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Now that we have seen how moraines
evolve at or near the edges of active glaciers where melting
and ice creation is balanced, let's look at a way in which landforms
develop as ice melts and retreats.
Picture your driveway during spring thaw. Water trickles through
the channels and crevices in built-up layers of snow and ice.
They slowly fill with a winter's worth of gravel and dirt from
your car. When the ice disappears, your driveway is covered with
tiny raised rivulets of dirt that are washed away with the first
spring shower.
Now picture the same thing happening in ice that is 500 metres
thick, with inner channels and tunnels as big as the Fraser or
Red rivers.
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An esker in north-central
Keewatin, N.W.T. It is part of an esker system that is several
hundred kilometres long.
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Just as rivers on land carry and deposit sediment, melt water that flows in the openings
beneath, above and within a glacier also carries and deposits
sediment. Tunnels near the base of retreating glaciers fill with
transported sediments, which remain as sandy or gravelly ridges
that look like upside-down stream beds after the glacier melts
away. The ice that formed the sides and roof of the tunnel subsequently
disappears, leaving behind sand and gravel deposits in ridges
with long and sinuous shapes. Eskers can be 500 to 600 kilometres
long and, depending on the pattern of the glacier's inner tunnels,
can interconnect in a pattern of central ridges and tributaries,
just like a branching river system. |
| Since eskers are made up of highly porous sand
and gravel, they are frequently excavated for construction. They
are considered an endangered geomorphological species in southern
Quebec since they have been used either to develop roadways --
offering natural elevated, dry terrain -- or they have been ripped
up for the gravel to build nearby roads. For centuries in northern
Canada, Inuit and wildlife have typically used eskers for high
and dry travel routes. More recently, eskers have been used in
the hunt for diamonds. Since they lie in the direction of glacial
flow, prospectors have used eskers to trace where minerals glacially
eroded from diamond-bearing formations have been transported.
They trace these "indicator" minerals "upstream"
in an esker until they abruptly disappear: this indicates the
diamond source is nearby. |
| The most extensive esker formations in the world
are found in the Districts of Keewatin in Nunavut, and Mackenzie
in the Northwest Territories, in Manitoba, northern Quebec and
Labrador. These regions were most affected by Canada's two major
ice sheets -- the Keewatin and Labrador portions of the Laurentide
Ice Sheet -- that melted away from eastern and western Hudson
Bay about 6,500 years ago. In fact, eskers and their tributaries radiate outward from the centres
of these two main glacial masses like the spokes of a wheel.
Esker networks can also be found in Quebec's Appalachian region,
as well as near Brampton, Ont., and throughout northern Ontario. |
An esker and kettle lake near the Manitoba, N.W.T. border.
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