Few natural processes on the Earth's surface can match glaciers in transporting debris long distances away from their source. Indeed, one of the most immediately obvious features of mountain glaciers is the amount of rubble and the huge blocks of rock that litter their surface. Commonly, the entire ablation area of a mountain glacier is completely debris-covered, although the melting of ice beneath the debris layer creates a very uneven and unstable topography that is arduous to walk over. Thus, a glacier may be considered as a sort of conveyor belt for rock debris, transporting material from all points along its length towards the snout. Typically, this material is carried on the surface (where it is referred to as supraglacial debris), or near its bed in basal ice (where it is described as basal debris). Large amounts of debris may also be ingested from both the surface and bed as englacial debris, giving the glacier a very dirty appearance. In addition to debris carried within the ice, debris is also transported within the deforming bed zone.
For a glacier to be laden with supraglacial debris, it normally requires rocks to be exposed above its flanks. Thus, ice sheets and ice caps, which submerge mountains, carry almost no supraglacial debris, whereas the lower reaches of mountain valley glaciers carry many boulders. The main sources of this supraglacial debris are rock-fall from frost shattering, aided by the unstable nature of the hillsides over-steepened by the glacier.