Along the entryway to the Leona Quarry development is a tumble of big decorative boulders, ready for deployment in the landscaping. When I drove by I made a mental note to check them out. I assumed they were chunks from the quarry itself. No such luck; they’re exotic blocks full of shell fossils, which are unheard of in Oakland. This is a view of the shell faces.

This boulder is almost what geologists call a shell hash or coquina, but the shells are mostly intact. I might call it a shell marl instead, but the default name would be fossiliferous limestone. I have no idea where it’s from—somewhere over the hills, or maybe in the Santa Cruz Mountains, but not Oakland.

Here’s a closeup of another boulder showing a cross section of the shells.

This boulder is closer to a shell hash; it’s mixed with sedimentary rock clasts. A few of the darker clasts have released iron, presumably from sulfide minerals. This formed in a very active coastal setting.

It’s a fascinating set of boulders, but an arbitrary one. It also includes big hunks of shale that have disintegrated in the sun and rain here. Who picked them, and why?

A few days ago I visited the former Leona Quarry, now slowly filling with townhomes. The view was great.

The rock slope on the north side looks rugged but is heavily engineered with many setbacks and retaining walls. But I was here to look at the rocks. There’s no trespassing on the high slopes, which is just as well because there is little bedrock to be seen. The whole hill is mapped as quartz keratophyre at the very bottom of the Great Valley sequence, of Jurassic age (no younger than 140 million years). It’s lava flows from an oceanic volcanic arc, like modern-day Japan or the Philippines, that have been heavily altered. It’s typically light colored with strong rust stains and full of fractures. You’ll see several different examples below.

Some outcrops show bits of less-altered gray lava, although this example is still full of veins.

This small boulder displays slickensides. The white mineralization includes quartz as well as carbonate minerals, sometimes layers of each in the same vein.

It’s pretty sterile, but life is taking hold amid the stones.

I took a couple hand specimens of the hardest, most pristine looking rocks. Even these have a greenish tinge from metamorphism. This specimen has faint banding in it, probably metamorphic rather than from flowing of the lava.

This kind of rock is almost impossible to study without using thin sections and a petrographic microscope. I don’t know what I would call it if the map didn’t say—probably just “altered lava.” Quartz keratophyre is specifically a metamorphosed trachyte, a highly alkaline high-silica lava. That’s meaningful to petrologists in a way that would put most of us to sleep if they were to tell us about it. I don’t mean that unkindly. But I’d hazard a guess that here it reflects some input from subducted continental sediments as well as the usual melted seafloor.

Rock from the Leona Quarry appears all around town in rustic walls, but I suspect that nearly all of its output went to crushed stone.

The original rainbow trout come from Oakland; did you know that? The species Salmo iridia was first described, in 1855, from San Leandro Creek. Redwood Creek is a branch of San Leandro Creek that still contains good spawning grounds for rainbow trout, and this nicely maintained fish ladder is here to help them upstream.

The rainbow trout has been spread all over the world, of course. This wasn’t the only locality in the Coast Ranges they came from. The Oakland fish aren’t superior to other local strains. Their only distinction is artificial: they’re merely the first to come to scientific attention. Naturalists name species simply by picking an individual and describing it in detail in the literature. It’s arbitrary, but the only way to begin.

That’s as arbitrary as the creationist’s crude notion that the species is simply something uttered by God and named by Adam. Creationists doesn’t care to ask why populations vary in their genetic makeup—or if they do they regard it as the necessary decay from perfection of all earthly things. Variation is useless in their theory. But to naturalists, the reality of the species includes precisely the variation that the creationist downplays. Variation is the first rung in the ladder of evidence showing how the tree of life fits together as the result of evolution. The creationist’s interest in that project is only to suppress it.

Today we lump Salmo iridia as a subspecies under the larger species Oncorhynchus mykiss. Maybe God isn’t clear, or maybe our ideas need work. Maybe both statements mean the same thing.

The new Christ the Light Cathedral is a beautiful structure, designed to guide the mind toward bliss, to allow the susceptible a glimpse of heaven. In early 2005 I sought a high place of my own type—a parking structure—to have a look at the cathedral’s construction site.

The site was at one time a glamorous high-Deco car dealership. Before that I don’t know, but the Oakland geologic map shows it as half fill and half “marine coastal terrace” deposits. The fill half would be on the lakeshore side, naturally. The terrace is basically a shelf of sediments deposited in San Francisco Bay during the last interglacial, more than 70,000 years ago, when the sea was a good five or ten meters higher than today. Only small, subtle bits of it are around today. The pit looks like it’s floored with nice clean golden sand. That might be aboriginal sediment, or it might be dirt from downtown hauled here to fill in the swampy lake shore, as it was around almost the whole lake. The downtown dirt is Merritt Sand, a widespread sheet of ancient windblown dune sand much like what underlies western San Francisco. That sand came here at the height of the last ice age, when the seas were very low, the weather was cold and the winds blew fine sand from the wide, exposed continental shelves onto the coastal hills.

If I had an hour to poke around these excavations! But only the geotechnical engineers get to do that, and maybe a touring group of their fellow professionals, all in hardhats. If any of those fine specialists are reading this, my email is geology at about dot com.

Oakland’s City Hall was the tallest building west of the Mississippi in 1914, when it was completed. It’s still an impressive structure, 324 feet high, covered with intricate stonework and flooding the plaza with warm reflected light around midmorning.

City Hall weathered the 17 October 1989 earthquake without collapse, although there was serious damage and it is said we nearly lost the clocktower. After the quake the city was motivated to retrofit the structure. In evaluating the possibilities, Charles Rabamad and Donald Wells write, “To minimize the amount of new construction, the existing structure was given credit for the strength it exhibited during the Loma Prieta earthquake. This performance-based approach required less strengthening than conventional, code-based design, which ignores the existing capicity of the building.”

