How do glaciers cause weathering and erosion?

ANSWER:

A glacier is nothing more than a frozen river still moving. It might only move an inch or two per year, but it still moves and this ice will erode the ground and rock below it faster and more agressively than if it was just water. Its because glaciers also pick up and move the rocks that they run over and this gravel (chunks of rocks) can carve mountains down and cut valleys miles deep.