 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).