Drone footage of several different valleys of different shapes and sizes.

Sam: What gives a valley its shape? There are two main ways that valleys form. Rivers in high mountains move very fast and have a lot of power to erode rock. Usually these rivers are very skinny and this results in a V-shaped valley with a narrow bottom and straight steep sides.

A steep sided V-shaped valley with a skinny river running through it.

Sam: When the planet gets colder, glaciers flow down from mountains like giant rivers of ice. The wide base of the glacier scrapes away the rock underneath. When the planet warms and the glaciers melt, it leaves behind a U-shaped valley with a wide base and curving sides.

A wide U-shaped valley with a wide river running through it. An image of a glacier fades in and fills the valley.