Notes

As the Pacific Plate moved north, the crust over which it moved was forced down by the North American Plate, back towards the Earth's core. However, as the plate closed in on the 1st Terrane, this land mass was too buoyant to be forced downward and so it was added onto the edge of the continent. This is where much of British Columbia joined North America. Along with this collision came intense forces compressing the already existing land mass. This brought on the first orogeny, known as the COLUMBIA MOUNTAIN RANGES(it formed the Columbia Mountains made up of the Caribous, Selkirks, Purcells and the Monashees).
The collision causing the Columbia Orogeny occurred about 175 million years ago, and as the shock wave moved eastward, it forced huge masses of rock to crack and slide up over its neighbours. This is known as thrust faulting and was instrumental in the formation of the ROCKY MOUNTAIN RANGES. The shock wave began piling up the western ranges, and then the main ranges, around 120 million years ago.Therefore two mountain ranges were formed by the collision of the N. American and Pacific tectonic plates.