Here's why:
* Glacial Erosion: Glaciers are massive sheets of ice that move slowly over the land. As they move, they carve out valleys by:
* Plucking: Glaciers freeze to bedrock and then pull out chunks of rock as they move.
* Abrasion: The embedded rock fragments in the ice grind and scrape against the valley walls, further eroding the bedrock.
* V-shaped vs. U-shaped: River valleys typically have a V-shaped profile. Glaciers, with their immense weight and erosive power, widen and deepen these V-shaped valleys, transforming them into the characteristic U-shape.
Other factors can influence the shape of a valley, but glaciers are the primary shapers of U-shaped valleys.
