What evidence of glaciation exists in the Great Lakes region?

The Great Lakes Basin was once covered by the ice of continental glaciers. About 15,000 years ago the last ice melted to expose the lake basin. There have been minor advances and retreats of glaciers since then causing the level of the water in the lakes to rise and fall. How do scientists determine these past lake levels? Use mapping skills to identify the evidence of ancient beach ridges and  become aware of the uses of ancient beach ridges today.