a linear trough in the Earth's crust in which rocks of Late Precambrian to Mesozoic age (roughly 600 million to 66 million years ago) were deposited along the western coast of North America, from southern Alaska through western Canada and the United States, probably to western Mexico. The eastern boundary of the geosyncline extends from southeastern Alaska along the eastern edge of the Northern Cordillera and Northern Rocky Mountains of Canada and Montana, along the eastern edge of the Great Basin of Utah and Nevada, and into southeastern California and Mexico.

The principal mountain-building phases of the geosyncline took place during Mesozoic time, but many earlier orogenic events have also been recorded. Deformation of the Cordilleran Geosyncline and the formation of the Cordilleran fold appear to be related to the development of oceanic trenches along the western margins of the North American continent, the underriding of the continental plate by oceanic crust, and the development of batholithic intrusions and extrusion of volcanic rocks associated with this movement.