Archive for the ‘Other topics’ Category

Lessons from the Carrizo Plain

15 April 2019

Last week I paid my first visit to the Carrizo Plain since 2005. David K. Lynch’s superb Field Guide to the San Andreas Fault says, “Nowhere in California is the San Andreas Fault more dramatically expressed than in the Carrizo Plain, a closed depression between the Temblor Range to the east and the Caliente Range to the west. Water drains in and evaporates leaving the glistening, usually dry Soda Lake. . . . There is little ground cover and the unobstructed views reveal countless tectonic features in all their glory.” That was true in 2005, when I came through in October and didn’t meet another soul. Not true last week — it was the peak of the wildflower season, Soda Lake was a by-god lake, and hundreds of car-driving, selfie-taking visitors were scattered across this wide, remote national monument. (For this post I’m offering some 1000-pixel images, just because.)

Topographically, the Plain is a basin with closed drainage, where all streams, such as they are, lead to Soda Lake. Geologically, the Plain is a sedimentary basin that until just a few million years ago was part of the Great Valley. Then the tectonic plates shifted slightly, the San Andreas fault was squeezed, and on its east side the rocks folded up to form the Temblor Range. Later the whole Plain was raised almost 2000 feet.

Coastal California has been going through rearrangements like this for some time, and Oakland’s younger rocks like the Claremont Shale probably formed in a basin the same way, one that was off the coast. North of the Plain, roadcuts in the Bitterwater Valley expose the kind of rocks being made in the Carrizo basin. They’ve been tilted nearly vertical by forces across the fault, just as their cousins in Oakland have been tilted by squeezing across the Hayward fault.

A place stuck between the Temblor and Caliente Ranges sounds kind of inhospitable, and even though the landscape resembled a gigantic Holi festival, a brisk and parching wind blew the whole time I was there. The Carrizo flowers are as tough as they are beautiful.

You like those purple Phacelias? Here’s a billion of them.

This is the view downvalley toward the San Emigdio Mountains, with the Caliente Range on the right. On the left, the peaks of the Temblor Range are nearly hidden by the lower range of the Elkhorn Hills, which are a large pressure ridge directly along the San Andreas fault. That’s where I drove next.

The most famous, geo-tourist-trappy place in the Elkhorn Hills is at their north end, where the fault has forced Wallace Creek to jog hard to the right. I didn’t go there last week, but this is how it looked from Elkhorn Road in the barren fall of 2005. The creek comes toward you on the right side, turns left behind the frontmost ridge, and cuts through that ridge on the left side. You’re standing on the Pacific plate, moving left about an inch and a quarter per year, and on the other side of that first low ridge is the North America plate.

I’ve shown you the same kind of stream displacement in Oakland, caused by the Hayward fault.

Anyway, down at the south end of the Elkhorn Hills the entire slope is warped by motion on the fault, and the spring vegetation helps bring out the distortion. Every little stream is curled to the left, like grass in a stiff wind. The expression of the tectonics in the landscape is so strong, just looking at this photo makes me clench my teeth. In person, in 3D, it’s even more uncanny.

The great earthquake of 9 January 1857 was centered near here. The ground cracked for some 200 miles. Shaking was felt the entire length of California and into Nevada. In the Carrizo Plain, the ground shifted about 30 feet. After that, the unnamed mountains to the east started being called the Temblor Range, and the San Emigdio Mountains also got their name, honoring the patron saint of earthquakes, at that time.

Ramón Arrowsmith, now at Arizona State University, has studied this region for decades. His 1995 Ph.D. dissertation includes a thorough backgrounder of the sciency side of this mighty, lovely land. But everything he’s doing in California is interesting.

GHADs—peculiar agencies that safeguard the land

12 November 2018

It’s been a couple years since I’ve visited and written about the Leona Quarry site. It continues to fill with houses. The plantings of local plant species are doing OK, though they’d be happier without the drought. Here’s an updated view from Burckhalter Park, which I’ve visited occasionally since 2003 just for this purpose. (The trees keep getting taller.)

The highest and final residential level, Skyview Drive, is being populated now. The rest of the land will always be open space.

However, the land won’t be exactly natural. It will be intensively maintained. That sturdy concrete flume in the foreground, for example, is there so rainwater runoff from the slopes won’t start digging gullies. The runoff goes to a collection basin at the bottom of the slope that keeps all the muddy sediment and sends the water on into Chimes Creek.

