Geoscience Reference
In-Depth Information
Depositional Glacial Landforms
Like many other Earth processes, the formation of glacial
landforms can be better understood when visualized in ani-
mated form. To do so, go to the
Geo Media Library
and se-
lect
Depositional Glacial Landforms
. This animation illustrates
the way moving and melting ice shapes the landscape. It also
contains a nice video of glaciers in Peru. After you complete the
interactivity, be sure to answer the questions at the end to test
your understanding.
VISUAL CONCEPT CHECK
17.3
Glaciers construct landforms in many different
ways, both in direct contact with the ice and in
close proximity to it. Given your understanding
of landscape evolution and glaciers, what kind of
landform is being constructed in this image? How
can you tell? What will the relief of this landform
be like when it is fully developed?
5.
Kames and eskers are distinctive meltwater landforms.
A kame is an irregularly shaped hill that essentially con-
sists of an alluvial fan or a deltaic deposit that forms
in contact with the ice. Eskers are winding ridges that
develop when sediment accumulates in a stream that
flows in a tunnel at the base of the ice.
KEY CONCEPTS TO REMEMBER ABOUT
GLACIAL DEPOSITION AND RESULTING
LANDFORMS
1.
Two primary kinds of glacial deposits occur: till and
outwash. Glacial till is deposited in direct contact with
the ice, whereas glacial outwash accumulates through
meltwater streams flowing in front of the ice.
History of Glaciation on Earth
The many examples of glacial landforms demonstrate that ice
sheets have been an important and widespread feature around
the world in the past. The presence of drumlins in places like
New York and Michigan, for example, indicates that ice sheets
once existed where they no longer do. You might wonder if it
is possible for such conditions to return at some point in the
future. This is a great question and provides the context for a
discussion of Earth's glacial history.
In order to provide the proper context about the glacial past,
some fundamental terminology must be established. Periods of
glacial advance, when glaciation is a dominant worldwide pro-
cess, are called
glacials
. During glacials, temperatures are cooler
on average, ice sheets expand and become larger, and sea level is
lower because more water is stored in the ice sheets. Periods of
glacial retreat, such as we are in now and have been for about the
past 10,000 years, are called
interglacials
. During interglacials,
average global temperatures are warmer, ice sheets and alpine
glaciers are smaller, and sea level is higher because meltwater
from the glaciers has returned to the sea.
2.
Glacial till is relatively unsorted and accumulates ei-
ther as basal (lodgement) till smeared under the bot-
tom of the glacier or as ablation till laid down as the
ice melts.
3.
A distinctive depositional landform created by glaciers
is a moraine (end, lateral, or medial), which is a ridge
of till that forms when the ice front or margin is in one
place for a relatively long period of time. Drumlins are
streamlined till landforms created by the weight and
pressure of the overlying flowing ice.
4.
Glacial outwash is relatively well sorted because it is
deposited by flowing meltwater in front of the ice. The
associated streams are typically braided and create a
broad, flat surface known as an outwash plain. Some-
times glacial outwash buries a block of ice that broke
off the front of a receding glacier. When this ice block
subsequently melts, it forms a water-filled depression
called a
kettle lake
.
