Geology Reference
In-Depth Information
(Figure 14.12c). Cirques are typically steep-walled on three
sides, but one side is open and leads into the glacial trough.
Some cirques slope continuously into the glacial trough, but
many have a lip or threshold at their lower end.
The details of cirque origin are not fully understood, but
they probably form by erosion of a preexisting depression on
a mountain side. As snow and ice accumulate in the depres-
sion, frost wedging and plucking, combined with glacial ero-
sion, enlarge and transform the head of a steep mountain
valley into a typical amphitheater-shaped cirque. Tension in
the upper part of the glacier may reduce the erosive power of
the ice on the immediate, downslope side of a cirque, leav-
ing a lip or threshold in the valley fl oor after the ice melts
away. Small lakes of meltwater, called
tarns
, often form on
the fl oors of cirques behind such thresholds.
Cirques become wider and are cut deeper into moun-
tainsides by headward erosion as a result of abrasion, pluck-
ing, and several mass wasting processes. For example, part
of a steep cirque headwall may collapse while frost wedg-
ing continues to pry loose rocks that tumble downslope,
so a combination of processes erode a small mountainside
depression into a large cirque.
Arêtes
—narrow, serrated ridges—form in two ways. In
many cases, cirques form on opposite sides of a ridge, and
headward erosion reduces the ridge until only a thin parti-
tion of rock remains (Figure 14.12c). The same effect occurs
when erosion in two parallel glacial troughs reduces the
intervening ridge to a thin spine of rock.
The most majestic of all mountain peaks are
horns
,
steep-walled, pyramidal peaks formed by headward ero-
sion of cirques. For a horn to form, a mountain peak
must have at least three cirques on its fl anks, all of which
erode headward (Figure 14.12c). Excellent examples of
horns are Mount Assiniboine in the Canadian Rockies, the
Grand Teton in Wyoming, and the most famous of all, the
Matterhorn in Switzerland.
extensive exposures of striated and polished bedrock. These
areas have deranged drainage (see Figure 12.19e), numerous
lakes and swamps, low relief, extensive bedrock exposures,
and little or no soil. They are referred to as
ice-scoured plains
(
Figure 14.13). Similar though smaller bedrock exposures
are also widespread in the northern United States from
Maine through Minnesota.
◗
What kinds of glacial deposits do geologists recognize, and
are they as distinctive as the erosional features? Both valley
and continental glaciers deposit sediment as
glacial drift
, a
general term for all deposits resulting from glacial activity. A
vast sheet of Pleistocene glacial drift is present in the northern
tier of the United States and adjacent parts of Canada. Smaller
but similar deposits are found where valley glaciers existed or
remain active. The appearance of these deposits may not be
as inspiring as some landforms resulting from glacial erosion,
but they are important as reservoirs of groundwater, and in
many areas, they are exploited for their sand and gravel.
One conspicuous aspect of glacial drift is rock fragments
of various sizes that were obviously not derived from the un-
derlying bedrock. These
glacial erratics
, as they are called,
were derived from some distant source and transported to
their present location (
Figure 14.14). Some erratics are gi-
gantic. For instance, the Madison Boulder in New Hampshire
and Daggett Rock in Maine weigh about 4550 and 7270 metric
tons, respectively. The glacial erratic shown in Figure 14.14b is
not the world's largest, but it is one of many in a narrow belt
of erratics stretching more than 640 km from their source.
As noted,
glacial drift
is a general term, and geologists
defi ne two types of drift: till and stratifi ed drift.
Till
consists
of sediments deposited directly by glacial ice. They are not
◗
Erosional Landforms
Areas eroded by continental
glaciers tend to be smooth and
rounded because these glaciers
bevel and abrade high areas that
project into the ice. Rather than
yielding the sharp, angular land-
forms typical of valley glacia-
tion, they produce a landscape of
subdued topography interrupted
by rounded hills because they
bury landscapes entirely during
their development.
In a large part of Canada, par-
ticularly the vast Canadian Shield
region, continental glaciers have
stripped off the soil and unconsol-
idated surface sediment, revealing
◗
Figure 14.13
An Ice-Scoured Plain in Northwest Territories of Canada This low-relief surface
is an ice-scoured plain in the Northwest Territories of Canada. Numerous lakes, little or no soil, and
extensive bedrock exposures are typical of these areas eroded by continental glaciers.
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