Here's why:

* V-shaped valleys are typically formed by rivers eroding downwards through a landscape, carving out a narrow, pointed valley.

* U-shaped valleys are formed by glaciers. As glaciers move, they carve out a wider, rounded valley with steep sides.

The Grand Canyon was formed by the Colorado River, but it was also shaped by the erosive power of glaciers in the past. This explains its U-shaped profile and its wide, flat bottom.