About 15,000 years ago, waves lapped against the upper reaches of Mill Bluff, which stood as an island in Glacial Lake Wisconsin, while this place lay on the bottom of the former lake. The lake formed when glacial ice plugged an ancient river at Wisconsin Dells and prevented meltwaters from draining south. It deposited deep layers of sand now known as the Central Sands of Wisconsin. At its largest extent, Glacial Lake Wisconsin was 72 miles long and up to 150 feet deep.