Monument Valley Navajo Tribal Park's monolithic sandstone formations are a must-see for any visitor to the U.S. Southwest. The entire valley was once a solid, 1,000-foot-deep basin of sandstone and limestone, laid down over the millennia by sediments eroded from the Rocky Mountains to the east. A slow and gentle uplift elevated the horizontal strata to one to three miles above our current sea level. The basin had now become a plateau. Over the next 50 million years, wind and water eroded the layers of hard and soft rock, leaving behind the harder Shinarump formation caps over the softer DeChelly sandstone layer. The latter forms most of the famous cliffs on the mesas and buttes. Softer Organ Rock shale at the cliffs' bases have eroded into stair-like horizontal terraces. The orange-red sandstone (called "Red Rock") cliffs are from the Coulter Formation of 160 million years ago. Also, volcanic activity is evident in the southern areas of the park.