The Appalachians have had 3 orogenic (mountain building) periods followed by significant erosion. What you see currently is the result of the last erosional period. Imagine something much closer to the Himalayas (similar continental convergence process) at their highest. Appalachian geology is rather complex compared to other mountain ranges. The Rockies aren't quite comparable since they formed due to eastward migration of a shallow angle subducting plate. Trust me, I'm a geologist.

Like the song says they're older than the trees. Kinda crazy to think trees hadn't even evolved yet when those mountains were at their peak.

Still pretty remarkable, just that they were so much bigger before. The wild thing is learning that most of the land east of them to the coast is sediment from their erosion. It makes sense once you think about it, but imagine the amount of time to do that.