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
Brazos Mountains the view from the west, including from Heron Lake. U.S. Route 64 crosses the southern end of the range at a 10,481 foot (3194 m) pass. Nearby are the photogenic Brazos Cliffs, headwaters of the Rio Brazos and part of the Rio Chama river system. These mountains overlook the Chama River, the largest tributary of the Rio Grande which lies to the east. In essence, these mountains are the southern extension of the South San Juan Range that lies in Colorado. The Brazos Mountains are the southwestern remnants of a Laramide peneplain resulting from the erosion of mountains formed 80