Archive for the ‘oakland soil’ Category

Sinking and rising

25 August 2010

Sometimes the changes we can deduce are simple. In geology, not always so. If you visit Jack London Square, you shouldn’t miss Heinold’s First and Last Chance Saloon:

heinolds

The building and its floor are canted, and there’s a step down as you enter. The tilt adds a certain funhouse spice to the smoke-dimmed fixtures and memorabilia. What happened here is that the bar was set on landfill, the kind of early landfill done by Oakland’s first developers using harbor dredgings and other waste. Horse and hands and probably some steam powered the work, and little of that hard effort could be wasted on tamping the muck down to be worthy of the ages. So, imperfectly compacted, the “made land” settled for many years. At the same time, layer after layer of new material was laid around the tipsy little landmark, and now it’s the sunken treasure we enjoy today.

Did the building sink or the land rise? Yes.

Parts of San Francisco have gone through the same history, well south of Market around 10th AvenueStreet. Old sinking marshland, topped with layer after layer of fill added to keep the streets draining properly, have left century-old homes almost half a story below street grade.

On the California coast, much of the land is gradually rising while, over geologic time, the sea rises and falls. The dance of land and sea sometimes enables the sea to cut deeply into the coast, creating wave-cut platforms that perch above the beaches today. In Oakland that sort of topography is either obscured or nonexistent, most likely because the bay shore has never been exposed to Pacific surf.

Fields of goldfields

31 May 2010

serpentine prairie

The efforts to improve Serpentine Prairie are paying off. Last year I mentioned that the heart of the prairie would be fenced in for a few years. They have also removed the non-native pines that were inappropriately planted. This photo, taken March 28, shows the prairie awash in goldfields. Early summer is the best time to see the rare Presidio clarkia in bloom up there.

Goldfields (Lasthenia spp.) is a very hardy genus that often carpets serpentine soils. Naturally it deserves its name for that alone, but it also has a deeper association with gold. The Mother Lode, running along the western flank of the Sierra Nevada, is a major fault zone, long inactive, that once marked a tectonic suture. Hot gold-bearing fluids rose along the fault and infused the country rock. Serpentinite came up the same way, in the same places. Roadcuts along state route 49 expose lots of it. So where the goldfields blooms, the odds of finding gold are significantly better than chance. Oakland has a great variety of rocks, but no gold that I know of.

Treasure!

12 November 2009

excavation

Where the subsurface is inaccessible, you learn to prize the opportunities to sample it. A house on the crest of Montgomery ridge is upgrading its foundation, and the contractor has helpfully kept the tailings in this handy bin. It’s mostly sand and mud, but there’s a good deal of the Franciscan sandstone in it as well, plus hunks of the lovely red chert that seems to be part of the youngest detrital beds all over this hill. I restricted myself to collecting just one chert piece to add to the pile in the front yard.

I keep thinking that a concerted effort by enough Oakland citizens could help ensure that these ad-hoc trenching studies are properly exploited for their scientific value.

A glimpse below

21 October 2009

christ the light cathedral foundation

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.

Geologists at work

31 August 2009

cpt truck

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.

Serpentine Prairie

18 May 2009

serpentine prairie

Serpentine Prairie is a segment of Redwood Regional Park right across Skyline Boulevard from the Crestmont neighborhood (where my very first post came from). It’s a small remnant of a serpentine barren, most of which is occupied by hilltop homes. This is the entrance from the parking lot at 11500 Skyline Blvd. Some large boulders of serpentinite are here to keep out vehicles, but they’re handy for studying this rock type. The lush greenery is foxtail grass, an alien invader that benefits from the nitrogen of dog urine near the path and the absence of fire.

serpentine prairie

This view shows how serpentine ground differs from sandstone slopes in the distance. Serpentinite yields a soil that is very high in iron and very low in calcium, phosphorus and nitrogen; it also includes high levels of chromium, nickel and cobalt that are toxic to most plants. The stony slope in the left distance is a true serpentine barren, with almost nothing growing on it. The near ground has many endemic grasses and flowers but also a lot of foreign plants. Now is the time to see it. I visited on May 15 and caught the endangered Presidio clarkia in bloom (see it on my fotothing site).

serpentine prairie

The serpentinite itself presents a variety of colors from brown to grass-green, but the bulk of it here is close to bluejean color, hard to capture on camera but quite striking in person. Here it is: California’s state rock with Eschscholtzia californica, the state flower.

Visit the prairie now, but respectfully: this fall the park plans to fence off 3 acres of the land and start protecting it from degradation. If you walk amid the fields, watch your step and pretend that you’re inside that fence. Of course, there is no picking the plants or collecting the rocks. There’s plenty of serpentinite that’s just as good along the roadside on Skyline.

Foundations

16 April 2009

sidewalk slump

Walking past the Oakland Museum of California on the 10th Street side on my way to the Coliseum yesterday, I saw that the extensive work going on now doesn’t seem to include fixing the sidewalk. There probably is no structural threat to the building; this most likely represents compaction. Try as we may, we can’t always make ground as good as nature alone does. She has the advantage of “world enough, and time.” Foundation work is expensive to do right, but even more so to do poorly.


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