Mount Everts, as seen from Mammoth Hot Springs near the North Entrance of Yellowstone National Park. Near the top of Mount Everts is an unconformity—a geologic boundary where two rock types of very different ages are in direct contact, and where geological history is "missing." Here, Cretaceous sediments that are at least 65 million years old make up the gray strata at the bottom, while the light red rock that makes up the top of Mount Everts is Huckleberry Ridge Tuff that formed as a result of the first caldera-forming eruption of the Yellowstone volcanic system about 2.1 million years ago. The sediment was deposited on the margin of a shallow inland sea in a setting that might have looked a little like the delta of the Mississippi River today. After the sedimentary rock formed, it was uplifted by tectonic forces into the mountain it is today, and it gradually eroded until the eruption that deposited the volcanic ash on top. At the interface between the sediment and ash, the thin white later is ash that fell before the main Huckleberry Ridge Tuff ash flow, representing the opening phase of that eruption. The thin black layer is that portion of the ash flow that quenched, or froze, against the ground surface when the flow was emplaced. The reddish hue in the sediments was caused by the extreme temperature of the overlying ash, which "baked" the underlying sediment when it was deposited.