Formation of the Great Lakes–St. Lawrence Lowlands: Seas, Sediments, and Glaciers

The Great Lakes–St. Lawrence Lowland region formed through a sequence of geological events over hundreds of millions of years:

1. Ordovician Sea (485–444 Ma): A shallow tropical sea covered the region, depositing thick limestone, shale, and sandstone (now the bedrock).
2. Post-Ordovician Uplift & Erosion: The sea retreated; rivers carved valleys into the sedimentary strata.
3. Pre-Glacial Drainage: Ancestral rivers (e.g., Laurentian River) flowed through what became the lake basins.
4. Repeated Glaciations (Pleistocene, 2.6 Ma–11.7 ka): Continental ice sheets (Laurentide) advanced and retreated multiple times. Each advance scoured bedrock, deepened valleys, and deposited till. The last (Wisconsin) glaciation carved the current lake basins and the St. Lawrence River valley.
5. Glacial Melt & Lake Formation (14–10 ka): Retreating ice dammed outlets, creating massive proglacial lakes (Lake Algonquin, Lake Iroquois, Lake Vermont) that preceded the modern Great Lakes.
6. Isostatic Rebound (Ongoing): Crust depressed by ice weight is slowly rising (up to 30 cm/century), tilting lake outlets and changing drainage patterns.

The result is a flat-to-gently-rolling lowland of fertile glacial sediments, rich freshwater resources, and strategic waterways—core to North America's industrial heartland.