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
Fossil Beach (Sedbury Cliffs) examples in the bulk of the mudstone: the cliffs are easily weathered, with parts subject to notable expansion and contraction, Sedbury Cliffs and the foreshore contain similar geological layering to the more famous and visible Aust Cliff the on the opposite site of the River Severn. However, at Sedbury the cliffs are wooded at the top and overgrown with bushes and trees at the base, with fallen blocks and old trunks amongst them. Farther along the beach from the grass, sandstone blocks become more common. The highest rock layer is the brown rocks of the Blue Lias formation, which was