Here’s more of that impressive drainage system, on a lower slope.

What about that high rock face standing in the back? Even from this distance, it looks a little ragged, a little menacing. During my last visit, in January 2016, I was up there and it looked like this.

That crumbling rock was pouring past this temporary barrier and overfilling one of those handsome concrete troughs.

It’s a big and constant job to maintain this infrastructure. There’s always something to do. Who does it, and what happens when something major happens, like a landslide?

In developments like these, a homeowners association typically handles repairs and upkeep for community amenities like paving, play equipment and landscaping. Those aren’t demanding tasks. But what do most homeowners know about geology? For the special hazards posed by geological forces, like erosion and landslides, there’s something better: a Geologic Hazard Abatement District or GHAD.

GHADs (pronounced like “gadzooks” without the zook) are peculiar entities, created under the Beverly Act of 1979, that are set up to handle the specialized job of dealing with geologic hazards for a specific set of landowners. Formally political divisions of the state independent of cities and counties, they’re exempt from bureaucratic headaches like reviews under the California Environmental Quality Act (CEQA) or hassles with the county’s LAFCO agency (and its wretched website). They have elected Boards of Directors and can own land, impose taxes, issue bonds, exercise eminent domain and possess other superpowers. There are dozens of GHADs now, and Alameda and Contra Costa Counties are their epicenter.

What keeps GHADs from running amuck or shirking their job is their “constitution,” a detailed Plan of Control drawn up by a licensed engineering geologist that ensures the Board will act with a prudent level of care. The program of regular monitoring and maintenance is spelled out in an Engineer’s Report, prepared by a licensed geotechnical engineer. It’s a simple but powerful program that spares taxpayers the geological risk inherent in developing sites like Leona Quarry. Learn more from the California Association of GHADs.

The Leona Quarry GHAD was formed in 2005, and by now it has saved up several million dollars of landowners’ assessments for maintaining the defenses — the retaining walls, drainage facilities, native plants and so on — and for major incidents like slides, wildfires and washouts. In emergencies, the GHAD can act with funding in place, trusted experts (from ENGEO) in charge and minimal red tape.

As spelled out in the Engineer’s Report, the concrete drains are cleaned and inspected four times a year, and checked whenever rainfall exceeds an inch in 12 hours. The collection basin is mucked out whenever it fills to a certain level, and the vegetation inside is cut low whenever it grows above 5 feet high. When a wildfire swept over the north end of the upper slope last year, the GHAD sent its plant specialist out afterward to keep the native habitat in good shape, which in turn helps stabilize the slope.

A second GHAD, the Oakland Area GHAD, was established in 2006 for the small Siena Hill development off Keller Avenue. You’ve seen its rugged retaining walls with the fake-stone finish looming as you head east up Keller from Mountain Boulevard toward Sequoyah Community Church.

The builders and the city probably wouldn’t have developed this daunting location without a GHAD that enabled the homeowners to cover their own risk. And now the residents pay their annual fee of about $1500 and the City Council, meeting as the GHAD Board of Directors, spends literally three minutes a year passing a budget and keeping its hands clean of liability. Success for the developers is not certain, though, with fewer than half the lots filled after 12 years.

However, neither has failure been declared. What’s there so far looks really nice.

And from my point of view Siena Hill is a success because it exposes the Leona volcanics well, like this pretty boulder at the far end of Siena Drive.

Farther up is a fresh roadcut that displays the chaotic nature of this intriguing geologic unit, which represents a volcanic island arc of Jurassic age.

The whole thing consists of the distinctive greenish celadonite-bearing phase that I’ve found scattered all over the East Oakland hills. This is the first proper outcrop of it I’ve seen. Well worth a visit.

But enough of this stone cheesecake.

The Oakland Area GHAD is more than just a minor political curiosity; it was established with an eye toward folding in other similar developments as they come up. And it will hit the big time as the Oak Knoll development takes place over the next decade, because a whole bunch of land there, including the open space, the oak woodlands and the streambed of Rifle Range Creek, will come under the GHAD’s purview. The City Council may have to take a few more minutes from now on to manage that.

Work at Oak Knoll began in September. I am looking forward to the day when that land opens up to visitors.

