A trail leads to a large boulder on top of a hill
Much of Yellowstone was shaped by glaciers. Glacial erratics are scattered across Yellowstone’s landscape.

NPS/Diane Renkin

Locations of ice over Yellowstone National park and the surrounding mountains
Light shaded areas bounded by black and red lines indicate the areas covered during the Pinedale and Bull Lake glaciations, respectively. Blue lines are contours in thousands of feet on the reconstructed Pinedale glacier surface. Black dashes with double-pointed arrows indicate main ice divides.

Modified by J. Licciardi in December 2015 from Licciardi and Pierce (2008)

Glaciers result when, for a period of years, more snow falls in an area than melts. Once the snow reaches a certain depth, it turns into ice and begins to move under the force of gravity or the pressure of its own weight. During this movement, rocks are picked up and carried in the ice, and these rocks grind Earth’s surface. Ice and water erode and transport earth materials as well as rocks and sediments. Glaciers also deposit materials. Large U-shaped valleys, ridges of debris (moraines), and out-of-place boulders (erratics) are evidence of a glacier’s passing.

Yellowstone and much of North America have experienced numerous periods of glaciation during the last 2.6 million years. Succeeding periods of glaciation have destroyed most surface evidence of previous glacial periods, but scientists have found evidence of them in sediment cores taken on land and in the ocean. In Yellowstone, a glacial deposit near Tower Fall dates back 1.3 million years. Evidence of such ancient glaciers is rare.

The Bull Lake glaciers covered the region about 151,000 to 160,000 years ago. Evidence exists that this glacial episode extended farther south and west of Yellowstone than the subsequent Pinedale Glaciation, but little surface evidence of it is found to the north and east. This indicates that the Pinedale Glaciation covered or eroded surface evidence of Bull Lake Glaciation in these areas.

The Yellowstone region’s last major glaciation, the Pinedale, is the most studied. Its beginning has been hard to pin down because field evidence is missing or inconclusive and dating techniques are inadequate. Ages of Pinedale maximum vary around the Yellowstone Ice Cap from 21,000 years ago on the east to 20,000 years ago on the north and possibly as young as 15,000–16,000 years ago on the south. Most of the Yellowstone Plateau was ice free between 13,000 to 14,000 years ago.

During the Pinedale glaciation, glaciers advanced and retreated from the Beartooth Plateau, altering the present-day northern range and other grassy landscapes. During glacial retreat, water flowed differently over Hayden Valley and deposited various sediments. Glacial dams also backed up water over Lamar Valley; when the dams lifted, catastrophic floods helped to form the modern landscape around the North Entrance of the park.

During the Pinedale’s peak, nearly all of Yellowstone was covered by an ice cap up to 4,000 feet thick. Mount Washburn was completely covered by ice. This ice cap was not part of the continental ice sheet extending south from Canada. The ice cap occurred here, in part, because Yellowstone’s higher elevation volcanic plateau allowed snow to accumulate.

Map of Yellowstone National Park with numbered markers around Grand Loop Road
Map of Yellowstone National Park with stops.


Where to See Evidence of Glaciers
  1. Mammoth Hot Springs: Thermal kames exist in this area. Mount Everts and Bunsen Peak were both overridden by ice. The top of Sepulcher peeked above the Pinedale ice
  2. Swan Lake Flat: These meadows and wetlands formed after the glaciers retreated. Electric Peak, to the northwest, was also carved by glaciers.
  3. Norris Geyser Basin: Several high peaks in the Gallatin Mountains were above the Pinedale ice cap.
  4. Madison Valley, west of Seven Mile Bridge: Glacial moraines, glacial outwash, and recent Madison River deposits can be seen.
  5. Fountain Flats: The Porcupine Hills and Twin Buttes are thermal kames.
  6. Upper Geyser Basin: Volcanic rocks and ice-water contact deposits. Deposits underlying this and other area geyser basins store the water necessary for geysers to occur and allow the water to percolate up from depth.
  7. Hayden Valley: The valley is covered with glacial till left from the most recent glacial retreat. It also has a variety of glacial and ice-water contact deposits. This glacial till contains many different grain sizes, including clay and a thin layer of lake sediments that do not allow water to percolate quickly into the ground. Thus, Hayden Valley is marshy.
  8. Tower Fall area: North of Tower Fall, sediments between layers of basalts may show evidence of the oldest known glaciation in Yellowstone. Plus, huge boulders (known as erratics) from the last major glaciation rest atop the youngest basalt.
  9. Lamar Valley: Huge boulders and ponds between the Lamar and Yellowstone rivers were left by melting glaciers, as were several moraines. Other ponds are within post glacial landslides.

More Information


Liccardi, J.M., and Pierce, K.L. 2008. Cosmogenic exposure-age chronologies of Pinedale and Bull Lake glaciations in greater Yellowstone and the Teton Range, USA. Quaternary Science Reviews 27: 817–831.

Pierce, K.L. 1979. History and dynamics of glaciation in the Northern Yellowstone National Park Area. US Geological Survey Professional Paper 729–F.

Pierce, K.L. 2004. Pleistocene glaciations of the Rocky Mountains in Developments in Quaternary Science, J. Rose (ed). 1: 63–76. Elsevier Press.

Last updated: August 20, 2019

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