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
Hole in the Rock (rock formation) them along the relatively benign terrain beneath the Straight Cliffs of the Kaiparowits Plateau. However, when this led them to the 1200-foot (400 m) sandstone cliffs that surround Glen Canyon, they needed a way to cross to the eastern rim. They found (and named) Hole in the Rock, a narrow, steep, and rocky crevice and sandy slope that led down to the river. Directly across the river was Cottonwood Canyon, a tempting route up to Wilson Mesa on the other side. They worked for months to prepare the road, using blasting powder to widen the upper section and hand chisels