Boulders of the Transbay Transit Center

17 September 2018

San Francisco’s new transit center — with the 5-acre garden park on top — is worth a leisurely visit, no matter where in the Bay area you live. Naturally I had to see it too, because a reader sent me photos of a large, alluring boulder that’s part of the complex. He couldn’t decide what rock type it is. “Gneiss,” I typed back, refraining from adding “Nice gneiss.”

There are different ways to ornament a large structure with boulders, seems to me. You could make them identical, for that unified look. The FDR Memorial in Washington DC does that well with slabs of red granite. You could make them vary, like Ruth Asawa’s landmark “Garden of Remembrance” at SF State.

The transit center and the park on its roof (to which Salesforce has purchased naming rights for the next 25 years) are studded with about a dozen large boulders, from 4 to 6 feet tall. The designer’s scheme for them mixes unity with variety.

The variety lies in the personality of the stones and the mix of rock types. The unity lies in their source and their surfaces. Let me show you some personalities. The first two are up in the park and the rest are at ground level.

Oh, I should warn the sensitive that all of the boulders have had an opening carved into them to hold a steel sign either pointing to or announcing that you’re in “Salesforce Park.”

As for the rock types, these are mostly gabbro (basically, quartz-free granite), some with differing degrees of metamorphism. But each one is distinctive in details that geologists appreciate: petrology, deformation, texture. A selection:

Some of these photos show the texture that made me say “gneiss” — a banded appearance with dark and light layers. With closer inspection, though, they aren’t layered enough. They’re just a little smooshed, not stretched out like taffy.

The other thing I mentioned is the similarity of the boulders. First, they are similar in their provenance — where they come from. The only information I’ve been able to glean is that they were sourced in “the Pacific Northwest.” That doesn’t sound like much to go on, but Oregon and Washington are mostly volcanic, and these gabbros are not volcanic; they’re once-molten rocks that cooled slow and deep.

Only three areas have such rocks: around Medford in southern Oregon, in the Blue Hills of northeast Oregon, and in north-central Washington. I favor Washington, and my evidence is in the second similarity: their surfaces.

I notice two things about these boulders in general. First is that their shapes are all natural — they’re field stones, not chunks broken in a quarry, and they show some degree of rounding.

Boulders don’t just round themselves; it took a very vigorous environment to make these. Something like the enormous snowmelt flows that once ran down all the rivers of the Sierra Nevada, leaving streambeds like this, in the upper Stanislaus River. The largest rock here is the size of an SUV.

However, the Sierra rocks are scrubbed fresh and smooth, and that’s the second thing about the Transbay Transit Center boulders: they were all tumbled to a rough rather than polished state, and their surfaces have since then been exposed at the Earth’s surface for a particular amount of time, not long enough to decay into clay minerals or crumble apart, yet long enough to acquire rusty colors from iron oxides. A short time, geologically speaking, measured in thousands of years.

This surface staining takes the form of red-brown streaks and spots, as in the photos above, and an all-over patina in some cases.

Here are two different versions of the altered surface, a crust a millimeter or so thick, formed on a coarse-grained gabbro, and a thin bronze sheen well developed on a fine-grained version. In both photos the underlying rock is exposed by chipping.

All this evidence points me to a scenario in which a deeply exposed body of gabbro was broken into large pieces, which were tumbled briefly and left piled upon each other, perhaps in a steep talus slope along the foot of a cliff. There the wind and weather gave them their delicate earth-tone finish. I picture a locality along the Columbia River in north-central Washington that was inundated, over and over, by the colossal Ice Age Floods that ended (for now) about 13,000 years ago.

These boulders are cool. Give them a pat as you take in San Francisco’s newest public park. The NL bus takes you right to it, a world-class ride.

Tracing the old Thorn Road

6 August 2018

Hiram Thorn took it upon himself in 1853 to build a road over the Coast Range hills from today’s Montclair to his redwood mill, which was either at the present site of Canyon or farther downstream where the former town of Pinehurst once sat. Thorn’s Road was a toll road for a long time, connecting Oakland to the Moraga Valley agricultural hinterland and beyond. “This was the main road into Contra Costa county in the early days,” wrote the Tribune in 1923, “and a daily stage ran over it to Walnut Creek, Danville and the top of Mount Diablo.”

This piece of the 1897 USGS topographic map shows the Thorn Road running from the lower left to the lower right corner.

