Yellowstone is a land of contrasts and extremes. Just as the internal fires of the earth bring boiling water to the surface as geysers and hot springs, the park’s high elevation and northern latitude also make it a land of deep snows and long winters. When more snow falls in winter than can melt in summer, ice begins to form under the weight of the snow and eventually begins to flow as a glacier. Though there are no active glaciers in Yellowstone today, such conditions have occurred here intermittently during the last two million years. 

Like any good sculptor working in stone, these giant glaciers left their imprint on Yellowstone in many ways, both subtle and harsh. The region’s most recent period of glaciation began about 50,000 years ago in the high mountains of the Absaroka Beartooth Wilderness, northeast of Yellowstone. With time, vast sheets of ice, thousands of feet thick, flowed from the mountains to converge over Yellowstone Lake, covering the Yellowstone Plateau and virtually all of the park and surrounding area. While thermal basins continued to seethe beneath the ice, this land of fire and brimstone was in a deep freeze for thousands of years. At the peak of this glacial era, roughly 25,000 years ago, prominent peaks like Mount Sheridan lay hidden underneath this icy blanket, while the tip of Mount Washburn and the thin ridgeline of the Absaroka Mountains barely peeked above this unrelenting sea of ice. For thousands of years, ice flowed in all directions from this immense ice field, carving, scouring, and sculpting the land. 

As this ice age slowly ended nearly 15,000 years ago, it left behind ample evidence of the trans-forming power of ice. Among the broad hills and benches of Hayden Va l l e y, lake sediments of silt, sand, and gravel, covered in glacial till, remain from a time when the valley was covered by an ancient lake formed by an early ice dam. Large river valleys like the Firehole, Madison, and Lamar were broadened and scoured by accompanying rivers of ice. Retreating glaciers and their meltwa-ters gradually dropped their load of rock debris. Having carried massive stones from high, far off mountains like the Beartooths, the ice melted and left behind fields dotted with large granite boul-ders, called glacial erratics, at places where granite is not found, like Canyon’s Inspiration Point and near Lamar Va l l e y. Glacial ponds, striated hillsides, chiseled peaks, and polished mountain faces, all fashioned by the hand of ice, create some of the finishing touches on the spectacular landscape we see today.