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