Ice at the land surface - Fundamentals of the Physical Environment - page 496

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.
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
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 turnover
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 15.9).
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 stress and pressure-melting, enabling
the ice stream to exploit weaker bedrock. Quarrying is self-enhancing, as it draws more
ice into the developing rock basin, and this increases dramatically in the outlet glacier
zone (Zone III).
In steady state the ELA is located relatively close to ice sheet margins, nourished and
melted in advection-driven mass and energy balance conditions. This transforms the ice
stream into a temperate, warm-based state and vigorous outflow draws down adjacent
inland areas of the ice sheet through outlet glacier troughs. They are the most impressive
of erosional landforms, excavated 1-5 km deep and breached clean through any
cordillera in their path regardless of subglacial topography (Plate 15.11). Transfluent ice
flow on this scale excavated fjords through the coastal mountains of southern Norway
(Plate 15.12), the South Island of New Zealand, British Columbia, Alaska, southern Chile
and - to a lesser extent - western Scotland. The Finger Lakes region south of Lake
Ontario marks transfluent ice flow towards the southern margin of the Laurentide Ice
Sheet. Beyond the constriction of outlet glacier troughs, ice fans out in the piedmont zone
(Zone IV). Where ice flow is still vigorous it erodes knock - and - lochan topography of
parallel roches moutonnées interspersed with shallow rock basins which become lake-
filled during deglaciation.