Here's why:

* Erosion: Mountain horns are formed when glaciers carve out valleys on all sides of a mountain peak. As glaciers move down the valleys, they erode the surrounding rock, leaving behind a sharp, pointed peak.

* Deposition: Deposition is the process of adding material to a landform. While glaciers do deposit material (like glacial till), this process doesn't create mountain horns.

In short: Glaciers are the primary force behind mountain horn formation through their erosive power.