Comments on: The real Claremont chert

By: Dave

Dave — Sun, 01 Aug 2010 15:19:45 +0000

Hi Guys,
Been a while, but I wanted you to know that my son and his partners got an A on the science project. It was a lot of work at the end to pull it all together (6th graders, go figure). They had to give a presentation about it that we didn’t realize until the morning of, but since I wanted them to do the work and understand what they were doing they were able to explain it well.

My son has had his fill, but I’m still interested in this stuff. Keep the posts coming. Nice to know what we have in the area and to be able to identify it.

Dave

By: Andrew

Andrew — Fri, 14 May 2010 15:52:15 +0000

Jim, I took my cue from Russ Graymer’s geologic map of the Oakland area (MF-2342), where he refers to “the formally accepted name Claremont Shale” citing Lawson 1914. Adding to the confusion is that the Claremont is actually a local piece of the enormous Monterey Formation.

As for blueschist, we do have it in Oakland. But I’m hoping to find lawsonite in large enough grains to identify it without a thin section, not having my own pet mike.

By: Jim

Jim — Fri, 14 May 2010 10:07:36 +0000

Forgot one thing. Your best best for Lawsonite is not in the east bay. It is almost always associated with the blueschist facies of metamorphic rock along with glaucophane. Although it does occur as a altered mineral in Gabbros and diorites, but that is much less common. Good luck finding it. Its cool looking stuff in thin section.

Cheers

By: Jim

Jim — Fri, 14 May 2010 10:04:00 +0000

I am a geologist at Berkeley and just completed a class studying the Berkeley hills and all the different strata. Just wanted to offer another perspective on the name. It is formally known as the Claremont Formation, but it is typically referred to as the Claremont Chert since chert is the main component of the formation. As you may well know in addition to the shale interbedding there are also some thin beds of limestone.

By: Chi-yuen Wang

Chi-yuen Wang — Sat, 24 Apr 2010 03:08:38 +0000

A hardness test using your pocket knife would also be helpful. If you scratch the surface of the rock with the tip of your pocket knife, it will not leave a clear mark on the fresh surface (i.e., not weathered) of a chert chert, but it will leave a clear mark on limestone.

By: Dave

Dave — Sun, 11 Apr 2010 22:48:46 +0000

Hi Andrew,
We took samples from thick and thin strata layers in a road cutout just to the left of the entrance to Huckleberry, on Skyline. The thin layered section had layers less than a centimeter thick while the thick ones were over 5 cm thick. We baked them in an oven (little weight loss, but it had been raining, playing it safe) and then broke them into small pieces of about a centimeter in size and soaked them all in hydrochloric acid for a week. None of them showed much weight loss but the thick layer of strata, upon breaking, was gray on the inside rather than the light orange to white on the outside. The thin layers were light orange to dark gray.

After a week in the acid all light color was removed from all samples. The thick layer was uniformly gray. The thin layer was composed of two rocks, tan and dark gray. The thick layer was the hardest of all, even after the acid. The dark gray was next, and the tan the softest, and was the only one that could be broken in your hands.

Based on these characteristics we believe that the three types of rocks are:

The thick stratum is chert- hard, gray, with a trace of limestone and iron for color.

The thin, flaky stratum was composed of alternating layers of dark hard siltstone (felt gritty when rubbed against teeth) and tan shale with a bit of limestone and iron for color.

We are going to characterize some rocks from other locations. One of the first we tried in acid was light orange and floated at first, fizzed, turned brown and sank. When we picked up the remaining brown rock and it promptly turned to mud in our hands, so we know there are some rocks in the area with a decent amount of limestone or dolomite in them.

One more weekend to go on the science project. You have been an excellent resource!

By: Dave

Dave — Wed, 24 Feb 2010 21:35:49 +0000

Hi Andrew,

I have a reference that says the yellow rock might be dolomite, I suppose with a bit of iron in it as well? The best way I know of to dissolve silica is HF. I don’t know if that’ll get the carbonates of the fossils, assuming that’s what the fossils are made of.

Of course, dolomite is a carbonate, so that’s an easy one to test. The dark gray shale, I think, is in pastry thin layers in between most of the yellow stuff. The exposed areas we’ve found have various degrees of weathering and some crumble to dust with little encouragement.

I’ll keep you appraised. Thanks for the tips!

By: Andrew

Andrew — Mon, 22 Feb 2010 05:03:53 +0000

That’s an interesting challenge. The simplest chemical test is the acid test for carbonates (vinegar works), but Oakland has almost none of those rocks. The other basic test suitable for children is hardness, in which case the shales along that stretch of road, if you can get a decent piece at all, are softer by far than the chert. Another useful thing to do might be crumbling the different sorts of rocks. Shales and sandstones would yield clay particles and sand grains, whereas the chert would need to be (carefully) hammered. Examine these particles with a magnifier and describe them, compare them with beach sand or washed soil sediments from the yard. Try chewing the shale–much of it is absolutely without grit.

If you have the materials to dissolve silica, you might demonstrate the presence of microfossils in the chert. I’ve never done it, but with vigilance it shouldn’t be difficult.

By: Dave

Dave — Sun, 21 Feb 2010 01:00:54 +0000

Hi Andrew,

I’m trying to lead my son and a couple of other kids on a science project (6th grade) to characterize several outcroppings of this chert on Skyline and Elverton (in between Thornhill and Shepard Canyon). Not quite outcroppings, they are exposed due to road building.

I was wondering if there are any methods for characterizing the strata that would be accessible to us. The crux of their project will be to determine what is making these rocks look different in appearance from each other (weathering, age, composition and so on). I’m a chemist by trade, so basic wet chemical methods are a possibility.

Any help you could give us on this would be appreciated. Thanks!

By: Michael Carey

Michael Carey — Tue, 19 May 2009 05:38:55 +0000

There are several prominent outcrops of very hard, high-grade metamorphic rock in the upper El Cerrito area. I don’t remember if the rocks had garnet in them, but they might, and anyway, they are very nearly eclogite.