The Great Lakes look like ice lobes, and so they have always been explained as ice lobes.
The ice advances are a unique phenomenon, in that they totally erase all that has gone before. So, one can say that the Great Lakes were born only after the retreat of the last glaciation, simply because that's all we have!
But to apply a bit more intelligence to the problem, we have to conduct an Einsteinian thought experiment. What if the Great Lakes didn't give a hoot about glaciation? What if temperate glaciers can only do a 'wash and wax' to the land, and are not the massive steel bulldozers they are imagined to be?
Before glaciation, we had a hundred million years of everyday erosion, after the last great split of the continents. We can imagine what the land looked like before the first ice cube, and for that we take the geological map.
I'm simply going to take a huge bite out of every soft, erodible rock there is. Oh look! I get the Great Lakes!
Now, I find a bedrock topography map.
Oh look, the soft crap has been worn away, by 100 million years of erosion, and not the stupid ice sheets!Finally, if the Great Lakes were scooped out by giant ice bulldozers, then we would expect a great hunk of dirt specifically at the end of each lake, but that doesn't happen, the dirt is evenly spread, and it really isn't that much. Something like an inch or two polished off the top of the whole glaciated area.
Oh, and I found this picture doing this.
I love it! This is the stuff they are going to use for our nuclear waste up a Bruce!
2 comments:
Glad to see you are working on the Great Lakes problem and that you are discovering there is no simple solution–I came to the same conclusion. My knols are written using a deep glacial erosion (and thick icesheet) paradigm, which is fundamentally different from most glaciation models (most models assume relatively minor glacial erosion by thin icesheets, except they want the thin icesheets to erode the Great Lakes basins). The deep glacial erosion paradigm assumes the Canadian Shield was covered with significant thicknesses of sedimentary rock that glacial erosion removed to produce the modern day geologic map. If so, prior to glaciation the regional geologic map and landscape would have looked very different than it does now. The paradigm I am using also assumes rapid melting of a thick North American icesheet, comparable in size and thickness to the present day Antarctic icesheet. Based on this paradigm I suspect the Great Lakes basins were eroded by immense melt water floods (most models do not recognize immense melt water floods of the size my paradigm assumes). I also suspect these immense floods moved through northern outlets (such as through the North Bay area) before isostatic rebound lifted northern Ontario. But there are problems with this hypothesis (some of the lake basins are too deep) and I need to figure out ways to move water out of the deeper Great Lakes basins to the Atlantic Ocean (or Gulf of Mexico or Hudson Bay) and/or to erode the Great Lakes basins after the glaciation that eroded the Canadian Shield. I can make a strong case for a subsequent glaciation with thin icesheets, although at this time I do not see how a thin icesheet that produced very little erosion in the northern plains region (where the bedrock is easily eroded) would erode deep basins in the Great Lakes region (where bedrock is much more resistant to erosion). Any ideas you or your readers have would be appreciated.
There is nothing fancy about the formation of the Great Lakes, but I have a lot of trouble finding the stuff I know is out there. We did a heck of a lot of geophysics in the lakes, and it is all sucked up somewhere. The same with a lot of earthquake studies. I just lay it out and forget about it, hope for future generations. :)
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