The park’s geologic chronology spans much of the earth’s history. Surrounding the Yellowstone caldera are stories of more ancient times that yield remarkable geologic treasures. The oldest rocks revealed in Yellowstone date back 2.7 billion years. These rocks are found in the northern mountains of the park and represent the very foundation of North America. Later, 500 million years ago, Yellowstone was a far different place than it is today. Covered by shallow inland seas, ocean sediments built up layer upon layer to form the common sedimentary rocks found in the park—lime-stone, sandstone, and shale. And the story continues with the latest deposits of travertine on the terraces of Mammoth Hot Springs. Standing in one place in Mammoth, a visitor can see some of the oldest and newest rocks on earth at the same time. Between the time of ancient seas and the caldera-forming volcanoes of the recent past, a great period of mountain building began as the North American Plate collided with the Pacific Plate 100 to 50 million years ago. A time of tremendous upheaval, this powerful tectonic activity folded, faulted, and compressed the earth, leading to the uplift and creation of the Rocky Mountain chain. In this unstable landscape, even more ancient volcanoes arose about 50 million years ago to form the Absaroka and Washburn mountains. Lying across Yellowstone Lake and bounding the park’s east side, the Absarokas are an imposing mountain range that formed from erupting volcanoes during a 15-million-year period. To d a y, they provide a wonderful backdrop to the waters of Yellowstone Lake. At the time of their creation, they ejected silica-rich lava and ash, which mixed with water to form mudflows. These mudflows surrounded redwoods, sycamores, magnolias, dogwoods, and other trees, preserving the world’s largest petrified forest as a record of an earlier climatic period. To d a y, these forests of stone can best be seen on Specimen Ridge near Lamar Valley.
