 The Chuska Mountains and the Defiance Uplift immediately to the southwest form one of the prominent uplifted highlands of the Colorado Plateau.The uplifted region is separated from the San Juan Basin to the east by the Defiance and associated monoclines.Relative uplift, basin subsidence, and monocline formation began in the early stages of the Laramide orogeny about 75 to 80 million years ago.