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The Great Lakes: Michigan The Great Lakes contain the largest
supply of fresh water on earth; 20% of the earth's total fresh
water; 9,402 miles of shoreline; and 94,710 total square miles of
surface area (about the size of Texas). The Great Lakes basin is a
295,200 square mile area within which all surface area drains into
the Great Lakes. It includes parts of Illinois, Indiana, Michigan,
Minnesota, New York, Ohio, Pennsylvania, Wisconsin, Ontario and
Quebec.
Lake Michigan is the third largest of the great lakes but is
second in holding volume of fresh water! It is the third largest
volume of freshwater in the world. But how did these great lakes
come to be?
Approximately 15,000 to 20,000 years ago, the last of the major
continental glaciers moved south, scouring the land and adding mass
to the landscape. The combination of mass and scouring grooved out
the lobe known today as Lake Michigan. These glaciers brought
sediments from far north and deposited them in piles called
moraines. As the glacier started to melt water filled the basin
nearly 20 feet higher than the level today and make one large lake
as shown in the top map.
At the time settlers moved to Chicago, the original shoreline
was nearly 4 miles farther west than it is today. The water level
was nearly 4 feet higher than today and the shoreline was thousands
of acres of bog and swampland.
After the Great Chicago Fire in 1871, the downtown shoreline was
used to dispose of the debris. Further south, great ports were
being established for shipping. Swampland was again filled to allow
for deep water passages along shore. The Great Lakes are often
referred to as ‘freshwater oceans’ because of their
massive size.
Maritime weather patterns originate over these bodies of water.
These freshwater maritime storms have caused more ships to flounder
and sink than both ocean coastlines.
The Chicago shoreline was subjected to erosion due to icebergs
floating down from the northern areas of the lake. These icebergs
would scour the shoreline. Blocks of ice would grind along the
shore and drag shoreline material out into the lake. During severe
winters, it is not uncommon to see ice sheets 10 feet thick and
nearly 100 feet across. During warm weather, waves generated across
the lake would grow. The longer a wind blows from a single
direction, the higher the waves can pile up. This is called fetch.
Waves nearly 30 feet tall would often crash along shore bringing
the landfill out into the water deep.
During the Great Depression, Civil Conservation Corps projects
were started to:
1) protect the shoreline and
2) bring work to the thousands of starving out of work
families.
Breakers were erected off the shoreline along the south shores.
Sea Walls parallel to shore were erected as well to slow the
erosion. Finally, great limestone blocks brought up the Calumet
River from the south were set in place along the shoreline and
serve as the current coastline for much of the south shore
areas.
The water level continues to drop over time for several reasons.
No more glacier runoff is filling the basin. Large cities draw
water from the lake and discharge waste water into rivers leaving
the basin. The lake bed is rising from isostatic rebound. The
glacial pressure that compressed the lake bed is gone and the
tectonic plate is once again rising slowly.
To log this earthcache, please email the following answers:
1) estimate the current lake level using your GPS unit. If the
'official' lake level is 581 feet recorded in 1907, how much has
the lake level declined in the past 100 years?
2) Look at the sea walls out in the lake. What is located at each
end of the openings? Why do you suppose those have been
erected?
3) What is the elevation of the top row of shoreline blocks? When
weather systems come across the lake generating 20 foot waves,
would you be safe on this row of shoreline blocks?
4) Each block is approximately 10 feet tall, 10 feet wide and 10
feet deep. Volume is length x width x height. Calculate the volume
of each block.
5) To determine the mass, you multiply the volume by the density.
The density of limestone is 150 pounds per cubic feet. Take this
number and divide by 2000 to get the number of tons. Calculate how
much does each block weigh in tons.
6) Look south from your location. What ‘scenery’ is as
far as the eye can see? Why has that scenery developed there?
7) Upload a picture of your GPS unit (you included would be a
bonus!) with the ‘scenery’ due south in the
background.
Congrats to mqbmusic on the FTF