Lake Warren was a proglacial lake that formed about 12,700 years before present in the Lake Erie basin after Lake Whittlesey fell and an advancing ice front blocked Lake Wayne’s lower outlet, raising water levels and trapping meltwater more effectively. It spread across much of the southern Lake Erie basin and into parts of the Lake Huron and Lake Ontario basins, draining through the Grand River to Lake Chicago, though an earlier stage used the Mohawk valley. Geologists identify three lake stages by elevation. Its importance endures in ancient beaches, ridges, deltas, and tilted shorelines that reveal changing ice margins, land rebound, and drainage at the end of the last glaciation.