The Rocky Mountains are actually a system or series of subranges. 3 major mountain-building periods account for most what we consider the Rockies. These orogenies are shown below with the earliest uplift occurring about 250 Ma.
    60 million years ago the rockies were like Tibet today, a high plateau some 20,000 ft above sea level. Since then erosion has stripped away the top rocks revealing the ancestral rocks from even earlier eras. More recently, it's been the numerous Pleistocene ice ages that have shaped the Rockies leaving many glacial landforms.
    It should be noted that a large part of the Rocky Mountain region is made up of lowlands - basins and fault-bounded troughs and trenches.