My favorite alberta rockies stomping grounds used to be shallow seabed on the coast of north america - as the continent moved with the pacific plate subducting under it, some larger islands that were on the pacific plate were "scraped off" before the pacific plate went under north america. these collided with the coast, forming the rockies (geologists
figure there were several collisions). so roughly - the bits that collided formed british columbia. Makes me think every time i see fossils while hiking (which is frequently)