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From: ran@ho95e.UUCP (ran)
Newsgroups: net.sci
Subject: Geology (Great Lakes) Question
Message-ID: <150@ho95e.UUCP>
Date: Mon, 5-Aug-85 16:19:54 EDT
Article-I.D.: ho95e.150
Posted: Mon Aug  5 16:19:54 1985
Date-Received: Tue, 6-Aug-85 12:48:31 EDT
Distribution: net
Organization: AT&T Bell Labs, Holmdel NJ
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Does anybody out there know about the origin of the Great Lakes
(and have any newer theories come out lately, particularly
with regards to plate tectonics and hot spots)?

I remember learning that they were supposedly formed
(though I don't know whether that means "created" or "put into
their present form") during the Ice Ages.  However, I have trouble
understanding this.

If you look at them (in particular, Superior, Michigan, and Huron),
they are *very deep*.  For instance, Superior is 1300 feet deep.
With the surface at about 600 ft, it extends 700 feet below sea level.
Michigan and Huron do the same (but not by as much).  How did glaciers
manage to dig these lakes so deep?????  Other glacial lakes
(Manitoba, Great Slave) are relatively shallow.

I'm wondering if some plate tectonics might be involved here.  The only
other inland areas below sea level I can think of had tectonic causes
(Death Valley, Dead Sea, Caspian Sea, Loch Ness).  Might the Great Lakes
be the remnant of a failed rifting (which was much later covered by ice)?

I'm struck by the three way shape of the 3 lakes, which is typical
of riftings (see the Horn of Africa, for instance).  However, I know
nothing of the geology of the area (except that they sit where
the Canadian Shield pokes through).  Does the geology blatantly contradict
the above guess, or is it a possibility?

Thanks.