Here's why:

* Crevasses: These are deep cracks that form in the glacier's surface due to the tension caused by its movement over uneven terrain. They typically form perpendicular to the direction of movement. The shape and orientation of crevasses can reveal the direction the glacier is flowing.

* Other Clues: While crevasses are the most reliable indicator, other features can also provide hints:

* Striations: These are scratches or grooves carved into the bedrock by rocks embedded in the glacier's base. They run parallel to the direction of movement.

* Terminal Moraine: The pile of debris (rock, sediment) deposited at the end of the glacier can indicate the direction of the glacier's advance.

* Lateral Moraine: The piles of debris deposited along the sides of a glacier, where the glacier meets the valley wall, can also give clues about the direction of movement.

Let me know if you have any more questions about glaciers!