Cryospheric systems - Fundamentals of the Physical Environment - page 366

Plate 15.12 Thinning, break-up and retreat of outlet glaciers near Angmagssalik at the eastern margin of the Greenland Ice
Sheet.
Photo: Ken Addison

rock surfaces are polished and striated by vigorous ice
flow. Streamlined roches moutonnées are abundant.
Valley networks may be rectilinear where glaciers flow
away from major morphotectonic watersheds such as the
Andes and radial from more isolated eminences, as in
the English Lake District. Glaciers remain confined to,
and accentuate, rock-wall channels even if ice growth
intensifies, although some minor transfluence crosses and
progressively erodes ice sheds. Erosive intensity over one
or more glacial stages is reflected by surviving pyramidal
summits and their narrow, precipitous connecting arêtes .
Less intense glaciation is also recorded by trimlines ,
marking the upper limits of smaller glaciers quarrying
their own diminutive troughs into the main valley.

growth, as late Pleistocene cordilleran ice limits show
( Figure 15.1 ). Instead, ice sheets envelop large inland areas
of lower-lying ground where low mass balance and turn-
over combine with gentler slopes in sustained glaciation.
Plateau ice caps such as Hardangerjökull in Norway,
transitional between alpine and ice sheet glaciation, act as
embryonic ice sheets early in glacial events.
The size and full range of thermodynamic conditions
of continental ice sheets are imprinted on four widely
recognized thermodynamic and landsystem zones ( Figure
15.13 ). Zero or low basal velocities and little meltwater in
the ice-shed zone of cold-based ice sheets severely hamper
quarrying and abrasion (Zone I). Such erosion as occurs
is inconspicuous and distributed uniformly. Only
nunataks emerge through almost total ice cover to provide
any scope for undercutting and supraglacial debris. Away
from ice dispersal centres, abrasive scour is more common
and there is evidence that basal ice begins to 'stream' at
depth, quarrying bedrock channels in the selective erosion
zone (Zone II). Local ice thickening increases basal shear

Ice sheet erosional landsystems
Alpine glaciers occupy a greater mountain land area and
extend piedmont lobes into surrounding lowlands during
glacial maxima. They rarely become the focus of ice sheet