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Monster Scouring Pads
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In the time of the glaciers, huge masses of ice formed over
the northern half of the North American continent. The massive
pressure caused the ice particles at the bottom of the ice mass
to become plastic and flow. The weight of the accumulated ice
also scraped loose rock particles and boulders, which acted like
the grit in sandpaper on the surface of the land. Any sediment
over top of the Canadian Shield rocks was scraped away and moved
far to the south, where it was left behind when the glaciers
withdrew.
The huge weight of these masses of ice pushed down the earth's
crust, which is still rebounding slowly back. Oddly enough, the
north shores of the Great Lakes are
rising faster than the south shores, so ancient shorelines of
the same age are more elevated from the lakes' surface on the
north shore than on the south shore.
When the glaciers melted, they left huge accumulations of
gravel behind in formations with names like moraine, esker and
drumlin. Channels mainly carved by glacial action are U-shaped;
valleys eroded by running water tend to be more V-shaped.
Kettle lakes are also remnants of glacial activity.
The weight of the glaciers depressed the Saint Lawrence
and Ottawa River valleys in eastern Ontario enough so that,
for a time, an arm of the ocean (Champlain Sea) flooded these areas.
Where did all the ice come from?
At the same time as the glaciers were advancing, the oceans dropped
world-wide by 100m (330 ft) which accounts for a lot of water and
would represent an even larger volume of ice. Glaciers advance
when winters with heavy
snow accumulation are followed by cool
summers. Since snow and ice reflect more of the sun's heat than bare
soil, a large glacier can modify the local weather patterns to its
advantage.
What difference does all this make?
Glaciers:
- deposited the soil
in the North American grain belt, the
midwest.
- drastically reshaped the continental surface where they passed.
- established fresh water sources and drainage patterns.
For example, the floods in the watershed of the Red River
are so extensive because the floodplain of this river is the bottom of an ancient glacial lake.
- will most likely return (though probably not in the next week or
two). Currently, glaciers cover about 2% of North America.
Global warming appears to have caused a rapid decline in glaciers throughout the world. Glacier National Park in the United States may be glacier-free by 2030.
Glacier Links
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