The Fall Line

In school I learned that major cities are where they are because when people and goods mostly moved by boat you needed to be on the ocean, or at least a river. But how do we explain new york being on the ocean while richmond is well inland? It turns out that there's a geologic feature called the atlantic seaboard fall line where the composition changes rapidly from metamorphosed Paleozoic rocks to post-orogenic, flat-lying late-Mesozoic and Cenozoic rocks. This makes waterfalls, or at least rapids, and tends to mark the farthest up the river you can get most shipping:

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