When Oakland was first settled, a large wetland extended inland from the tidal channels east of Alameda Island. Samuel Merritt took it upon himself to improve the marsh with a dam, and the resulting brackish water body was named Lake Merritt. Today the lake is closely regulated with a water gate beneath 7th Street, and this creek runs both ways depending on the tide. Not even the Oakland watershed map gives this stream a name, so I’ll call it Merritt Slough. Click the photo for a 800×700 version.

The bank on the right side is the south bank, or the left bank. It’s nearly in its natural state, whereas the other bank is part of an area of made land a couple hundred meters wide. I’m standing just off E. 10th Street, next to the Civic Auditorium.

Just northwest of Lake Chabot are some tiny areas mapped as “Irvington Gravels,” high above the Sheffield Village neighborhood in the Dunsmuir Ridge Open Space. They caught my eye because Irvington (part of present-day Fremont) is the site of a famous set of Ice Age fossils, from which the Irvingtonian age of North American land mammals was established. Yesterday I checked the area out, in case there were some sabertooth-cat fangs lying around. This entry has a lot of photos.

You get there starting at the access at the end of Covington Road, a dirt fire road that goes straight up a steep hillside. The Hayward fault crosses the road partway up, at a little level spot at the edge of the woods. To the west of the fault, the rocks are mapped as San Leandro gabbro (Jurassic rocks of the Coast Range Ophiolite), but it’s really hard to tell:

Across the fault the rocks change to Late Jurassic volcanic rocks of the Great Valley Sequence, the same stuff exposed in the big Leona quarry:

Higher up are three small terraces where the gravel is mapped. This is looking south from the northernmost one:

It looks like a hopeless task to find rocks here. Luckily for me, the fire roads have recently been graded, so there was a window into the substrate. As I approached the terraces, the roadbed started to display river cobbles, quite unexpected in this setting:

I made a point of crossing the grassy slope to the other two terraces, looking for stones the whole way. Nada. From the southernmost terrace, here’s the view north. Click on the picture for a stereopair:

There’s a house on a knoll at the same height as the terraces. The upper part of the Knowland Park Zoo land also lines up with the terraces. No gravel is mapped at either place, but there might be some.

Now the cobbles start to look interesting:

Above is another, higher terrace. It’s over 500 feet above the starting point and a bit of a trudge.

Just below it are scattered outcrops of the volcanic bedrock:

The roadbeds on the upper terrace also have interesting cobbles. I took a few home to clean up and photograph. Remind me to bring them back on my next visit.

Russ Graymer, who prepared the Oakland geologic map, describes the suite of cobbles thus: “Cobbles . . . consist of about 60 percent micaceous sandstone, 35 percent metamorphic and volcanic rocks and chert probably derived from the Franciscan complex, and 5 percent black laminated chert and cherty shale derived from the Claremont Formation.” He holds that these little terraces started out near Fremont and were carried here by the Hayward fault. They started out at a much lower elevation too, I would think; just a sign that fault movements are not straightforward.

You don’t see a lot of crystals in Oakland rocks. But stopping to examine the boulders in the walls at 166 Tunnel Road, just over the Berkeley line along the Hayward fault, I was arrested by this perfect quartz crystal in a coarse-grained marble block. It’s maybe a centimeter long.

I brought up the topic of suiseki last summer, and now it’s that time again. This weekend, Saturday and Sunday, is the annual suiseki show at the Lakeside Garden Center from 10 to 5, no charge.

This was one of the stones on display at the 2005 show. It’s part of my gallery of Earth art on About.com, too.

Unfortunately I must miss this show. Fortunately, I will be touring the gold mines of the Sierra foothills instead.

I visited Sibley Volcanic Reserve on Sunday and was transfixed by this:

The light-colored blobs are amygdules, or fossil bubbles. Many of the lava flows that issued from a small volcano here about 10 million years ago were full of gas bubbles. Later those filled with minerals, and today the amygdules are weathering out. The minerals involved include quartz, its noncrystalline variety chalcedony, and various zeolite minerals. Read a little more about them on my About.com site. Amygdules were named by a Berkeley geology professor, Joseph Le Conte, in 1878. He surely saw them in these very hills.

I also sought out the labyrinths and found three localities. The first one is on the north side of Round Top, hidden in a sweet spot surrounded by vegetation and butterflies.

It was a good day yesterday to visit the Hayward fault at the top of Chabot Road. The previous night’s rain softened the ground and left the cut weeds smelling like fresh hay. This is the view north from the ground above the end of the road.