Today City Hall rides on a grid of 113 big, fat rubber-and-lead base isolators 19 inches high and either 29 or 39 inches wide. These soften the shaking and allow the building to be strengthened with the least impact on the historic building’s interior. The building will shift back and forth as much as 17 inches. It’s designed for a magnitude-7 earthquake on the Hayward fault, after which some cracking, fully repairable, is expected. Completed in 1995, the retrofit was the world’s first base-isolation project for a high-rise building, setting the precedent for many more retrofits including several at UC Berkeley. Now Oakland City Hall is in all the engineering textbooks.

The Earthquake Engineering Research Institute has a publication on the whole thing, and a 15-page paper by Mason Walters with the dirty details was presented to the Third Seminar on Utilization of Strong-Motion Data in 2003.3

Back before it became a tony townhome colony, the Leona Quarry was Oakland’s largest quarry and a major eyesore. I took a photo through the fence in January 2003, shortly after work began on the redevelopment:

Here’s a more distant view taken that March, from Burckhalter Park:

And this was in September 2005.

The city has a long page of documents about the site. The most recent vegetation report, from 2007, notes that most of the hundreds of trees planted here are doing well, although some species shouldn’t even have been considered for the dry, west-facing rocky location. It also notes several notorious invasive plants taking over, including stinkwort (“smells like Noxzema”). Stinkwort is the only green weed at this time of year, a formidable competitive trait.

I would have preferred reforestation without new homes. The smooth amphitheater is an unnatural landform, and the things it does to sound from the freeway must be disconcerting to residents. You might think that the Hayward fault is nearby; not so much. It runs nearly a mile away. Well-built new homes should be fine here, meaning they won’t kill you in the next big earthquake.

We all admire the Bay when we visit high places, whether it’s here at Dunsmuir Ridge (click for a 2X version) or elsewhere in the Oakland Hills. But why is there such a big basin here?

One ready answer comes when you contemplate the major faults, the San Andreas and the Hayward, that bound the bay on the west and east sides respectively. They aren’t quite parallel, but fan slightly apart as you progress to the north. As the crust moves along these faults, the part between would sink, just like what happens at the smaller scale when a sag basin forms. Maybe it does. Why, then, are the Oakland Hills still rising from compression across the Hayward fault? Well, maybe things shift direction slightly from time to time and the sense of stress across the fault changes. So today we have compression while other times we have extension. But maybe the analogy of sag basins is the wrong one.

Geologists have the same question when we consider the Great Valley, that vast trough between the Sierra and the Coast Range. Most sedimentary basins are depressed by their load of sediments, but not to the point of lying at sea level like the Great Valley. The current working hypothesis about the Great Valley is that it’s a captured slab of oceanic lithosphere, a dense bottom layer, with thick sediments on top. It naturally, persistently rides lower than the continental rocks around it. Might the San Francisco Bay basin be a chunk of the same slab? That’s one hypothesis; we don’t really know.

The roots of the Bay are pretty deep for active-source seismic imaging, the kind of technique used at the Garrido property in Antioch to study a crime scene. The Bay is also pretty small for the passive-source seismic studies used to study the crust and mantle at the regional and continental scale. If you gave me a few million dollars, I could make a stab at an investigation, but no one can spare that for an idle question. So we’ll keep wondering as we stand on the high places.

Sibley Regional Volcanic Preserve has some spectacular places like this spot at the north end of the park, where you can visualize when this was a working volcano. Click the view below for a bigger version.

This set of rock layers was laid down flat, but since then it has been tilted counterclockwise to nearly vertical. The red zone appears to be the baked top of a sediment bed that was buried in lava. The sediments were themselves deposited on an earlier lava flow, at right. I didn’t inspect this close up, though, for a couple of reasons. One, I was out for a walk and didn’t have my boots on. For another, this is easy to look at but extremely steep and hard to get close to. Finally, it’s hazardous, being prone to rockfalls:

A local gas station has been undergoing remodeling, but one day it had geo-specialists pay a visit. On the left is a small drilling rig, and on the right is the typical panel truck used for cone penetrometry testing (CPT). It’s propped up on stout legs to make a stable platform, and as I peeked underneath I could see the penetrometer shaft sticking into the ground like an ovipositor.

CPT consists of pushing a steel shaft with a standard cone-shaped tip straight into the ground. Sensors on the truck measure how much power it takes to do this in the tip measure the pressure, which varies as the tip penetrates different types of soil. Sensors in the tip also measure the electrical conductivity of the soil, pore-water pressure, and other things. In deep ground like this part of Oakland, a CPT tip can be pushed hundreds of meters down, but here they’re probably going down no more than 10 or 20 meters, just deep enough to see if the underground fuel storage tanks will be stable there. In chronically wet ground, empty tanks have been known to rise out of the ground during, say, earthquake shaking. Learn a little more on my About.com site.

This is hardhat work, but it’s not very dirty. Geotechnicians can get steady and varied work doing CPT with no more than a high-school education. The crew of this truck included a woman, too.

By the way, I have entered the “Blog Your Way to Antarctica” contest, which runs through September. Please see my entry here, and if you like the idea, give me a vote—the earlier the better. I know it would mean a break in this blog, but I’d make it up to you.

The Elks Order has a picturesque plot in Mountain View Cemetery on a south-side hilltop. This elk-topped crypt is surrounded with rugged boulders of serpentinite, probably of local origin. The stone is well chosen, being inhospitable to plants and moss, and adds a bit of miniature grandeur.