We process how ALL of these Great Lakes are freshwater glaciers that melted where they landed 10 to 12.000 years ago What immense NATURAL climate change could have caused not only the original freezing but the eventual melt?
Indonesian volcano Tobu which erupted some 70.000 years ago and thrust Earth into a multi thousand year Ice Age almost wiping out humankind. A more current example of this was Tambora, which erupted in 1815 and caused Year without a summer in 1816, decades before first smokestacks which appeared around 1870. Look it up…
https://www.visualcapitalist.com/mapped-the-maximum-depth-of-the-great-lakes