Plate tectonics acting over hundreds of millions of years, as in the Canadian Cordillera, have enormous potential to completely scramble the geology and paleogeography. Recall that an amount of ocean floor equal in length to one-third of the Earth's circumference has gone down below western North America in the last 150 million years. This means that rocks on the ocean floor from potentially thousands of kilometres away may be brought to the continental margin and stuck to it during the process of subduction. Formerly continuous rock bodies may be broken apart and moved to somewhere else along the plate boundary by transform faults; southwestern California moving northwards on the San Andreas Fault may eventually be lodged in southern Alaska. In many cases, we can identify the tectonic setting in which the rocks were formed with reasonable confidence, but not WHERE they were at the time they formed, because of the lateral mobility that is possible in a world whose geology is governed by plate tectonics. How do we sort out this apparent geological chaos, which is a four-dimensional, space-time, jigsaw puzzle? We must do this if we wish to learn how the Cordillera was made.

The first thing to do is to identify the pieces of the Cordilleran jigsaw puzzle. For many years, geologists working in the western Cordillera have recognized that the succession of strata or the nature of the rocks may remain relatively constant along or across the trend of the Cordillera for some distance, and then change abruptly across a big fault, even though rocks on both sides of the fault may be the same age. Such regions of internally consistent geology are called "terranes", and we can map terrane boundaries by finding major discontinuities in the geology. In the Cordillera, "terrane" is used for a piece of the Earth's crust with a geological record distinct from records of adjacent terranes. ??? and from rocks deposited on and adjacent to the old North American continent.

The bumping and grinding that occurred as the terranes were stuck (or accreted) to the backstop of the old continental edge probably formed many features of the Canadian Cordillera, such as the Coast and Omineca belts of formerly deeply buried rock. Along the old continental edge were deposited the Proterozoic to Late Jurassic (700 to 160 million years), within-plate, continental margin deposits that now make up the Foreland Belt. The oceanic and volcanic arc terranes mainly in the Intermontane Belt, but overlapping into the Omineca Belt to the east and the Coast Belt to the west, were together and accreted to the ancient continental margin by the Middle Jurassic (about 180 million years ago). Terranes in the Insular Belt, that in places extend eastward into the Coast Belt, were accreted to the terranes in the Intermontane belt in the mid-Cretaceous, about 100 million years ago. Terrane accretion is responsible for building the continental margin oceanwards from the position it occupied about 180 million years ago in southeastern British Columbia to its present position west of Vancouver Island.