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Hayden Valley is a gorgeous expanse of grassland and meadows located right in the center of Yellowstone National Park.  It is a haven for wildlife and a popular spot for viewing some of Yellowstone’s most iconic animals.  But why does this meadow exist in the midst of what is otherwise a high-altitude forest of lodgepole pine trees?  The area’s geology holds the key

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

While Old Faithful might be the most visited individual site and the most famous hydrothermal feature in Yellowstone National Park, no less famous a locale is Hayden Valley, situated between the north shore of Yellowstone Lake and the Grand Canyon of the Yellowstone.  The valley is home to an incredible diversity of animal life, including bears, wolves, and huge herds of bison that graze on the tall grasses and rest on the banks of the meandering Yellowstone River.

Violet Creek in Hayden Valley, Yellowstone National Park
A small stream, Violet Creek, winds its way through Hayden Valley in Yellowstone National Park.

The valley, named for Ferdinand V. Hayden, who led an expedition to the region in 1871, seems an oddity in Yellowstone.  While much of the park is occupied by forests of sturdy lodgepole pine trees, Hayden Valley is nearly devoid of trees except for a few isolated “islands” here and there.  This condition reflects the geology of the valley.

Yellowstone has been impacted by two ice ages in the past 150,000 years.  As the older of these—the “Bull Lake” glaciation—gradually ended, the receding ice deposited fine-grained sediment in the area of Hayden Valley.  An ice dam also formed across the Yellowstone River just to the south of the current location of Canyon Village.  This caused the river to back up and flood the valley floor with what is known now as ancestral Yellowstone Lake, a body of water much larger than today’s Yellowstone Lake.  The lake probably existed for several thousand years before around 100,000 years ago, since there is evidence that the Hayden Valley rhyolite lava flow, which formed at about that time, interacted with lake water during its eruption.

The presence of the lake means that fine-grained sediment accumulated on the lake floor.  But the story doesn’t end there.  A younger ice age—the “Pinedale” glaciation—occurred about 22,000 to 13,000 years ago.  As the Pinedale ice receded, more glacial sediment was dropped in Hayden Valley, creating a geologic layer cake of fine-grained glacial sediment atop fine-grained lake sediment atop even more fine-grained glacial sediment.

These geological conditions are ideal for grasses, which can outcompete trees for water in fine-grained soils.  Elsewhere in Yellowstone National Park, loose soil composed mostly of rhyolite lava flows is dominated by lodgepole pine communities, which thrive in the nutrient-poor environments and are resistant to drought.

The Hayden Valley rhyolite pokes out of Hayden Valley, to the west of the Grand Loop road.  The lava flow is easily identifiable due to the trees growing on its surface.  In fact, Hayden Valley itself is surrounded on all sides by tree-covered rhyolite flows.

Yellowstone is a geological wonderland, and that geology controls the flora and fauna that visitors enjoy today.  The geologic conditions are shaped not just by volcanic eruptions, like the famous large explosions and lesser-known lava flows, but also glaciers and lakes.  Every landscape in the park has a story to tell! 

Panorama of Hayden Valley, Yellowstone National Park
Panoramic photo of Hayden Valley looking east toward the Yellowstone River from the Mary Mountain trail.

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