The eastern part of North America contains rocks that tell the detailed story of the closing of the Iapetus Ocean during the assembly of the Pangean supercontinent in the Paleozoic that was the subject of . Since about 200 Ma, eastern North America recorded the rifting of Pangea and evolved from a divergent tectonic boundary during the rift phase into its current passive margin at the margin of the modern Atlantic Ocean (seeand ). The western parts of North America contain a geologic record that reflects a similar history but essentially flipped in time. While major orogenic deformation was impacting eastern North America in the Paleozoic, western North America was a passive margin on which a thick sedimentary sequence was deposited on a Proterozoic and older basement.