Re: How exactly did glacial ice dig such deep and large lakes?

Each lake has its own unique history, but here's the general story:

The Great lakes formed not just by the gouging action of ice, but also from 
more subtle effects of glaciation such as crustal depression and flexure.

To start, the Great Lakes were formed as glaciers retreated northward from 
their maximum extent 12,000 or so years ago.  Large glaciers such as the 
ice sheet that covered Canada and northern North America contain immense 
volumes and weights of ice and they actually cause the lithosphere (the 
stiff outer shell of the earth) to flex and to sink downward.  This 
down flexing occurs over an area bigger than the glaciers causing it.  This 
results in melt water accumulating in a depresssed area right in front of 
the glacier, forming large lakes.  These lakes can be made even bigger if 
ice dams up their natural drainages. In some parts of the world, incredible 
floods resulted from the breaking of those ice dams (the channeled 
scablands of Washington state, which formed when glacial Lake Missoula 
burst its ice dam, are the best known example).  At the web site below, a 
short animation shows how the lakes changed as the glacier retreated:
 http://www.seagrant.wisc.edu/communications/greatlakes/GlacialGift/

Even after the ice melts completely away from an area, it will remain 
depressed for many thousands of years becuase the lithosphere is stiff and 
only bounces back very slowly.  The modern Great Lakes area is still 
rebounding from its glacial load and so it is still lower than surrounding 
areas, leading water to drain into the depression.

The Great Lakes also lie at the boundary of old geologic provinces, with 
the hard rocks of the Canadian Shield to the north, the much softer rocks 
of the Appalachian/Alleghanian Basin to the south and the complex rocks of 
the Mid-Continent rift and the iron range to the west.  The differences in 
hardness led the glacial ice to preferentially gougue out certain areas, 
leaving deeper depressions there.  Not all of the Great Lakes are as deep 
as L. Michigan, either.  L. Erie is quite shallow, by comparison. To learn 
the details of each Great Lake, you will need to do some more research, 
perhaps at the web sites of state geological surveys.

The great Lakes are not unique, however.  There are major glacially-related 
lakes all over the glaciated parts of the world.  Even the Great Salt Lake, 
now in a desert, is the remains of a much larger glacial-age lake (L. 
Bonneville).  Others are forming and growing today.  See this news story 
about problems in Nepal due to lake growth:
 http://ens.lycos.com/ens/jun99/1999L-06-25-01.html

I hope this helps answer your question,
David Smith, Geology and Environmental Science
La Salle University, Philadelphia, PA