Why are the Appalachian Mountains lower and more rounded than the Rocky Mountains?

The Appalachian Mountains are lower and more rounded than the Rocky Mountains primarily due to their extreme age and the long-term effects of erosion.

The Appalachians are among the oldest mountains on Earth, with their primary mountain-building events—such as the Alleghenian orogeny—occurring hundreds of millions of years ago. Over these vast spans of geologic time, the ceaseless action of water and ice has continuously sculpted the landscape, wearing down once-higher peaks into the more subdued, rounded forms seen today. Because they formed so long ago, they are often classified as residual mountains, which remain despite a very long history of erosional forces.12

In contrast, the Rocky Mountains are significantly younger, with much of their major mountain-building occurring during the Laramide Orogeny roughly 65 to 35 million years ago, and some ranges, like the Teton Range, attaining their height less than 10 million years ago. Their relative youth means they have been subjected to less total erosion, allowing them to retain more jagged, high-relief features and greater elevations.32