For 1.6 million years, glaciers spread and retreated across Illinois, leaving layers of rock when they stalled and melted, while wind-blown dust from glaciers to the west added to each layer. After the last glacier retreated over 10,000 years ago, grasses and flowers began to grow, and some developed roots up to 15 feet deep to survive the region's dry summers. When these plants died, their roots, leaves, and stems formed a thick nutrient-rich layer atop the glacial deposits. The resulting soil under this prairie is some of the richest in North America.