There are a few things to point out. Kohler Creek is called Temescal Creek today, but back then Temescal Creek went straight uphill from the Lake Temescal reservoir. That streambed was obliterated by the later construction of upper Broadway, Route 24 and the Caldecott Tunnel bores. The dashed line from top to bottom is the county boundary, and the thick dot-dot-dash line running up the canyon along with the road is the boundary between Vicente and Antonio Peralta’s shares of the San Antonio rancho, the immense royal land grant made to their father in 1820. The Thorn Road was also the official line between the Oakland and Brooklyn Townships of Alameda County.

This 1878 map made by Malcolm King shows the landmarks at the time, including the location of the toll gate about where the Thornhill Coffee House stands today.

In the 1880s it was already being referred to as “the old Thorn road.” In 1889 the Tribune reported on the Alameda County Board of Supervisors meeting of 1 April: “The Committee of the Whole, to whom had been referred the petition asking for the placing in good repair of the Thorn road to Moraga valley presented a report saying, that upon examination of the ground, they were satisfied that it would be utterly impossible to ever make the same a good road on account of the steep grade.” They recommended surveying “a road to the summit over a new route and on an easy grade.” That new road was the Snake–Skyline–Pinehurst Road route. Between it, the existing Redwood Road, the Kennedy tunnel to the north and the Oakland Antioch & Eastern railway to the south, which went up Shepherd Canyon and cut through the hills to Eastport, the Thorn Road was no longer the best way over the hills.

Nevertheless, the 1936 street map showed Thorn Road still following its old route up to the Huckleberry saddle.

The 1947 topo map shows that the top segment of the Thorn Road, and all of it on the far side, had been abandoned. The newly named Thornhill Drive took a zigzag route incorporating what had been Idlewild Drive, and the part of the Thorn Road left behind was named Sobrante Road.

Here’s the modern Google map just to give an idea of the streets and terrain.

The Thorn Road took the gentlest way up Thornhill Canyon to the topographic saddle where the entrance to Huckleberry Preserve is today. It was still a very challenging grade near the top, about a 36% grade or 20 degrees, according to my phone compass. (This would rank among the steepest streets in notorious San Francisco.) It was even steeper on the Contra Costa County side.

For a while after the 1947 topo map was published, street maps connected Sobrante all the way to Skyline, but as of 1967 the upper end of Sobrante had been cut off. However, there’s still a right-of-way and a sewer line running down it.

That’s where I took a walk last week. This is looking back at the end of Sobrante and across Thornhill Canyon.

There are remnants of the old grade, but no path bigger than a game trail. I think a footpath should be built here, as an emergency route if nothing else.

Underfoot is Claremont chert, not a surprise because this is right next to, and a hundred feet downhill from, the endangered chert roadcuts of Elverton Drive.

The habitat has possibilities. The ground was wet during my visit, thanks to fog drip. But crews have dumped a bunch of eucalyptus slash, which not only obstructs passage but also presents a fire hazard.

Also prominent in the human litter is a bunch of slash consisting of For Sale signs. There are still lots available up here.

On the far side of the ridge, in Contra Costa County, a stub of the Thorn Road got the name Winding Way. It was known as a shortcut for motorcylists when CHP Captain George Kallemeyn, chasing a group of hotrodders down the road, went over the edge and died in July 1959. Winding Way was still shown as open as of 1967, though it went only a short distance down the canyon.

Some time after that a landslide took out the highest segment of the road, and today the Huckleberry Path edges around the scar. About a hundred yards down the trail, a bench marks the spot where the old roadbed, heavily eroded and overrun in roadcut rubble, picks up again. It’s passable on foot all the way down to the hairpin turn of Pinehurst Road.

I recommend visiting this end of the old road starting down at Pinehurst. You can park beside Pinehurst a little bit downhill from the hairpin, where the old rail tunnel came out. (The cut is still there, filled with rubble and leaking a steady stream of groundwater.) But just as convenient, and more tempting, is the new Wilcox Station staging area, an access point to Sibley Volcanic Preserve’s eastern annex where the Eastport station once stood.

The road starts out along San Leandro Creek, then soon starts to climb.

It’s a steady grade, but the road was never more than one lane wide. As you walk it, imagine the work it took to trailblaze by pickaxe and oxteam. Imagine driving the daily stage to Danville over it. Between raveling ground on the uphill side and landslides on the downhill side, this road, once a vital link in the commerce of the redwood era, is reduced to a precarious trail today.

At any time an earthquake or rainy winter could cut it off, either until repairs can be made or once and for all.