Here's why:

* Stream valleys are V-shaped: Rivers erode downwards, creating a V-shaped profile.

* Glaciers are powerful: Glaciers are massive and exert immense pressure as they move.

* Glaciers carve out valleys: The weight and movement of a glacier erode and widen the existing V-shaped valley, carving out a wider, more rounded U-shape.

* Glaciers level out the valley floor: The glacier's erosive power also flattens out the valley floor, contributing to the characteristic U-shape.

Key features of a U-shaped valley:

* Steep, nearly vertical sides: These are formed by the glacier's erosive power on the valley walls.

* A wide, flat valley floor: The glacier scours the valley floor, leaving a relatively flat area.

* Hanging valleys: Smaller tributary valleys, often carved by smaller glaciers, are often left hanging above the main U-shaped valley, creating waterfalls.

So, the next time you see a valley with a distinctly U-shaped profile, you can be sure it was carved out by the powerful force of a glacier!