As a glacier moves, particularly a warm glacier, it causes erosion of the underlying surface. However, ice doesn't seem as if it should be a particularly effective material for wearing away hard rocks, so how do glaciers manage to produce such large erosional features?Material from underlying bedrock or sediment is picked up by the glacier and 'held' in the ice as it moves. Material falling onto the surface (often the result of freeze-thaw activity, or frost shattering, on the surrounding rock walls) is also transported, and often finds its way down through crevasses to the base of the glacier. Material held within the glacier is called 'englacial moraine'. It is this material trapped in the ice, that allows the glacier to erode its surroundings.