Rocks at has a lot of nice articles about rocks and rock formation by Andrew Alden. Already this morning, I’ve learned how to tell siltstone from shale or sandstone or mudstone, what a thunder egg is, that agate and chert are both types of chalcedony, that alabaster is a kind of travertine, and how banded iron formations were formed. They date back to when the earth’s atmosphere mostly free of oxygen, and any oxygen produced by photosynthetic bacteria quickly bonded with iron in the earth’s crust.

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