Most people, including me, think of mountains as rising up due to geologic forces, and that is often the case. However, you can't apply that generalization to the mesas and buttes of Monument Valley. In fact, the key to understanding the landscape is to realize that the story is what is not there, rather than what is not. The valley's mesas did not rise out of the surrounding landscape, they are all that is left of that landscape, as it existed eons ago. If you are standing on the valley floor, looking up at a mesa, realize that the valley floor used to be at the top (or higher) of that mesa. Everything around it has eroded away, to the point where you are standing several hundred feet below what used to have been.