Diatom time

Today’s EPOD is of a diatom. Seems like as good an excuse as any to share a couple photos of a big brass diatom sculpture from the American Museum of Natural History in New York.

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Finger for scale, but not really, since the sculpture isn’t to scale.

Folds of New York

Thursday is ‘fold day’ here at Mountain Beltway.

Let’s take a look at some folds I saw last weekend in New York City. We’ll start with a bunch seen in the Manhattan Schist in Central Park. Here’s an example of the foliation in the schist. It’s got finer-grained regions and coarser, schistier regions with big honking muscovite flakes. Metamorphic petrologists: Does this correspond to paleo-bedding? (i.e. quartz-rich regions that metamorphose less spectacularly, and mud-rich regions that converted more totally to muscovite during metamorphism?)
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Anyhow, here’s what it looks like when it’s folded (accented with a small granite dike):

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And another, with some boudinage thrown in for flavor:

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This was one of the best outcrops I saw that weekend (on the edge of the ‘lake’), but it was inaccessible to closer photography. Sorry about all the branches in the image. What you’re looking at here is a series of folds with axes plunging at ~45° towards the lake:

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Crudely annotated version:

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Granite dike:

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Boudinaged granite dike:

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Folded and boudinaged granite dike #1:

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Folded and boudinaged granite dike #2:

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Lastly, here’s a couple of folds from inside the American Museum of Natural History. A metaconglomerate:
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A little model mountain belt made out of compressed sand layers:

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The thing that really struck me about this sand model is the folds visible in the green and yellow central part of the mountain belt: There are refolded folds there. The lower-central antiform with dark green atop yellow is the best example. I had the idea in my head that two generations of folds meant two generations of deformation, but here you’ve got two generations of folds resulting (presumably) from a single episode of ‘mountain building.’

Such beautiful complexity! I want a sand model like this for my lab.

Pyrolusite on a pterosaur

All the photos I posted over the weekend here were via iPhone, and hence not particularly high-quality, despite their excellent geological content. Now I’ve downloaded the photos from my real camera, and have a few good ones to show. Here’s a succession of photos of the same specimen of Pterodactylus longirostrus, each progressively more zoomed in than the last. It’s a late Jurassic pterosaur (140 Ma) from the Solnhofen limestone of Germany.

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I mainly took these for the pyrolusite dendrites rather than the fossil itself…

Rusty weathering rind

On a granite block

Giant ground sloths

In the American Museum of Natural History:

These mylodontids reminded me of Puerto Natales

Where I am today

Graphics by USGS, after Schuberth, 1967.

Travertine nubbins on a bridge

Another in the Geology Of Central Park series…

Plumose structure

Propagation direction: upper left towards lower right:

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!!!

Glacial striations, southern Central Park

New York City has some cool geology: Paleozoic metamorphics scraped by Pleistocene glaciers.

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