Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 209

contain large amounts of fine glacier-derived rock powder in suspension (called glacial
lour ), especially if a glacier still exists in the cirque. Lakes frequently form in the de-
pressions behind the treads downvalley from the cirque lakes. These lakes often occur
in “chains” along glacial troughs, and are called paternoster lakes because of their re-
semblance to beads on a rosary (Fig. 4.42).
FEATURES RESULTING FROM GLACIAL DEPOSITION
Sooner or later, a glacier must put down the load of earth and rock that it has picked
up or that has fallen onto its surface. The landforms created by glacial deposition, less
spectacular than the features caused by glacial erosion, are nonetheless distinctive.
Most glacial deposition takes place upon melting and retreat of the ice. Moraine materi-
al is deposited directly from the ice, while glaciofluvial material is deposited by meltwa-
ter streams. Moraines typically consist of large and small particles mixed in an unsorted
matrix. They may occur along the sides of the glacier as lateral moraines, or around
the end of the glacial tongue as a terminal moraine or, as it recedes, as recessional mo-
raines. In other cases, the moraine may be less distinct, occurring as a jumble of rocky
debris like the tailings from a deserted strip mine. Lateral and terminal moraines can be
quite impressive (see Fig 4.38, for example), reaching heights of 100-300 m (330-1,000
ft) or more.
The larger rock debris can only be transported directly by the glacier or by ice rafting
(blocks of ice floating in water), but the smaller material may be carried considerable
distances by wind and glacial meltwater streams. The winds that blow off the glacier in
summer are often very effective at picking up and transporting the finely ground rock
particles produced by grinding and scraping during glacial transport (glacial flour). In
some valleys where glaciers exist, the development of such winds is an almost daily oc-
currence during clear weather in summer. Ecologically, the deposition of this silt (called
loess ) is beneficial, as it expedites soil development and greatly improves local pro-
ductivity. Several major agricultural regions have developed on loess soils (e.g., the Pal-
ouse region of eastern Washington State).
Glacial melt streams are the main mechanism for transport of the smaller material.
The amount a stream can carry depends primarily upon the stream's velocity, which in
turn depends, among other factors, upon the volume. Glacial streams, of course, display
great fluctuations in flow between winter and summer as well as between day and night.
Such volume fluctuations produce an irregular pattern of erosion and redeposition; dur-
ing periods of high velocity, the stream erodes and carries a large load of material,
only to drop it again as the water volume subsides and the velocity decreases (Price
1973). Glacial streams are characteristically choked with sediment, much of which is
eventually deposited near the glacial terminus. Such deposits, called valley train, gla-
cial outwash plain, or sandur (Church 1972; Sugden and John 1976; Martini et al. 2001;
Benn and Evans 2010) create flat-floored valleys and may reach considerable depths
and extend for several kilometers beyond the glacial terminus. An extreme example
is Yosemite Valley of California, where seismic investigations reveal that over 600 m
(2,000 ft) of deposits cover the original bedrock floor excavated by the glacier (Guten-
berg et al. 1956). Glacial and glaciofluvial deposits are important ecologically because
soil and vegetation develop much more rapidly on aggregate material than on bare rock.