Page 100 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 83

Plate 3.4 Striated limestone bedrock near the snout of Saskatchewan Glacier, Canadian Rockies.
(Photograph by Mike Hambrey)

being constant, the abrasion rate increases as the on the downstream side of bedrock obstacles. Large
basal pressure rises. Eventually, the friction between blocks are entrained as the ice deforms around them and
an entrained debris particle and the glacier bed rises engulfs them.
to a point where the ice starts to flow over the Moving ice is a potent erosive agent only if sediment
glacier-bed debris and the abrasion rate falls. And, continues to be entrained and transported (Figure 3.15).
when the pressure reaches a high enough level, debris Subglacial debris is carried along the glacier base. It is
movement, and hence abrasion, stops. (5) In warm produced by basal melting in ‘warm’ ice and subsequent
glaciers, the basal water pressure, which partly coun- refreezing (regelation), which binds it to the basal ice.
teracts the normal stress and buoys up the glacier. Creep may also add to the subglacial debris store, as
(6) The difference in hardness between the abrad- may the squeezing of material into subglacial cavities
ing clasts and the bedrock. (7) The size and shape in warm-based glaciers and the occurrence of thrust as
of the clasts. (8) The efficiency with which eroded ice moves over large obstacles. Supraglacial debris falls
debris is removed, particularly by meltwater. on to the ice surface from rock walls and other ice-free
areas. It is far more common on valley and cirque glaciers
Quarrying and abrasion can occur under cold glaciers, than over large ice sheets. It may stay on the ice sur-
but they have a major impact on glacial erosion face within the ablation zone, but it tends to become
only under temperate glaciers where released meltwater buried in the accumulation zone. Once buried, the debris
lubricates the glacier base and promotes sliding. is called englacial debris, which may re-emerge at the
ice surface in the ablation zone or become trapped
Glacial debris entrainment and transport with subglacial debris, or it may travel to the glacier
snout. Where compression near the glacier base leads
Two processes incorporate detached bedrock into a to slip lines in the ice, which is common in the ablation
glacier. Small rock fragments adhere to the ice when zone, subglacial debris may be carried into an englacial
refreezing (regelation) takes place, which is common position.