Figure 3. By around 700 million years ago North America had rifted away from Gondwanaland (Antarctica and Australia), and western North America became a passive margin trailing the eastward motion of the continent. (Source: R. Blakey, Northern Arizona University, Paleogeography of the Southwestern U.S.: http://jan.ucc.nau.edu/~rcb7/paleogeogwus.html)