Monument Valley (Navajo: Tsé Biiʼ Ndzisgaii, meaning valley of the rocks) Tribal Park is a region of the Colorado Plateau characterized by a cluster of vast sandstone buttes, the largest reaching 1,000 ft (300 m) above the valley floor. On the border of Arizona with Utah, the Park was a vast lowland basin before humans appeared. For millions of years, materials that eroded from the early Rock Mountains deposited layer upon layer of sediments which cemented a slow uplift generated by pressure from below, lifting these horizontal strata uniformly to form a plateau. Wind and water that eroded the land spent the last 50 million years peeling away at the surface of this plateau.