Glacial Lake Iroquois

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It has been suggested that Iroquois Shoreline be merged into this article or section. (Discuss)

This map of Lake Ontario shows the valley of the Mohawk River, through which Glacial Lake Algonquin crossed the mountains to the Hudson River Valley.

Stages of great lake development.

Glacial Lake Iroquois was a prehistoric proglacial lake that existed at the end of the last ice age approximately 13,000 years ago. The lake was essentially an enlargement of the present Lake Ontario that formed because the St. Lawrence River downstream from the lake was blocked by the ice sheet near the present Thousand Islands. The level of the lake was approximately 30 m (~100 ft) above the present level of Lake Ontario.

The lake drained to the southeast, through a channel passing near present day Rome, New York. The channel then followed the valley of the Mohawk River to the Hudson River.

The lake was fed by Early Lake Erie, as well as Glacial Lake Algonquin, an early partial manifestation of Lake Huron, that drained directly to Lake Iroquois across southern Ontario, along the southern edge of the ice sheet, bypassing Early Lake Erie.

The subsequent melting of the ice dam resulted in a sudden lowering of the lake to its present level, and setting off the Younger Dryas episode.

The prehistoric shoreline, marked by a ridge known as the Iroquois Shoreline, can be discerned in places around Lake Ontario.This can be seen, for instance, in Toronto parallelling Davenport Road near Spadina Avenue, and also nearby in Scarborough, Ontario, where the prehistoric shoreline takes the form of earthen cliffs at the modern lakeshore (called the Scarborough Bluffs).