Straddling the Arizona-Utah state line, Monument Valley Navajo Tribal Park, is known by the Navajo as Tse’Bii’Ndzisgaii. Natural forces of wind and water that eroded the land spent the last 50 million years cutting into and peeling away at the surface of the plateau. The simple wearing down of altering layers of soft and hard rock slowly revealed the natural wonders of Monument Valley today. The valley’s vivid red color comes from iron oxide exposed in the weathered siltstone. The buttes are clearly stratified, with three principal layers.