Yellowstone is covered with evidence of glaciers. About 22,000 to 14,000 years ago, during the most recent ice age, ice draped the region to a thickness of a kilometer—that’s over half a mile! When the ice melted, it left debris everywhere, mantling much of the surface of Yellowstone National Park. For example, Hayden Valley, located between Yellowstone Lake and the Grand Canyon of the Yellowstone, contains glacial and lake sediment associated with multiple ice ages.