Environmental Engineering Reference
In-Depth Information
Figure 3.43.
Diagrammatic section along a valley glacier (based on Blyth and de Freitas, 1989).
3.12.1
Glaciated valleys
Figure 3.43 is a diagrammatic section along a valley glacier during a period of its
“advance” down the valley. It can be seen that the ice tends to move over irregularities in
its path, behaving in a viscous manner near its base and failing in shear and tension at
higher levels. The resulting valley floor is uneven and often contains deep hollows eroded
by the ice. Many such hollows are now occupied by lakes, the water is dammed partly by
ridges of rock and partly by glacial debris. Streams of meltwater flow beneath the ice and
exit at the snout.
During periods in which winter snows do not survive through summer the glacier
diminishes in size and its snout “retreats” up the valley.
Although glacial valleys are generally U-shaped when viewed broadly, in detail the val-
ley floors are often highly irregular as at Parangana Dam in Tasmania (
Figures 3.44
and
3.45
). The shape of the buried valleys at this site and nature of the infill materials, indi-
cates that they are remnants of pre-glacial river valleys. Figure 3.44 illustrates the way in
which such a pre-glacial river channel can be preserved under glacially derived deposits.
In other cases locally deeper valley sections under glaciers may have been differentially
eroded by ice or meltwater streams.
3.12.2
Materials deposited by glaciers
The general generic term for material deposited by glaciers is “till”. Deposits of till are
often referred to as “moraine”. A further term “drift” has been used extensively and
loosely to describe surficial deposits of glacial, alluvial or colluvial origins.
Figure 3.46
shows mechanisms of accumulation, ingestion and transport of debris by a
glacier. Upslope from the equilibrium line (Figure 3.46a) windblown dust and rock debris
is buried within the snow which feeds the glacier and is transported downglacier forming
the basal debris zone and thin basal traction layer. The rock particles in the traction layer
abrade, polish and groove the rock floor, generating fine rock particles known as rock
flour. The ice and rock blocks within it also pluck or “quarry” rock from the floor and
sides. The resulting material developed in contact with the rock floor is known as lodge-
ment till (Figure 3.46b) and usually contains a wide range of particle sizes. The lodgement
till is formed under relatively high effective normal pressures and consequently is usually
compacted to at least stiff consistency. Its upper surface is grooved or fluted parallel to the
direction of flow of the ice.
