Nipissing Great Lakes was a prehistoric proglacial lake. Parts of the former lake are now Lake Superior, Lake Huron, Georgian Bay and Lake Michigan. It formed about 7,500 years before present (YBP). The lake occupied the depression left by the Labradorian Glacier. This body of water drained eastward from Georgian Bay to the Ottawa valley. This was a period of isostatic rebound raising the outlet over time, until it opened the outlet through the St. Clair valley.