alexander McAdam (1854?-1920s) was a minor character in Oakland’s history who left a highly visible mark in our cityscape. A Canadian farmboy who was orphaned at a young age, he came to California after apprenticing as a wheelwright, and after eight years he saved enough money to buy a farm “at the head of Thirteenth avenue,” according to a short biography by James Guinn in 1907. “He was successful in this occupation, but in the meantime had discovered a sandstone quarry on his property. Upon the sale of his farm he acquired considerable financial returns. Stone from it has been used in many of the largest buildings of Oakland, among them being the Unitarian Church, the last buildings of the deaf and dumb asylum, numerous retaining walls, and for many other purposes.”

This caught my eye because I have long thought that Oakland’s rocks were exclusively used as crushed stone. Yet here in the First Unitarian Church, ashlar blocks of genuine Oakland sandstone form the dignified cladding of this important cultural monument and civic institution, built in the early 1890s under the energetic leadership of a leading Progressive of his time, Rev. Charles Wendte.

Rev. Wendte oversaw the building project from his home across the street. The stone cladding was the costliest item in the project, and he singled it out in his memoirs: “Our employment of stone led to vexatious complications. Quarrymen were unable to deliver this material in sufficient quantities, workmen struck for higher pay in handling it. Contracts were broken or remade.”

I had to track down this stone somehow. The documentary clues are slim, and any signs of the quarry appear to be lost. But first, there is the stone itself.

It’s a fairly sound stone of an even consistency with a warm grayish-brown color and massive (i.e., absent) bedding. The block serving as a lintel over the doorway probably broke during the 1906 earthquake, when most of the cladding along Castro Street and the top of the tower collapsed. (The tower was rebuilt without any stone, a smart move.)

A closer look shows that the stone actually varies (although some of that may be substitute stone from another source, as Wendte’s wording suggests), and that a century of exposure has caused a fair amount of spalling. No wonder there were quality problems during construction.

A still closer look reveals it as a medium-grained wacke (“wacky”): a sandstone with grains no larger than a millimeter and a large component of minerals that are not quartz. The black grains are mostly biotite mica; without a microscope I’m limited in what more I can say.

It’s familiar to me. It’s not the Franciscan sandstone produced by the dozen or so quarries in and around Piedmont. I can rule that out categorically. It’s from the high hills on the far side of the Hayward fault.

All of this is consistent with the documentary evidence placing the source in Montclair. The “head of Thirteenth avenue” is where Park Boulevard, the former 13th Avenue in Brooklyn Township, meets Mountain Boulevard. It’s the intersection at the bottom of this excerpt from the 1897 topo map.

To orient (or disorient) you, here’s the same area today.

The “XII Report of the State Mineralogist,” published in 1894, said the following about McAdam’s quarry: “It is in Medos Cañon, back of Piedmont, and is a small quarry, producing sandstone for rubble and ashler [sic]. It is not worked regularly.” The official who wrote that description, a busy guy on a quick visit to cover the whole county, wrote down “Medos Cañon” when someone said “Medau’s canyon,” meaning the valley of present-day central Montclair where the dairy farm of John H. Medau once lay. I believe that if the site had been in Shepherd Canyon, his informant would have said so as that name was in wide use at the time.

All of this means that the quarry could have been a good exposure of the Redwood Canyon Formation, a wacke of Late Cretaceous age, that forms part of the east side of Montclair’s valley along the Hayward Fault. It’s the unit marked “Kr” on the geologic map, below. The lithological description of the unit, and the composition data from Jim Case’s 1963 Ph.D. thesis, are close enough to the stone in the church.

But also likely is the Shepard Creek Formation (Ksc) and even the Oakland Conglomerate (Ko), when you consider that the units are only subtly different, variable in composition and not well mapped despite the best efforts of competent geologists.

In any case, I had a good time visiting these rock units along the Montclair Railroad Trail the other day. There’s a lovely outcrop of the Redwood Canyon Formation above the trail along the route of the recently upgraded powerline, southwest of the word “grade” on the map. That warty weathered surface, reminiscent of the Incredible Hulk’s hide, is one of this unit’s distinctive features.

But the rock there’s not a good match.

Neither is the rock in the landslide at the upper end of the trail.

And just for good measure, here’s a chunk of sandstone from the Oakland Conglomerate. The material is coarser and wacke-er, but again under the 10X hand lens it’s not like the church’s stone.

Nowhere in this area, in many years of visits, have I seen a body of rock big enough and sound enough to support a quarry capable of producing ashlars — not on this side of the Hayward fault. The nearest quarry site is down Park Boulevard where the Zion Lutheran Church sits today, the former Heyland/Diamond Cañon/Bates & Borland quarry on the side of Dimond Canyon. But that produced crushed Franciscan sandstone, something quite unlike McAdam’s stone.

I can only conclude that McAdam found a lucky hillock on his farm and made the most of it, one that’s been obliterated during the waves of development since 1890. And the site of his farm is, as we say, poorly constrained. Even his life dates are fuzzy. But his accomplishments include making a profit from farming, acquiring a large home in Temescal, serving two terms on the City Council in the nineteen-oughts, and equipping an important building with a handsome exterior (despite the vexation he caused Rev. Wendte). I can’t confirm when he died or where he’s buried, so this building surely is his monument.

While I was researching this post, the papers covered a lovely story about how archeologists used advanced geochemistry to pin down the source of Stonehenge’s biggest stones, a peculiar sandstone known in Britain as sarsen. The New York Times version was my favorite writeup, and the hardcore details are in Science Advances in an open-access paper.

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.

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.