What would make a mountain top become flatter and smoother over time?

What is this process called?

Erosion

A mesa is a flat-topped mountain or hill, they are formed by erosion, when water washes smaller and softer types of rocks away from the top of a hill. The strong, durable rock that remains on top of a mesa is called caprock. A mesa is usually wider than it is tall and are usually found in dry regions where rock layers are horizontal

reclamation

Explanation:

claiming something back or of reasserting a right.