Answer:
Geologic age
Step-by-step explanation:
The eastern parts of the United States and Canada are much older than the western parts. A clear example of this difference in age can be seen when comparing the Rocky Mountains to the Appalachians.
The Rockies are still rising due to subduction off the Pacific Coast of the Juan de Fuca plate underneath the North American plate. As a result, the Rockies are a very young range. The Appalachians, on the other hand, are ancient mountains that have been heavily eroded. They are on a passive continental margin and low ago stopped rising.