#1-18,000 years ago
1 of 11The Ottawa River as a Glacial Spillway
The Laurentide ice sheet started to retreat about 22,000 years ago. I chose a view of 18,000 years ago to explain some of the processes that are caused by the weight of multiple kilometre thicknesses of ice over 90,000 years. Both of the following processes are explained in Wikipedia
  1. Eustacy means that for 84,000 years this ice stored all the precipitation that fell on it lowering the Ocean's level over 100 metres.
  2. Isostacy means
    • that the weight of that ice warped the land's surfaces (i.e. it pushed the land down about 200 metres around central Ontario) when and while it covered it
    • and the same land slowly rebounded raised when the retreating ice cap uncovered it (this has been measured and is recorded.) Actually it is still rebounding around Central Ontario and nobody knows when it will stop!
  3. Newton's law (equal and opposite reaction) applied. Down near Windsor/Detroit the land was raised and it has been sinking since the ice sheet retreated! That is why Lake Erie drained into Lake Huron and out the Mattawa/Ottawa valley from 7,600 to 4,700 years ago. After that the waters started to drain over Niagara and out the Saint Lawrence.

Click for #2-12,000 years ago