More than 10,000 years ago, enormous rivers of ice covered this land, gouging the earth like plows and leaving piles of sand and gravel called moraines. As the glaciers thawed, their deep tracks became large lakes of meltwater. One of these, Lake Nipissing, gradually became Lake Michigan as its water level fell. Meanwhile, waves eroded beaches and bluffs, and currents carried sediment along the shore to form sandbars. By about 2,000 years ago, these currents had deposited enough material on either side of an island now known as Alligator Hill to landlock Glen Lake. Over time, grasses and trees took root, and today this forested sandbar is where the town of Glen Arbor stands beside a beach of ancient sand.