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. Although the Chuska Mountains can be considered part of the Defiance Uplift, they stand higher. They are capped by an erosional remnant of Chuska Sandstone, a unit locally more than 500 meters thick. The flat-lying Chuska Sandstone rests unconformably on Mesozoic rocks deformed in the Defiance monocline. Biotite in layers of altered volcanic ash within the Chuska Sandstone has yielded radiometric ages of 35 and 33 million years by argon-argon dating. The Chuska Sandstone is formed of sand dune deposits, and it appears to be a remnant of a huge Oligocene sand sea, the Chuska erg. The erg hypothesis is consistent with major exhumation of the central Colorado Plateau in the late Oligocene and early Miocene (e.g., from about 26 to 16 million years ago). If so, then major uplift of the central Colorado Plateau may postdate the Laramide orogeny.