Admiralty Lake was a proglacial lake

Proglacial lake

In geology, a proglacial lake is a lake formed either by the damming action of a moraine or ice dam during the retreat of a melting glacier, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around the ice...

 in the basin of what is now Lake Ontario

Lake Ontario

Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north and southwest by the Canadian province of Ontario, and on the south by the American state of New York. Ontario, Canada's most populous province, was named for the lake. In the Wyandot language, ontarío means...

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The shoreline of Admiralty Lake was about 20 metres (65.6 ft) lower than Lake Ontario. The shoreline of Glacial Lake Iroquois

Glacial Lake Iroquois

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...

, an earlier proglacial lake was much higher than Lake Ontario's, because a lobe of the Laurentian Glacier blocked what is now the valley of the St Lawrence River. Lake Iroquois drained over the Niagara escarpment

Niagara Escarpment

The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs westward from New York State, through Ontario, Michigan, Wisconsin and Illinois...

, and down the Mohawk River

Mohawk River

The Mohawk River is a river in the U.S. state of New York. It is the largest tributary of the Hudson River. The Mohawk flows into the Hudson in the Capital District, a few miles north of the city of Albany. The river is named for the Mohawk Nation of the Iroquois Confederacy...

. When the lobe of the glacier retreated the weight of the glacier kept the outlet of the St Lawrence River lower than the current level. As the glacier continued to retreat the region of the Thousand Islands

Thousand Islands

The Thousand Islands is the name of an archipelago of islands that straddle the Canada-U.S. border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario, the...

rebounded, and the lake filled to its current level.