 In the central Canadian Rockies, the main ranges are composed of the Precambrian mudstones, while the front ranges are composed of the Paleozoic limestones and dolomites.Volcanic rock from the Cenozoic (66 million–1.8 million years ago) occurs in the San Juan Mountains and in other areas.Millennia of severe erosion in the Wyoming Basin transformed intermountain basins into a relatively flat terrain.