Why do they call Yellowstone Yellowstone?

The name "Yellowstone" is thought to have originated from the yellow rocks found in the Grand Canyon of the Yellowstone River. The rocks are a type of volcanic rock called rhyolite, which is rich in sulfur and iron. When these rocks are exposed to water, they oxidize and turn a bright yellow color. The first recorded use of the name "Yellowstone" was in 1805, when the Lewis and Clark Expedition passed through the area.