Archive for July, 2008

Shutter ridges

31 July 2008

A reader was unclear on the concept of shutter ridges, so I thought I’d try to show it as well as tell it. Look closely at this excerpt from the Oakland geologic map covering Lake Temescal.

shutter ridge map

The lake is the blue blob near the top. The Hayward fault slashes through it and across the map from top to bottom. The left (west) side moves north with every major earthquake on the fault. The blue area labeled KJfm (Cretaceous-Jurassic Franciscan mélange) is part of what I refer to as the Piedmont block; it makes up the ridge you see across the lake:

lake temescal

As that ridge moves north, it cuts off the course of Temescal Creek and forces it to flow north to get around it. That’s where the “shutter” term comes from—the ridge barrier moves like the shutter of an old-fashioned box camera. You can see on the map how Temescal Creek flows today, after tens of thousands of years of this process: it comes downhill on the right edge of the map, jogs to its right for almost a mile, goes through Lake Temescal, then turns left around the curve of Route 24 (the double purple line) in a culvert to resume its course to the bay. See the ridge from another perspective in this post.

Another excellent example is on the fault just north of the Oakland Zoo, where Arroyo Viejo comes down Golf Links Road and makes a similar jog around the hill of Toler Heights before resuming its bayward course under 82d Avenue:

Rocks of Upper Rockridge

29 July 2008

During my search for Rockridge Rock, I’ve been finding some fine Franciscan knockers doing their understated bit to add character to the neighborhood. Call this part 1, because I’ll surely find more as I pursue my search. These five rocks are shown in descending order of elevation. The first is at Buena Vista Avenue and Hill Road, a red chert.


This gray chert is on Bowling Westminster Drive, near the purported Rockridge Rock site on Glenbrook.

bowling drive

This altered chert is next to Romany Road.

romany road

This rugged outcrop is in a private yard.

broadway terrace yard

This greenstone boulder is at Cross Road and Broadway Terrace.

cross road


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