An Elverton update

After a visit five years ago, I had high praise for Elverton Drive: “From end to end, it offers the best exposures anywhere of the Claremont chert.”

This stuff, as seen a few weeks ago during a return visit.

Those of you who’ve followed along know the amazing striped chert of the Claremont Shale, which crops out in a belt from Claremont Canyon along a couple miles of Skyline Ridge to Huckleberry Botanical Preserve and beyond in the hinterland. The fat pale stripes are layers of microcrystalline silica — chert — and the thin dark ones are layers of claystone — shale.

During this visit I walked from the south end of Elverton past the newest set of houses, near Huckleberry, and had a good stop in the old borrow pit. The wall has crumbled a bit since five years ago, opening this fine exposure.

I was hoping to find pieces of dolomite rock, which are present as an uncommon third ingredient, so I gave the rubble a good look. None of that there, but I was interested to see some extra-thick pieces of the chert and shale.

The chert, in fact, was very light. It was barely changed from its original state as diatom ooze on the seafloor, almost the balsa-wood lightness of the Pinole diatomite. I did not expect that.

At the other end of the pit is the same big-ol’ boulder that was lying there in 2013. This is not a decorative rock placed there to look good; no, it fell here from the beetling cliff above and stopped rolling just short of the roadway. I recalled writing in 2013, “if you feel an earthquake while you’re there, step the hell back.”

Every time I visit the high hills, the pleasure of geologizing gives way, sooner or later, to a sense of dread at the state of the roadcuts. The eucalyptus roots in this scene were exposed as the hillside crumbled away, and behind them is a concrete cast meant to slow down a landslide.

But thinking ahead I looked forward to admiring this again after five years away. Google Street View still shows it.

Instead, it’s being shored up and fitted with a shotcrete shroud.

And another splendid exposure farther along is being smothered too, with no finesse.

In fact, not long afterward I started to despair of Elverton Drive. Is this the point of occupying such a spectacular setting? To cover it with property? To look outward and not downward?

The Claremont chert isn’t as solid as it might seem. Given the tendency of these young rocks to crumble, there’s no guarantee a new house in the high hills will survive its first mortgage. Or that the road will last that long.

Look out. Don’t look down. Elverton Drive is falling apart while it’s still filling up.

I already miss the place.

4 Responses to “An Elverton update”

  1. mpetrof Says:

    Is the Claremont Chert all or mostly diatomaceous, not radiolarian? I really appreciated the bit about the dolomite bands from a related post.

  2. Andrew Says:

    It’s entirely diatoms, like the rest of the Monterey Formation.

  3. William Forester Says:

    From 1936 until today I’ve called the Elverton borrow pit “the old Quarry” What a crime nobody knew squat about Geology. Thanks for edifying me about Claremont Chert. For a few years after 1936, our family drove jalopies both up and down the steepest part of Sobrante — from Oakwood to Skyline. It was indeed steep. We came down slope in Low-Gear to relieve the stress on marginal brakes. I recall the winter freshet that scoured a good part of Sobrante road away; to the extent that the Oakland Street Department abandoned it. Yet, when we kids chose to glean huckleberries, we legged it up and down the former road to pick & sell Huckleberries.

  4. Andrew Says:

    Thanks for that, William. It occurs to me that the drain down the right-of-way is simply a storm drain rather than a sewer line. And you may be correct that an actual quarry used to exist up there. I’ll keep my eyes open.

    More drivers should use their low gear instead of their brakes!

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