Geology Reference
In-Depth Information
Leaving It All Behind: Glacial Deposits
In the previous section, I describe features created by the removal of rocks and sedi-
ment by glacier ice. All that earth material carried by glaciers has to end up somewhere.
In this section, I describe landscape features created by the
deposition,
or leaving be-
hind, of earth materials by glaciers.
Because glacier ice can carry large amounts of sediment and large sized rocks, when
these materials are left behind they create easily recognizable features. Such features
tell us that ice once covered the land in places you would not imagine ice existing today,
such as New York City or London, England.
Glacial drift
describes any sediments left behind by melting glacial ice. Before scientists
understood how ice sheets covered the land, they proposed the hypothesis that ice-
bergs had drifted these materials into their present location and melted, leaving them
behind. They were only partly correct. Glacial drift is the result of movement and depos-
ition by ice but not by floating icebergs; these sediments were deposited by ice sheets
and glaciers.
Depositing the till
Glacial till
is any rock or sediment that has been carried and left directly by glacial ice.
Because ice can carry rocks and sediment of various sizes without separating them by
size, till deposits are often a mix of different sized rocks and sediment. These mixed de-
posits are called
poorly sorted
and are different from the well-sorted deposits (those of
similar-sized rocks or sediments) typical of wind or water deposition (see Chapters 12
and 14).
Moraines
Glacial till often piles up alongside the edges of a glacier or at the end of one. These
piles of poorly sorted sediments are called
moraines.Terminal moraines
or
end moraines
occur at the end of a glacier as it recedes, melting and leaving the till behind.
Lateral mo-
raines
occur along the edges of a glacier.
Medial moraines
occur in between two glaciers
(where two lateral moraines come together). Each of these moraine types is illustrated
in Figure 13-4.
Figure 13-4:
Differ-
ent types of glacial
moraines.