During the period from 700 to 250 million years ago (the Late Proterozoic and Paleozoic) the Rodinia supercontinent split into a number of pieces, the pieces moved independently, and then gradually came together to form a new supercontinent, called Pangaea. Ian Dalziel has a series of maps showing the positions of the continents during this interval. (Click on the numbers at the bottom of his webpage.) Malcom Ross's animation of continental drift at the University of Pittsburgh website illustrates the process of the continents coming together. [CAUTION! This animation is slow to load--5 minutes or more depending on the speed of your modem. Choose the "Forward in Time (550 Ma to 270 Ma)" option.] Notice how through this whole process the western margin of North America (called Laurentia) remains open, bounded by ocean.

Plate Tectonics of Montana (Page 7 of 14)