Want to know something crazy? The Appalachian Mountains are actually shrinking. Due to the fact that it's one of the oldest mountain ranges (or oldest anythings, for that matter) on earth, the range is shrinking by "0.03 millimeters per year" (2.15.8).
One can see this effect in action at the famous Delaware Water Gap. The Water Gap is a tiny stream that bisects two mountains—a microcosm of the way that water and wind are slowly cutting the Appalachians down to size.
These mountains were also formed in large part by glacial movement. In fact, there was a time when most North America was covered with glaciers.