Yellowstone National Park was shaped by many natural processes. The park is often called, the land of fire and ice and there is truth to that nickname. While the underlying geology of Yellowstone is rooted in volcanism, much of what we see today was shaped by glaciers.

Glaciers develop when more snow falls in winter than melts in summer. Over time, the bottom layers of snow become compressed into ice and begin to move due to the forces of gravity. While there is debate on their dates, this region has been covered by at least three major glaciations.

That most recent period, called the Pinedale Glaciation, was responsible for carving Yellowstone Lake into the shape we see today. Ice as thick as 4,000 feet once covered the area. It is believed that the Pinedale ended nearly 14,000 years ago. Volcanic explosions, earthquakes, wind and erosion also played a role in the lakes formation.

During the last glacial period, an ice dam blocked the flow of the Yellowstone River and as the water rose, Hayden Valley flooded and became part of Yellowstone Lake. When the valley drained, the force of the water helped form the Grand Canyon of the Yellowstone. Lake sediments that make up much of the soil in Hayden Valley are partly responsible for the open geography we see today.

When a glacier moves, the weight of the ice sculpts the landscape, often wiping away evidence of past glaciations. V-shaped valleys formed by water become U-shaped as the ice scours away the rock. 

On your next trip to Yellowstone watch for small lakes, or kettles that formed when chunks of ice were left behind by receding glaciers. Large boulders, called glacial erratics were also deposited throughout the park. One of the largest erratics can be seen along the North Rim of the Grand Canyon of the Yellowstone.

There are no glaciers in Yellowstone today, but a few remain just south of the park in Grand Teton National Park. Those formed during the Little Ice Age between 1400 and 1800 A.D.

As our climate changes, natural processes will continue to shape the Yellowstone of tomorrow. This truly is the land of fire and ice.

Video © 2008 Yellowstone Park Foundation