Here's why:

* Glaciers are powerful agents of erosion: Glaciers are massive bodies of ice that move slowly over the land. Their weight and movement carve out the landscape.

* How glaciers shape valleys: As a glacier moves, it scrapes and plucks the rock and soil beneath it, widening and deepening the valley. The characteristic U-shape is formed because the glacier's weight erodes the sides as well as the bottom of the valley.

In contrast, deposition is the process where material is added to a landform. While glaciers can deposit material (like moraines), the formation of a U-shaped valley is primarily a result of erosion.