All of this land is suspect today, and the rocks cannot be trusted. The high ground I was standing on is the rubble pile built to support Route 24. The high ground on the right is an old excavation or rubble pile, I’m not sure which, supporting the loop linking Route 24 west and Route 13 south. The flat ground is the former roadbed of the Oakland & Antioch Railway. The trees in the distance are on a rocky slope that roughly marks the Hayward fault, but it may well have been quarried in the past. The nice thing about a fault, for producers of crushed rock aggregate, is that it pre-crushes the rocks. But the fault is somewhere in this view, although it’s poorly mapped between the Claremont Resort and Montclair.

With that preamble, I feel free to speculate that the fault trace could possibly nip Chabot Road at its farthest end. We see displacement of the curbs, and at the farther joint we see evidence of compression. In both photos the near side would be west of the fault, moving leftward.

But just as likely, this trodden, retreaded land is shifting and settling all by itself. There may be slow landsliding involved. Also, heavy trucks and other vehicles could well have done this damage. The truth may come out after the next big earthquake ruptures the fault here. It’s one place I want to check out in the aftermath, if I’m lucky enough.

I was asked the other day about the safety of living below Oakland’s dams. We have two of them, both of which I happened to photograph in March 2003 (back when I was still using film). Anthony Chabot built them both. This is the dam at Chabot Reservoir, at the very south end of Oakland. The Hayward fault is a couple hundred meters downstream.

This is the dam at Lake Temescal.

Both are earthen dams, basically massive piles of clay and boulders. The first was built in 1874-75 and the second in 1868. As the “great San Francisco earthquake” occurred on the Hayward fault in 1868, seismic safety was high in people’s minds. Here’s a page about their construction. These dams are generally considered sound and able to withstand another big one. Lake Temescal straddles the Hayward fault, but the dam is so massive and the water it holds so modest that even a 2-meter displacement on the fault will not lead to a dangerous failure, as I understand it.

The Calaveras Dam, farther south near Milpitas, is also of earthen construction. It crosses the Calaveras fault and is being replaced with a safer design; in the meantime the Calaveras Reservoir has been drained to half its volume.

Here’s an assortment of serpentinite from the Serpentine Prairie. The first three show the polished surfaces that form so easily on this soft rock type, as well as the variety of blue-green colors. Remember, serpentinite is a metamorphic version of peridotite, the deep-seated, low-silica rock that forms the upper mantle and the bottom of oceanic plates.

Darker bits may reflect more iron-rich protoliths, or maybe rocks that are less altered.

And remember, no collecting in the park. If you like serpentinite, do what I did and collect from the roadside just to the west of the park, or down in the Crestmont neighborhood.

Learn more about serpentinite on my About.com site.

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.

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).

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.

Eldridge Moores is a grand old man of California geology, and indeed of American geology. That’s him talking about structural geology with his arms, at the intersection of Tunnel Road and Caldecott Lane in early May 2005. He played Virgil to John McPhee’s Dante in Assembling California. Lately Eldridge has been pushing the State of California to do something very simple and obvious: recognize high-school geology as a subject satisfying the lab requirement for California college admission.

It’s obvious because geology is an applied version of every other science: rocks are chemicals, geologic processes follow the laws of physics, fossils are a branch of biology. Indeed, every lab science originated in practical problems with the world around us. The world is still there, and geologists are its intimates.

It’s simple because we have a national and state consensus around supporting science, especially environmental science, with strong leadership from President Obama and Governor Schwarzenegger. Eldridge has lobbied every year for adding Earth science to the lab science requirement. This year both the National Association of Geoscience Teachers and the National Earth Science Teacher Association are joining his efforts, and so am I, and I would love it if you did too.

It’s important because you may notice that only one person in this photo was younger than 25. Somewhere between the age when we’re all dinosaur fanatics and adulthood, geology seems to get lost. Part of fixing that is giving high schools a bit more incentive to offer Earth science courses.

The procedure is to write a letter (email or paper) to the UC Academic Council, which is right here in Oakland, and other UC officials. They are laying groundwork for the next set of curriculum guidelines for high schools, and time is critical. The call to action, talking points and a sample letter are all posted by “Geotripper” Garry Hayes on the NAGT Far West Section blog. Garry puts it well here: “Earth Science has always taken a back seat to chemistry and physics, and yet is most vivid example of chemistry and physics at work in the real world. We need to support the teaching of the earth sciences at the secondary level.”