Randall Carlson

If you look at a map, you can see Lake Superior on there.

Lake Superior was...

was one of the major lobes of what is called the Laurentide Ice Sheet.

There was two great ice sheets over North America, separate from Greenland.

Greenland was kind of its own ice sheet, but the Laurentide was about the size of the ice sheet that now covers the South Pole.

Its center was up by Hudson Bay, and it reached all the way to the flanks of the

Rocky Mountains in Western Canada.

Then over Rocky Mountains in Western Canada, you had a secondary ice sheet called the Cordieran.

The Cordieran then reached from like Northern Idaho and Washington all the way up to Alaska.

It was about the size of the Greenland ice sheet today.

So you had basically two separate ice sheets in North America, Laurentide.

See, you've just asked one of the fundamental unresolved mysteries, unanswered questions.

I'll do my best.

There are many theories as to what happened, and a lot of the older theories that were pretty much accepted.

have had to be abandoned or drastically modified because of the time scale involved.

If we go back to like when I was a kid, we're talking 50s and 60s, the idea was that it took

tens of thousands of years, probably more like 100,000 years for the ice to appear, to grow, and then to disappear.

And it's based upon what are called the Milankovitch forces because the relationship between the earth and the sun is constantly undergoing these subtle changes.

For example, the earth's orbit around the sun is elliptical.

It's not circular.