Between 10,000 and 12,000 years ago, glaciers of the Wisconsin glaciation filled the valley. Otsego Lake was formed when a tongue of the glacier scoured out the Susquehanna River Valley. As the glaciers melted slowly, they filled in the valley they carved out. The lake takes its name from the Iroquois Indians, who inhabited the area around the lake in and before the 17th century. The name "Otsego" is from a Mohawk or Oneida word meaning "place of the rock", referring to the large boulder near the lake's outlet, today known as Council Rock.