Linear trough in the Earth's crust in which rocks of Late Precambrian to Mesozoic Age (roughly 600–65 million years ago) were deposited along the western coast of North America.
The principal mountain-building phases of the geosyncline took place during Mesozoic time, but many earlier events have also been recorded. Deformation of the Cordilleran Geosyncline and the formation of the Cordilleran fold belt appear to have been related to the development of oceanic trenches along the western margins of the North American continent.