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From: doug@escher.UUCP (Douglas J Freyburger)
Newsgroups: net.sci
Subject: Re: Geology (Great Lakes) Question
Message-ID: <45@escher.UUCP>
Date: Fri, 9-Aug-85 16:58:21 EDT
Article-I.D.: escher.45
Posted: Fri Aug  9 16:58:21 1985
Date-Received: Tue, 13-Aug-85 00:51:49 EDT
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Organization: NASA/JPL, Pasadena, CA
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> 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.

Sorry, but most of the Great Lakes are *very shallow*.  It
is hard to find a place in Erie, Superior, and Huron more
than about 100 feet deep, and most of Michigan is less than
200 feet deep.  Only Ontario is *very deep*, common several
of hundreds.  Given this shallowness, glacier dredging
seems reasonable.  On a clear day, I remember being able to
see the floor the Erie (in the 70s, but not the 60s).

> 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)?

Well, there used to be a mountain range there a VERY long
time ago.  The Catskills (sp?), the Boston Hills on the US
side, and something else in Western Ontario.  I might be
missing badly here, this is from my high school geography
course.  There is a nice ridge that runs long the southern
shore of lake Ontario, though.  It goes from somewhere in
New York or Vermont that extends through the start of
Niagara Falls' gorge.  There are different types of rock on
each side of this escarpment.  Was there a meeting an older
Canadian shield with an older American shield VERY long ago
to form that mountain range, and the escarpement that
exists there now?  If so, it would make the depth of lake
Ontario easier to explain.

Doug Freyburger		DOUG@JPL-VLSI, DOUG@JPL-ROBOTICS,
JPL 171-235		...escher!doug, doug@aerospace,
Pasadena, CA 91109	etc.