Conneaut Lake is a kettle lake formed by a receding glacier during the last Ice Age about 12,000 years ago. As the glacier melted, a large block of ice partially embedded in accumulated sediment formed the depression that became the lake. Prehistoric Mastadon and Woolly Mammoth bones have been found preserved in layers of silt in and around the lake, providing evidence of their existence during this period. The glacier changed the water flow from north to south, making the lake part of the Mississippi River drainage system. Conneaut Lake is Pennsylvania's largest natural lake, and four smaller kettle lakes exist in the area.