Archive for August, 2010

Joaquin Miller Formation, Lake Chabot

31 August 2010

The south shore of Lake Chabot—the whole eastern side actually—is a good place to study the Joaquin Miller Formation. Outcrops of this shaly unit are hard to come by, so the shoreline and the roadcuts along the trail are useful. Even so, you realize that Earth isn’t always tidy when you come upon this gulch full of siltstone boulders.

chabot cleft

Massive (that is, unbedded) siltstone isn’t part of the Joaquin Miller Formation’s description, which is “thinly bedded shale with minor sandstone,” grading into “thinly bedded, fine-grained sandstone near the top of the formation.” Oakland’s collection of Great Valley complex rocks, which includes the Joaquin Miller Formation, is crisply marked on the geologic map. But unlike your standard street or topo map, a geologic map is an exercise in vision and interpretation and approximation. The lines might be moved by the next geologist, with complete respect paid to the previous mappers. The photo is near the “70” mark on the map below.

chabot geologic map

Might this belt of stones actually be in the overlying Oakland Conglomerate? I don’t think so, because there are no coarse grains in them. They’re just an unusual element in the Joaquin Miller, to be appreciated but ignored in the larger context. We can’t forget about the hundreds of meters of soft, obscure rocks that surround them.

This valley has its head in a recently populated part of northernmost Castro Valley, which means that it poses a risk of allowing urban runoff into the reservoir. Clearly it carries a lot of water at certain times, even if those occasions are rare. I hope to revisit and get to know it better, if only because living bedrock appears to crop out up there.

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:


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.


Get every new post delivered to your Inbox.

Join 8,875 other followers