Montgomery Ridge

bowlofchert.jpg

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.

howegreenchert.jpg

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.

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4 Responses to “Montgomery Ridge”

  1. christie Says:

    there’s a pile of very angular, well sorted red chert on top of Ring Mt – I bet it’s derived from the Marin Headlands as I think the military was building on both sites at the same time. Your post reminded me of the Kronkite beach mystery: why so many colors of chert/jasper/carnelian etc. in the pebble beach when I have only ever seen radiolarian red in local outcrops?

  2. Andrew Says:

    Ah yes, the glorious pebbles of Fort Cronkhite/Rodeo Beach! My theory is that the offshore seds of the Marin Headlands are carried back and forth by strong currents around the Golden Gate, so they’re well mixed. But I only know the one beach, so I could easily be wrong. Also, maybe the other colors are from inaccessible outcrops.

    But like your Ring Mountain deposit, I can’t really be sure that this chert is truly natural (more precisely, autochthonous). A lot of human activity has affected my area for the last 150 years.

  3. Silver Fox Says:

    When I was young, I collected rocks and wanted one of every color. It seemed that a green one was the hardest to find, and I finally found a green pebble or stone in someone’s backyard in downtown Sacramento. Your green chert reminds me of that rock – although it seems unlikely that I really could remember it very well.

  4. Andrew Says:

    Well, there are serpentinite belts in the Sierra Nevada foothills, so some cobbles of it might have gotten to Sacramento. Or some of this chert. It really is remarkable stuff, very hard and beautiful. Probably involved in high-grade subduction.

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