The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch 3,000 miles in straight-line distance from the northernmost part of western Canada to New Mexico in the southwestern United States. The Rockies formed 80 million to 55 million years ago during the Laramide orogeny, in which several plates began sliding underneath the North American plate.

Source: Wikipedia