Here's why:

* Glaciers erode and transport material: As glaciers move, they carve out valleys and pick up rocks, soil, and other debris.

* Moraine formation: This material is deposited at the glacier's edge, forming a ridge called a moraine.

* Terminal moraine: The terminal moraine marks the furthest point a glacier reached during its advance. It's the outermost moraine, indicating the glacier's maximum extent.

Think of it like a "high-water mark" for the glacier.