A Fifth of the World's Fresh Water

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

None of the five Great Lakes formed the way a lake usually does, by a river filling a natural basin. All five are the direct product of continental glaciation: over the last roughly two and a half million years, repeated ice ages sent glaciers a mile or more thick grinding south across the region, gouging out and deepening basins that had already been weakened by older river valleys and softer rock, then damming the resulting depressions with glacial debris as the ice retreated for the last time, roughly fourteen thousand years ago. That single shared origin explains why the five lakes, despite ranging enormously in size and depth, from Lake Superior largest and deepest to the much shallower Lake Erie, sit in a connected chain rather than as unrelated bodies of water, each basin the product of the same ice sheet working the same regional geology at a slightly different point in its retreat. The result is water in a scale almost impossible to picture: the five lakes together hold roughly a fifth of the world's surface fresh water, more than enough, if spread evenly, to cover the entire continental United States to a depth of about ten feet.