Here's why:

* Glaciers and Debris: Glaciers are slow-moving rivers of ice that carry rocks, dirt, and other debris called glacial till.

* Lateral Moraines: As a glacier moves through a valley, it erodes the sides and picks up debris, forming lateral moraines along its edges.

* Medial Moraine Formation: When two glaciers meet, their lateral moraines merge to form a medial moraine, a dark stripe of debris running down the center of the combined glacier.

Think of it as two rivers meeting, each carrying its own sediment load. When they combine, the sediment is carried down the middle of the larger river.