Archive for the ‘oakland soil’ Category

Longridge loess

21 August 2012

I was walking up Longridge Road and spied an excavation, where a homeowner was replacing some water lines and renewing a driveway. Naturally, I sidled over and took the rare chance to look beneath the skin of Oakland’s Pleistocene fan. The material was massive—unbedded—and clean. I pried off this little piece . . .

longridge

. . . and nibbled on it. It was firm, but crumbled like Necco wafers and turned creamy on the tongue with just a hint of grit. Not sticky or chewy with clay. Not indurated like hardpan. No sand or pebbles to be seen. The more I thought about it, the more peculiar this sediment seemed, until I had a wild surmise.

Alluvial sediment is never very well sorted, because it’s carried short distances and laid down by streams. Longridge Road is, as the name suggests, a ridge road running up the crest of a ridge between parallel stream valleys along Trestle Glen and Mandana roads. The crest of a ridge should not be made of this fine silt. But it’s downwind from downtown, which is Pleistocene sand dunes (the Merritt Sand), cousin to the dunes of San Francisco. Dune sand is very fine sand, and the fraction that blows away from the sand is finer still. So my wild surmise is that the fan, at least this part of it, is dusted with a layer of windblown glacial silt—i.e., loess. It’s remarkable stuff, and something I never expected to see in Oakland.

Pebbles and perils

3 March 2011

pebbles

I was writing a post on Pebble Beach for the KQED Quest site. The pebbles there are special because they represent a huge variety of source rocks, pieces of which ended up offshore in deep-sea rocks back in the Cretaceous. Those rocks were pushed up above the sea and eroded again, freeing the pebbles for several more rounds of polishing and recycling. Then they were sifted in the surf to isolate them in a pure deposit.

Oakland wasn’t lucky that way. The sediments here derive mostly from finer-grained rocks, and the big exception, the Oakland Conglomerate, didn’t include such a variety of gravel clasts. And even then, our sediments don’t seem to have enjoyed a vigorous winnowing and polishing in the Pacific waves. Rather than shiny pebbles, our shores are mostly clay and mud.

But not entirely. The Franciscan block that underlies Piedmont and neighboring parts of Oakland contains the grab-bag of seafloor rocks typical of melange, and the debris it has shed down Oakland’s creeks has made it to the bay. So have bits of the basalt in the high hills. One place that these pebbles appear is in a little spot along the shore of Lake Merritt.

Unfortunately the waves of Lake Merritt will never turn them into a dazzling display. However, neither will Oakland ever be ground into bits like the retreating San Mateo coastline. Our dwellings will never fall down the eroding coastal bluffs like they do in Daly City. I can live with that.

Bird islands

13 January 2011

Lake Merritt is formally a wildlife sanctuary, declared in 1870, but the land itself is artificial and needs maintenance.

artificial land

The Parks and Recreation Department website says that the first bird island was built in 1925 and the other four were added in 1956. As long as I can recall (since 1989) they have been a tangle of thick foliage and tall snags, but right now the islands are undergoing a makeover. Anyone have more information?

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.


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