Glaciation
Water from glaciers melting 12,000 years ago formed ancestral Lake Ontario, which covered this entire area. A large ice lobe, roughly 20 metres thick, blocked the lake from draining eastward, leaving water levels high as the lake slowly drained south to what is now the Mississippi River. The ice lobe finally retreated, draining the lake to the St Lawrence River and forming the Great Lakes as we see them today.
Glaciation occurred when average annual temperatures were only 2 - 5 °C lower than present. The small increase to our current temperatures caused big changes to the landscape. Increases in average annual temperatures now may seem small, but they could cause major changes to the natural environment in the near future.
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