Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

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.

More evidence of the glacial history of the region is in the “erratics” that can be found especially in the northern half Yellowstone National Park. Erratics are rocks and boulders that were eroded from mountains, then transported by glaciers (sometimes over very long distances), and finally dropped on the land surface as the ice melted. These erratics can be huge. One such boulder, near Inspiration Point on the north rim of the Grand Canyon of the Yellowstone, and measures 7.3×6.1×5.5 meters (24×20×18 feet) in size and is estimated to weigh over 500 tons (450,000 kg).

This huge boulder was dropped by a retreating glacier on the north rim of the Grand Canyon of the Yellowstone in Yellowstone National Park, and it is a testament to Yellowstone's icy past/USGS, Mike Poland, May 29, 2019.

Another landform that is evidence of past glaciation is called a kame. Kames are created when sand and gravel accumulate in low spots within a glacier, and also sometimes along a glacier’s front and edges. When the ice melts, the sediment is dumped on the ground, forming a small hill—a kame—in an area that might otherwise be flat.

Kames can be unstable since they are usually composed of loose debris. When the ice melts, support is removed, and they sometimes collapse, spreading debris across the landscape.

At Yellowstone, there is a twist on the typical kame. Even though ice covered the region during multiple ice ages, there were still active thermal areas beneath the ice in places like Norris Geyser Basin and the Upper, Midway, and Lower Geyser Basins along the Firehole River. Because of the warm ground, the glaciers were a bit thinner in these areas, forming depressions along their surfaces as the thermal areas melted the ice from beneath. Rocky debris accumulated in these depressions, and when the ice melted kames were left behind as the debris piled up on the land.

Because these kames formed in thermal areas, hot water that rises from depth and that is enriched in silica leached from surrounding rocks circulated through the glacial debris, depositing silica and cementing the debris together. These “thermal kames” are unique to the Yellowstone region.

You can find thermal kames in many of the major thermal areas of Yellowstone National Park. In Norris Geyser Basin, the Ragged Hills, in the center of the basin, are an exceptional example of a thermal kame. And there are numerous examples along the geyser basins of the Firehole River, including Porcupine Hills and Twin Buttes in Lower Geyser Basin. The kames are poorly sorted, with material ranging from sand-sized particles to massive boulders, many of which were picked up locally from Yellowstone’s thick rhyolite lava flows.

Photograph of Twin Buttes in Lower Geyser Basin looking to the northwest, with Excelsior Geyser of Midway Geyser Basin in the foreground. Twin Buttes is a thermal kame that formed when glaciers covered the area, and hydrothermal activity below the ice led to melting and the deposition and cementation of glacial sediments. A large hydrothermal explosion occurred in the area following glacial retreat. Excelsior Geyser, the large steaming spring in the foreground, was formed by a series of small hydrothermal explosions in the late 1800s/USGS, Lauren Harrison, July 27, 2021, under permit YELL–2021–SCI–8158.

Because the thermal kames of Yellowstone are cemented by hydrothermal activity, they can be quite tall. The Ragged Hills rise 30 m (100 feet) above their surroundings in Norris Geyser Basin. Twin Buttes is even taller—100 m (328 feet) above the rest of Lower Geyser Basin!

Even though they are hydrothermally cemented, thermal kames can still collapse. There is evidence that Twin Buttes fell apart at some point after the ice melted at least 14,000 years ago. The collapse, which might have been triggered by an earthquake, may have suddenly and catastrophically lowered the pressure on the hot-water plumbing system beneath the buttes, causing a large hydrothermal explosion.

Thermal kames are just one of numerous outstanding geologic features in Yellowstone—an amazing wonderland where you can also find incredible animal and microbial life forms, exceptional landscapes, and the densest concentrations of geysers in the world, and where even a small unassuming hill has a fantastic story to tell!