Archive for March, 2008

Montgomery Ridge

16 March 2008

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St. Mary’s Cemetery, north of Mountain View Cemetery, is on a ridge that runs toward the bay and peters out at the Kaiser hospital on Macarthur and Broadway. The ridge is on bedrock at the high end and changes to old alluvial fan sediment just past Pleasant Valley Boulevard. I call it Montgomery Ridge because Montgomery Street runs approximately up its crest. My yard lies on the edge of this ridge down near its end. I find these Franciscan chert cobbles scattered thinly in the dirt, and I’ve been putting them aside. They are rough, but not jagged, so I take them to be natural, in-situ alluvium rather than fill or crushed rock. That’s where things stood until the other week, when I found a cutbank on upper Howe Street dug into the ridge, and the same chert was tumbling out of the hillside from a layer just beneath the topsoil. Walking down Montgomery, I saw more chert chunks in the soil by the road at the corner of John Street. My favorite pieces are the greenish ones, like this one by the side of upper Howe Street.

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This chert comes from the Piedmont block, but the geography is different today. Today, streams have incised the old fan and they’re too feeble to carry this kind of material. I picture much drier conditions, and flash floods strewing the chert across the surface of the ancient fan. The next thing is to see where else it occurs. Let me know if you find it in your neighborhood.

Rockridge Shopping Center quarry

10 March 2008

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Halfway up Broadway, where it crosses 51st Street/Pleasant Valley Avenue, there used to be the big Bilger quarry. The Oakland Paving Company mined a body of traprock (mapped as Franciscan quartz diorite, a near-basalt), crushed it, and shipped it out via a rail spur that crossed Broadway at 42nd Street and curved toward the former depot at 41st and Shafter streets (you can see the trace in Google Maps). This is what’s left. I’m standing at the very edge of the Catholic cemetery overlooking the Rockridge Shopping Center. Beyond it is a remnant of the original hill where California College of the Arts sits. It was a family estate before it became a school; perhaps they couldn’t stand the quarry operations and decided to give the place away. Still, the views from there are very nice, and the old quarry walls beautifully display the sandstone that adjoins the traprock as well as the faulted contact between them. Click the photo for a bigger version.

The old pit holds water from the Rockridge Branch of Glen Echo Creek after it traverses the Claremont Country Club golf course. A glory hole at the other end carries the overflow into a culvert that runs down the west side of Broadway, daylighted in two small places (by the new Kaiser parking structure and along Brook Street) before it joins Glen Echo Creek proper just north of 30th Street. The photo below shows where the creek joins the pit. I think the little waterfall on the left must be drainage from the country club grounds. It’s still pretty, and both are seasonal hidden treasures. The creek is barely visible from the other side—you have to stand by that little promontory at the front corner of the Longs store.

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