Rocky Mountains.
Primary formation (the Laramide orogeny) occurred roughly 80–55 million years ago.
A landscape with a history.
Formed through a broad, unusually far-inland compressional event, likely caused by a shallow-angle subduction of an oceanic plate beneath western North America, transmitting stress much further inland than typical subduction-driven mountain-building. This makes the Rockies geologically distinct from more "textbook" subduction ranges like the Andes.
What makes it remarkable
Spans a huge portion of western North America, from New Mexico to British Columbia, with significant regional variation in rock type and age along its length.
Another skyline. Another story.
Himalayas
Began forming approximately 50 million years ago; still actively rising today.
Andes
Formation began roughly 200 million years ago, with the modern range's major uplift occurring within the last 25 million years.
Appalachian Mountains
Began forming approximately 480 million years ago.