Geoscience Reference
In-Depth Information
(a)
Plate 15.10 A small thrust moraine extruded beneath
almost stagnant ice at the Lewellyn Glacier terminus on the
eastern flank of the Juneau Icefield, northern British Columbia.
Photo: Ken Addison
(b)
across the North York moors, but the recognition of other
former ice-dammed lakes from meltwater channels alone
may be spurious. Break-out in spring can be catastrophic
if sufficient meltwater build-up bursts through still frozen
channels, and the resulting jökulhlaup is a powerful ero-
sive agent. Meltwater deposi-tion is subject to normal flu-
vial 'rules'. It is restricted to ice-walled and bedrock
channels throughout the glacial plumbing system but is
free to develop unconfined deposits beyond the ice mar-
gin. Glaciofluvial sediment budgets are distinguishable by
their high point-source concentrations of glacially derived
debris, their high spring melt regime and by a high sus-
pended load of fines which gives meltwater streams a dis-
tinctly milky appearance.
Meltwater is increasingly seen as a more subtle
influence on processes regarded formerly as glacial in
nature. Its presence as a widespread water film at the
glacier bed is determined by the glacier's thermodynamic
character and pressure-melting regime. This facilitates
sliding, and thereby glacial abrasion, and exerts a major
quarrying influence as a hydraulic jack in bedrock
discontinuities. High pore water pressure leads to
instability in all forms of glacial debris and promotes
deformation, slumping and flowage.
Plate 15.9 Foliation and shear planes in the Tsidjiore Nouve
glacier, Switzerland, highlighted by contrasting clean and
debris-rich ice (a) and a melt-out version of similar structures
in glacier sediments (b).
Photos: Ken Addison
subglacial channels which feed discharge portals at the
terminus ( Plate 15.11 ). Water in englacial and subglacial
domains is liable to be under high pressure and capable
of maintaining ice-walled phreatic channels below the
glacier water table. Where it incises bedrock, subglacial
channels contain uphill segments cut through bed
irregularities.
Surface water is inhibited from entering the glacier dur-
ing winter freezing of the plumbing system in temperate
glaciers and more general freezing in cold glaciers. It then
cuts lateral ice-marginal channels and becomes ponded
in depressions at the glacier margin, often developing over-
flow channels. Newtondale, for example, marks the spill-
way of a former ice-marginal lake draining southward
GLACIER EROSIONAL
LANDSYSTEMS
Distinctions between cold and temperate glacier systems
allow us to model the erosional and depositional land-
 
 
 
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