Here's why:

* Erosion is the process of wearing away and transporting sediment from one location to another. Think of wind blowing sand, or a river carrying rocks and soil downstream.

* Deposition is the process of laying down sediment in a new location. This happens when the erosive force (like wind or water) loses energy and can no longer carry the sediment.

So, while erosion is involved in moving sediment, it's deposition that actually lays it down.