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glacier is a stream of ice, bounded by precipitous rock walls, that
flows down valley from a snow accumulation centre near its head.
Like rivers, valley glaciers can be long or short, wide or narrow,
single or with branching tributaries. Generally, the widths of alpine
glaciers are small compared to their lengths. Some extend for just
a fraction of a kilometre, whereas others go on for many tens of
kilometres.
In contrast to valley glaciers, ice sheets exist on a much
larger scale. Although many ice sheets have existed in the past,
just two achieve this status at present: in the Northern Hemisphere,
Greenland is covered by an imposing ice sheet that occupies 1.7
2
mln km and in the Southern Hemisphere, the huge Antarctic ice
sheet attains a maximum thickness of almost 4300 m and covers
2
nearly the entire continent, an area of more than 13.6 mln km .
Along portions of the Antarctic coast, glacial ice flows into
the adjacent ocean, creating features called ice shelves. They are
large, relatively flat masses of floating ice that extend seaward
from the coast but remain attached to the land along one or more
sides. The shelves are thickest on their landward sides, and they
become thinner seaward. They are sustained by ice from the
adjacent ice sheet as well as being nourished by snowfall and the
freezing of seawater to their bases. Antarctica’s ice shelves, for
2
example, extend over approximately 1.4 mln. km .
In addition to valley glaciers and ice sheets, other types of
glaciers are also identified. Covering some uplands and plateaus
are masses of glacial ice called ice caps. Like ice sheets, ice caps
completely bury the underlying landscape but are much smaller
than the continental-scale features. Ice caps occur in many places,
including Iceland and several of the large islands in the Arctic
Ocean.
Often ice caps and ice sheets feed outlet glaciers. These
tongues of ice flow down valleys extending outward from the
margins of these larger ice masses. The tongues are essentially
valley glaciers that are avenues for ice movement from an ice cap

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