Laboratory Manual for Introductory Geology 4e

15.3.2 Depositional Landscapes
Landscapes formed by continental glaciers contain several types of depositional
landforms. We will look first at features made up of till deposited directly from a
melting glacier, then at landforms deposited by meltwater.
15.3.2a Landforms Made of Till Moraines form when melting ice drops the boulders,
cobbles, sand, silt, and mud that it has been carrying. Some moraines are irregular
ridges; others are broad carpets pockmarked by numerous pits separated by isolated
hills. The type of moraine—ridge or carpet—depends on whether the front (terminus)
of the glacier is retreating (i.e., the ice is melting faster than it is being replaced) or stagnating
(i.e., the terminus is in a state of dynamic equilibrium in which new ice is replenishing
what is melting so that the position of the terminus doesn’t change).
Till deposited by a retreating glacier forms an irregular carpet of debris called
a ground moraine, which may be hundreds of feet thick. Large blocks of ice are
isolated as a glacier retreats, and some may be buried by outwash or till. When a
block melts, the sediment that covered it subsides and forms a depression in the
ground moraine called a kettle hole. For this reason, the irregular surface of a
ground moraine is often referred to as knob-and-kettle topography.
The terminus of a stagnating glacier may remain in the same place for hundreds
or thousands of years, and the till piles up in a ridge that outlines the terminus. This
ridge, called a terminal moraine, shows the maximum extent of the glacier.
FIGURE 15.6 shows some of the depositional features created at the termini
of continental glaciers. Use these diagrams to identify landforms in the following
exercises. A glossary of glacial depositional
and erosional features can be found in
Appendix 15.1 at the end of this chapter.
Drumlins are streamlined, asymmetric
hills made of till that commonly occur in
large groups. Four of the largest swarms of
drumlins are in Nova Scotia, in southern
Ontario and northern New York State, in
southeastern New England (most famously
Bunker Hill), and in Wisconsin, Iowa, and
Minnesota. Their origin is debated, but their
shapes clearly reflect the dynamics of ice flowing
through till, and their shapes and orientations
are good indicators of glacial direction.
FIGURE 15.6 Formation of depositional landforms by continental
glaciers.
Two lobes join.
Proglacial
lake
Tunnel
Tunnel
Outwash
Braided streams
choked with
outwash sediment
15.3.2b Landforms Made of Outwash A
melting continental glacier generates an
enormous amount of water, which flows away
from the terminus carrying all the clasts the
water is powerful enough to carry—mostly
sand and gravel, with perhaps a few small
boulders. Meltwater streams round and
sort this sediment and eventually deposit it
beyond the terminal moraine as a generally
flat outwash plain (see Fig. 15.6). The flat
outwash plains visible in the southernmost
part of Figure 15.7 differ dramatically from
the adjacent terminal moraines and from
the knob-and-kettle topography of a ground
moraine.
Drumlin
Kettle lake
Till
Ground
moraine
Recessional
moraine
Proglacial
delta
Kettle lake
Eskers
Glacial
lake bed
Terminal
moraine
Outwash plain
Stratified
outwash
sediment
Erratic
15.3 LANDSCAPES PRODUCED BY CONTINENTAL GLACIATION
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