There is something almost comically variable about the ever-changing landscape of Monument Valley, whose red sandstone mesas and buttes can look entirely different from one hour to the next. No wonder it winds up in the hallucinatory finish to '2001: A Space Odyssey.' It looks ever so much like something from another planet. And it is inimitable: when Sergio Leone made 'Once Upon a Time in the West,' his tribute to the American western, he shot the exteriors in Monument Valley. When he returned to Italy to shoot the interiors on a sound stage, he had to import the red dirt from the valley because no dirt in Italy quite measured up chromatically.