Archive for February, 2012

Clinton terrace

29 February 2012

Oakland has some small areas mapped as marine terraces. They run about 20 to 40 feet above sea level and they’re very flat. Here’s a look down 5th Avenue toward the bay that shows the terrace well, beyond the dip in the road. I was standing at E. 20th Street, on the Pleistocene fan; the low point in the road is at E. 18th Street.

clinton terrace

I name it Clinton terrace because it underlies the former town of Clinton. The terrace runs along the foot of the fan as far as Fruitvale. Here it is on the geologic map, unit Qmt, “Quaternary marine terrace deposits (Pleistocene).” I was standing at the asterisk.

clinton terrace map

In the map description, Russ Graymer correlates this terrace with other terraces along San Pablo Bay at Lone Tree Point and Wilson Point. Those have been dated at about 125,000 years old using uranium-thorium dating of oyster shells. That time was a well-known highstand of sea level, and coeval terraces occur up and down the Pacific coast. So we imagine the bay waves lapping against the older alluvial fan, nibbling off sediment and spreading it around to build a nice flat terrace. It was probably a lovely tidal marsh until the glaciers resumed, the sea fell and land vegetation moved in. Later, creeks dug into it along Park Boulevard, 14th Avenue and 23rd Avenue.

You can see there’s a bit more of the terrace under Lakeside Park. A final piece underlies the Valdez Street area beyond the upper left corner.

Gudde Ridge

19 February 2012

The basalt of the Moraga Formation is spectacularly exposed on both sides of Route 24 east of the Caldecott Tunnel. From along the highway you can get an excellent view of its makeup and structure, but this view from Radio Tower Hill shows how the rock unit makes up Gudde Ridge.

gudde ridge

Click the photo for the 1000-pixel verson. Gudde Ridge runs just east of Round Top all the way down to Canyon Road, on the back side of Moraga. The town of Canyon is on its west flank. And it’s Moraga basalt the whole way. In this photo you can see the underlying Orinda Formation to the right of the basalt. It’s gray conglomerate as opposed to the red-brown basalt.


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