WATER AND ICE - Physical Geography - page 108

abrasion is very fine and when suspended in water is known as
glacial flour. Meltwater at the bed of a glacier can also cause
erosion by mechanical or chemical processes as with a normal river
system. Glacial meltwater is good at erosion as it tends to flow
quickly and contains lots of abrasive sediment. The rate at which
glaciers and ice sheets erode varies depending on temperature, the
rate of ice movement and the bedrock. On average across the
planet it seems that a rate of 1 metre of erosion for every 1,000
years is a reasonable estimate for ice masses.
Weathering action such as freeze-thaw and undercutting by the
glacier results in material falling from rock slopes onto the surface
of glaciers where it is transported downstream by the ice mass. If
the material falls onto the glacier in the upper accumulation zone,
it will become buried within the glacier. The material will be
transported either within the main volume of ice or it may slowly
descend to the bed eventually assisting scouring. If material falls
onto the surface in the ablation zone it will usually remain on top
of the glacier. There have been some interesting finds of material
that have been incorporated and preserved by glacier ice. For
example, in Siberia, an extinct woolly mammoth was found frozen
inside a glacier, while in the Alps a prehistoric human was found.
The features formed by glacial erosion include large scale land-
forms such as U-shaped valleys, with steep ridges and horns between
the valleys. Here, ice masses flowed over the landscape, scouring it as
they moved, leaving angular summits exposed that protruded above
the ice. Glacial U-shaped valleys or troughs (note that river cut
valleys are usually V-shaped) are mainly the product of abrasion, but
rock fracturing and plucking downstream of smoothed obstacles are
also important. While most glaciers follow existing river valleys, they
act to deepen, widen and straighten them. Lakes often form in the
eroded valley left by a glacier and these lakes slowly infill with sedi-
ment over time. If the glacier has eroded the valley to below sea
level then the valley may get flooded after the ice has melted to form
a fjord . Some infilling of the valley bottom occurs after ice melt as
the gravels produced by meltwater downstream of the retreating
glacier, combined with lake sediments, act to flatten the floor of the
valley, assisting the production of the U-shape.
Often side valleys to the main glacial valley are truncated,
leaving hanging valleys . These hanging valleys were occupied by