Cascades.
The current volcanic arc has been active for roughly the last 5–7 million years, though volcanism in the region extends further back.
A landscape with a history.
Formed above the Juan de Fuca Plate subducting beneath North America — the same basic mechanism as the Andes, on a smaller and less continuous scale. The range consists of a chain of individual stratovolcanoes rather than one continuous uplifted mass, distinguishing it visually from fold ranges like the Rockies or Appalachians.
What makes it remarkable
Includes Mount St. Helens (notable for its explosive 1980 eruption) and Mount Rainier, both classic stratovolcanoes as described on the Volcanism page.
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.
Rocky Mountains
Primary formation (the Laramide orogeny) occurred roughly 80–55 million years ago.