Here's why:

* Glacial Erosion: U-shaped valleys are characteristic landforms created by glaciers. As glaciers move, they carve out the valley by:

* Abrasion: Glacial ice, laden with rocks and debris, grinds and scrapes the valley floor and sides.

* Plucking: Melting water at the base of the glacier freezes and expands, breaking off rock fragments from the valley walls.

* V-shaped vs. U-shaped: Before glacial activity, valleys are typically V-shaped, formed by rivers. Glaciers widen and deepen these V-shaped valleys, creating the characteristic U-shape.

Deposition in glacial valleys occurs as well, but it's a separate process. Glaciers deposit sediment, forming moraines, outwash plains, and other features, but the formation of the valley itself is primarily an erosional process.