Tectonic Evolution of the Southern Appalachian Mountains—500 Million Years Ago
diagram of ancient ocean basin
500 Million Years Ago—Iapetus Ocean Opens. Continental blocks destined to become North America and Gondwanaland drift apart. The eastern edge of ancient North America developed into a passive continental margin, similar to the modern East Coast. As the margin cooled and subsided, thick sedimentary strata covered the earlier mountains and rift valleys.
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illustrations above modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.