Nacimiento Formation Nacimiento Formation The Nacimiento Formation is a sedimentary rock formation found in the San Juan Basin of western New Mexico (United States) and named for the Nacimiento Mountains. It is a heterogeneous nonmarine formation composed of shale, siltstone, and sandstone, deposited in floodplain, fluvial and lacustrine settings, and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. It was deposited mostly between ~64.5 and ~61 million years ago, during the early and middle Paleocene. The climate was humid and warm to hot. This unit interbeds with the
Roan Cliffs In addition, the name Book Cliffs is often applied to both landforms, with Roan Cliffs being an alternate name. Moreover, because of the how far they are spread between Utah and Colorado, they are sometimes mistakenly believed to be separate landforms of the same name within each state. The Roan Cliffs are situated north of (in back of) and above, but run roughly parallel to, the Book Cliffs. Stretching nearly from west to east, the Roan Cliffs begin on southern edge of the West Tavaputs Plateau, on the eastern edge of the Willow Creek (and the US‑191 corridor), north of