Glaciation

Glaciation

This is an example of scrape marks made as the glaciers moved south. The view is
looking south. Notice not all the scrapes are in precisely the same direction.
The different advances of the glaciers came at slightly different angles.

This is an example of "chatter" caused by glaciation. The glacier moved up in
the picture (south) pushing a rock over the bedrock of the island. The bouncing or
jerking of the rock caused the marks.
There is a quarter in the upper right of the picture to help one get a sense of scale.

You may have encountered the phenomena of "chatter" when writing on the blackboard.
If the chalk is held the wrong way, it can do a kind of skip and bounce across the board.
Chatter is also a continuing concern in machine design. The elastic interaction of the
cutting tool on the material being cut can produce chatter which may result in tool
breakage and damage to the part being machined.

As a side note, it is worth noting that the south side of the island has quite a bit of
loose rock in the water. We tried to build a harbor there by making a crib out of logs
and filling it with rocks in about 1963, or well before the harbor was blasted.
The ice took it out. There is also a lot of loose rock just outside of the harbor but all of
that is a result of dynamiting the harbor. It could be that the rocks on the south side
remained from the glaciation because the island sheltered them. It would be interesting
to survey other islands in McGregor Bay to see if they also had a preponderance
of loose rock on the south sides.

Isostatic Rebound
All the land in this area is rising at a rate of abut 1 foot per century relative to the
water in lake Huron due to Isostatic rebound from the weight of the glacial ice
melting off the area. This is an important fact to consider when tracing the routes of
the voyagurs who passed this way from the mid 1600’s to the mid 1800’s because
the water around the island was about 4 feet deeper in the 1600's then in the 2000's.
When looking for canoe routes that is a lot. I explored a Voyageur route along the
French River with Dave east of Killarnie. It did not look too promising now but
a couple of feet more water would have made it quite attractive.

This rebound is from recorder history ('tidal gages'), but is also seen
in elevated beaches. There is some indication that the rate of rebound
is slowing down, that it was greater shortly after the ice receded.

Some alternate sources of glaciation information is:
http://en.wikipedia.org/wiki/Post-glacial_rebound

A report by the Natural Resources Canada shows the rebound
around Little Current to be 27 cm (10.6 In) /Century

The title of the report is Apparent Vertical Movement
Over the Great Lakes - Revisited and it is dated November 2001
I can no longer find it on the web.

http://igs.indiana.edu/geology/ancient/freezeframe/index.cfm
Its principal emphasis is further south but it has good info.

http://culter.colorado.edu/~saelias/glacier.html
has information about what causes Ice Ages.

The Glacial Lakes around Michigan by William R. Farrand, University of Michigan,
Published in 1988 by the MI Department of Environmental Quality, Geological Survey
Division. This is a good overview of the work of the glaciers in forming the
Great Lakes.

Rev 8/6/12