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The rocks highlighted here are some of the oldest in the United States. They formed in the Cambrian Period, between 560 and 504 million years ago, and the Ordovician Period, between 504 and 438 million years ago. The Cambrian Period is the earliest division of the Paleozoic Era, followed by the Ordovician period. During the Cambrian, life began to bloom in the Earth's oceans. By the Ordovician, marine life was very diverse, with creatures like primitive fish, corals, and trilobites.
Cambrian and Ordovician features of the United States:
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