The North American Cordillera began as an island arc of volcanic origin. At the same time the western edge of the Canadian Shield was the western coast of what we now know as North America. Over time the cordillera grew and the area between it and the Canadian Shield became lowlands and this area is now known as the Western Sedimentary Basin. This basin extends from the Gulf of Mexico to the Arctic Ocean. At various times throughout prehistory this area has been both above and below sea level and, consequently, experienced periods of erosion, and sedimentary deposition. Here are a few links that should help you. Best of luck with it.