Geoscience Reference
In-Depth Information
Bismarck
Minnesota
South Dakota
Wisconsin
Pierre
Minneapolis
Direction
of ice flow
Iowa
Chicago
Nebraska
Omaha
Des Moines
Illinois
Kansas City
Kansas
St. Louis
Missouri
0
100
200
km
0
100
200
mi
(a)
(b)
Figure 17.14 Plucking and erratics of Sioux Quartzite.
(a) The Sioux Quartzite is Precambrian bedrock that crops out in southwestern
Minnesota. Rocks from this outcrop were plucked by glacial ice about 600,000 years ago and carried as far south as northeastern Kansas.
(b) This Sioux Quartzite erratic lies in northeastern Kansas. It is highly resistant to erosion, which is why it remains a prominent feature even
though it has been resting there for about 600,000 years.
Direction of
ice movement
mountainous regions, resulting in very dramatic land-
scapes that are easy to interpret. In general, alpine gla-
ciers modify a landscape by rounding parts of it at the
same time that other aspects are sharpened. To see how
a preglacial mountainous landscape is transformed by
glaciation, refer to Figure 17.16 during the following
discussion.
The most obvious landform created by glacial
erosion in alpine regions is a cirque, which, as de-
scribed earlier, is a broad amphitheater that forms
on the flanks of a mountain. Cirques typically have
very steep side and head walls and a floor that is flat
to shallow sloping (Figure 17.17a). Remember that
cirques are source areas for alpine glaciation and are
enlarged through the combined processes of glacier
plucking and mass wasting of the adjacent walls.
Sometimes the ice in a cirque melts completely away,
leaving water in the quarried depression. Such a small
lake is called a
tarn
(Figure 17.17b).
Stoss side
Lee side
Bedrock
Surface polished
smooth by abrasion
Surface pitted and
rough due to plucking
(a)
A small lake that forms within a glacial cirque.
Tarn
Figure 17.15 Roche moutonnée.
(a) A roche moutonnée
is created through the combined processes of abrasion on
the stoss side and plucking on the lee side. (b) A roche mou-
tonnée formed in the Manhattan Schist in New York City's
Central Park. The stoss side is on the right-hand side of the
image.
(b)