Archive for the ‘Oakland’s soil’ Category

Lakeside Park clay

31 October 2016

qmt-1

They’ve been building some new parking in Lakeside Park, near the boathouse, and the work opened up the subsoil to view. You’re looking at a hard, pure marine clay from a time when the sea rode high, late in the Pleistocene Epoch.

Lakeside Park sits on the flat surface of a terrace a little over 20 feet elevation. It matches other surfaces in the Uptown/Valdez area and in old Clinton, all of which are assigned an age of around 125,000 years.

The scenario is this: As the great continental glaciers melted well beyond their extent today, the sea brimmed over and rose about 6 meters above its present level. The advancing waves carved their way into the Oakland landscape, creating a wave-cut platform, and then all was peaceful for a few thousand years. That’s when the nice clay topping was laid down. Only the finest sediment could get this far away from the new shore because all the stream valleys were drowned.

Later the Earth’s orbital cycles proceeded, the climate cooled, the glaciers grew back and the sea retreated. On this side of the bay, far from the Pacific surf, the process was gentle enough to leave behind much of the evidence. Elsewhere in the East Bay, the clay beds from this time were thick enough to support a brick industry.

The Merritt Sand

22 August 2016

merritt-terrace-madison

Downtown Oakland sits on an unusual bit of geology — a large dune field mapped as the Merritt Sand. San Francisco is famous for its sand dunes, of course, and Points Reyes and Año Nuevo have some too, but the dune fields of Oakland and Alameda are the only ones within the bay. Here they are, labeled “Qds” on USGS Map OF-00-444, which shows the young (Quaternary) deposits of the Bay area.

dunesandmap

They’re just like the dunes in San Francisco. They formed during the ice ages, when the shoreline was out near the Farallon Islands, the Bay was totally dry and the Sierra was full of permanent glaciers (on not quite the same schedule as the great continental ice caps). The rivers carried huge amounts of fresh-ground rock dust from the glaciers to the Bay and beyond, and the Pacific beaches of the time must have been formidable. Think of the brisk summer days we have when the sea wind is being sucked into the Central Valley, and now multiply that. Those winds blew all that sand here.

Brian Atwater of the U.S. Geological Survey mapped ice-age deposits in the Delta, and in 1982 he summarized the overall picture as “a stage on which three related and repetitious plays are presented simultaneously. In one play, wetlands and flood plains appear and expand as tidwater invades from the west, then become sites of erosion after the tidewater retreats. In another play, glacially eroded detritus from the Sierra Nevada builds alluvial fans and, reworked by wind, creates extensive sand dunes. In the third, little-understood play, streams draining the Sierra Nevada foothills and Coast Ranges episodically build alluvial fans. Spanish- and English-speaking persons enter during a major incursion of tidewater and find most of the stage covered with tules.”

Our dunes may sit higher than the buried dunes of the Delta because the conditions that built them were more stable here. There was always a wind gap at the Golden Gate and always lots of sand available on the other side.

In detail, the Merritt Sand (Qms) reaches the edge of Lake Merritt.

merritt-lake-geomap

It differs from the marine terrace deposits (Qmt) that I’ve described before. It consists of very fine sand, with no clay. It’s also higher and less flat. Apparently the original, undisturbed surface featured yardangs — elongated ridges of sand running in the direction of the wind — whereas the dunes of Alameda were the more typical arc shapes known as barchans. All of that is obliterated today.

You can see the edge of the Merritt Sand platform from across the lake where the streets rise abruptly away from the shore. Snow Park is probably the least disturbed exposure.

merritt-terrace-snowpark

Another exposure stands out between Jackson and Madison streets, although it probably has also been excavated.

Merritt terrace at 160-17th St

It’s the back yard of an apartment building at 160 17th Street. The view is nice from there.

merritt-terrace-view

That same agreeable elevation attracted Oakland’s early elite, who put up a row of mansions overlooking the lake. Of those great homes, only the Camron-Stanford House survives.

Oakland alluvium

1 August 2016

Qhaf-1

When someone opens up the ground in Oakland, no matter where, I think it’s interesting. This construction site on Telegraph Avenue between 29th and 30th Streets exposes alluvium, the stream-laid sediment that once supported productive farmland throughout Oakland’s flats. Mapped as “alluvial fan and fluvial deposits (Holocene)” or unit Qhaf on the geologic map, it covers more area than any other geologic unit.

The uppermost part, the brown stuff that the excavators have turned over in curls, is rich in organic matter and clay. A little deeper it turns tan as the organic matter thins out. It’s dense and firm, good ground for building.

The nearer you get to the bay, the finer grained this material gets — more clay, less sand and gravel. Streams have carried it down from their canyons in the hills over the last few hundred thousand years, pushing back the sea. And by “streams” I mean floods. The clear trickling streams we know are actually asleep. Floods are the one day in a thousand when streams awaken, picking up and carrying alluvium from place to place.

Occasionally the streams themselves jump their tracks. If you visualize the land in super-fast motion over geologically recent time, our streams whip back and forth over the coastal plain like firehoses out of control, winnowing the alluvium again and again. From the hills outward they build up low, cone-shaped piles of sediment called alluvial fans. Downstream, these coalesce into an alluvial plain.

The “h” in “Qhaf” refers to the Holocene time period. The Holocene (“fully new” in scientific Greek) began when the latest pulse of the ice ages ended, about 12,000 years ago. It’s been a mostly pleasant time. Many geologists argue, with good reason, that the Holocene has given way to a new permanent state of wrenching climatic changes. Because the natural balance of climate is strongly influenced by human activities, they argue, the climate system today is a writhing firehose we may be able to control. They propose to call our new era Anthropocene time.