Wednesday, October 28, 2009

Continental Drift

There are good maps of the prehistoric continents at the Paleomap Project, Mollewide Plate Tectonic Maps, and Paleogeography and Geologic Evolution of North America. The Silurian is particularly alien-looking, the continents assumed forms roughly recognizable to us after the K/T extinction, and the next supercontinent will form in approximately 250 million years.

It looks like what is now the Ohio River valley (west of Appalachia) rose from the ocean sometime in Devonian and early Carboniferous, after the Appalachian mountains formed some 450 million years ago. The limestone exposed at the Falls of the Ohio formed in a shallow sea during the Devonian about 380-325 million years ago. Interestingly, it appears much of southeastern Indiana, central Kentucky and Tennessee, and northern Alabama was the site of a large island (or peninsula) on the arid southwestern shore of Laurussia, the Old Red Continent, that ephemerally rose and sank into the ocean for 100 million years or so in a position roughly analogous to contemporary Patagonia.

The intrusion of the Western Interior Seaway during the Cretaceous is the closest this area came to sinking since then.

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