The major contributions by the Geological Survey of Canada in the Canadian Cordillera—systematic mapping and definition of the regional geological framework—led directly to tectonic syntheses that attempted to explain its origin. From the late 1800's until the 1960's, Cordilleran mountain building was viewed as the end result of geosynclinal deposition. Early workers felt that a connection existed between mountain building and the Pacific basin, but its nature was never clear because little was known about ocean basins and their relationships to continental margins. In the 1950's, the nature of the oceanic lithosphere and ocean–continent relationships became better known; the knowledge led to formulation of the plate-tectonic hypothesis in the 1960's, a time fortuitously coinciding with completion, by the Survey, of most of the regional geological mapping (scale 1: 250 000). Geosynclinal rock units were reinterpreted in terms of their possible modern analogues (oceanic, island-arc, continental shelf – slope assemblages) and paleontological and paleomagnetic studies were used to support a mobilistic view of Cordilleran paleogeography, rather than the relatively fixed paleogeography tacitly assumed in earlier interpretations. In contrast with deterministic geosynclinal theory, it was recognized that plate-tectonic processes applied over a long time have enormous potential to create disorder; the result is an orogenic collage to be analyzed as a series of time – space events each of whose geodynamic settings may have been very different from one another. This aspect is especially important in the long-lived Cordillera, which was initiated as a continent–ocean boundary in latest Proterozoic time, evolved through the entire Phanerozoic, and today is a convergent-transform continen – ocean plate margin. Interactions between various oceanic plates and the North American plate, shown by the Cordilleran record of accretionary complexes, as well as results of possible arc–continent collisions and continental margin arc magmatism, formed new continental crust.