a glacier made of ice can erode a landscape made of rock because there can be water held within the rock that gets frozen, and because water expands when it is frozen the rock breaks apart and the glacier grinds its' way 'downhill' (down a mountain range, a good example of this is the Appalachian mountain range) and carves out a significant amount of material that is below it, as well as the materials frozen inside of the glacier. so glaciers can freeze the rocks, and then scrape away any material that is softer than rock.
