The birth of the Cordillera occurred when the supercontinent Rodinia split apart about 700 million years ago. The break was probably not clean, but probably involved necking and thinning of the crust by both normal faulting and plastic stretching. Pristine, unthinned old continental crust probably occurs as far west as the southern Rocky Mountain trench, below Golden, near the boundary between the Foreland and Omineca belts. We take this as the western margin of the old continent. Stretched and thinned old continental crust may extend west of this at the base of the crust of the Omineca and Intermontane belts. For over 500 million years, until latest Jurassic time (about 160 million years ago), the initial split marked the boundary between continental crust of normal (about 40 km thickness), and thinner continental crust and/or oceanic crust to the west.

West of this boundary, a thickness of perhaps 15 km of sedimentary rock accumulated between late Precambrian and Late Jurassic time (700-160 million years ago) in a continental margin setting, similar to that of the present Atlantic margin of North America. The oldest rocks are late Precambrian Windermere strata comprised of coarse sandstones, shales and slates, and very minor volcanic rocks that were laid down during the initial rifting stage. These are overlain by the thick limestones, shale and lesser sandstone of Cambrian to Jurassic age. These strata form much of the Foreland Belt.

The Paleozoic rocks pass laterally westward into shales, minor sandstones and rare volcanic rocks that presumably were deposited in deeper water on stretched, thinned continental crust that was well removed from the old continental margin. Highly deformed and metamorphosed beyond recognition, these western rocks underlie much of the Omineca Belt.