At the western end of Lake Ontario, the Dundas valley, a major re-entrant in the Niagara Escarptment, was formed by river dissection of a more highly fractured zone at the crest of an anticline whose strike is nearly that of the present valley. The culmination of this erosion was the formation of the awesome Dundas gorge, the upper part of which was later modified by glaciation.

A continental ice sheet deposited drift in the valley and scoured the bedrock above the escarpment. The receding Port Huron lobe formed stratified drift at the head of the valley and a moraine system in a semi-circular pattern around the re-entrant. As these features were forming, they were reworked by Lake Warren, a large proglacial lake in the Lake Erie basin, which remained west of the valley except for a stage of high-level beach development. Further ice retreat caused the water to lower and become separated by the ice-contact ridges and moraines, the western section lowering rapidly, and the section between these deposits and the ice front forming a series of proglacial lakes. Three stages are recorded, each change in water level caused by the exposure of a new outlet as ice retrated from the Lake Ontario basin: a continuously lowering Hyper-Lake Iroquois stage, a continuously rising Lake Iroquois stage, and a rising Lake Ontario stage.

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