Glaciation is the modification of the land’s surface by the
action of glacial ice. When glaciers move over the land’s surface,
they plow up the soils, abrade and file down the bedrock,
carry and transport the sedimentary load, steepen valleys, then
leave thick deposits of glacial debris during retreat.
In glaciated mountains, a distinctive suite of landforms
forms from glacial action. Glacial striations are scratches on
the surface of bedrock, formed when a glacier drags boulders
across the bedrock surface. Roche moutonnée and other
asymmetrical landforms are made when the glacier plucks
pieces of bedrock from a surface and carries them away. The
step faces in the direction of transport. Cirques are bowlshaped
hollows that open downstream and are bounded
upstream by a steep wall. Frost wedging, glacial plucking,
and abrasion all work to excavate cirques from previously
rounded mountaintops. Many cirques contain small lakes
called tarns, which are blocked by small ridges at the base of
the cirque. Cirques continue to grow during glaciation, and
where two cirques form on opposite sides of a mountain, a
ridge known as an arete forms. Where three cirques meet, a
steep-sided mountain forms, known as a horn. The Matterhorn
of the Swiss Alps is an example of a glacial carved horn.
Valleys that have been glaciated have a characteristic Ushaped
profile, with tributary streams entering above the base
of the valley, often as waterfalls. In contrast, streams generate
V-shaped valleys. Fiords are deeply indented glaciated valleys
that are partly filled by the sea. In many places that were formerly
overlain by glaciers, elongate streamlined forms known
as drumlins occur. These are both depositional features (composed
of debris) and erosional (composed of bedrock).
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