14000 years ago the entire Great Lakes lowlands were under a glacier that began to recede at the end of the Ice Age

   As the glacier receded, it left indentations in the ground that began to fill up with the water melting off of the glacier

   As the glacier continues to recede more indentations are made and filled and start to become visible as the Great Lakes of today

   The glacier has fully receded and  the Great Lakes are completely filled and are completely recognizable as the current Great Lakes.