The Nacimiento Formation is a heterogeneous nonmarine formation composed of shale, siltstone, and sandstone,[2] deposited in floodplain, fluvial and lacustrine settings,[3] and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east.

[9] Fossils belonging to a number of different organisms have been found here, including: plants (mostly dicotyledonous angiosperms),[6][10] gastropods, freshwater bivalves,[11] cartilaginous fish and bony fish, salamanders, turtles, champsosaurs, amphisbaenians, lizards, snakes, crocodilians,[12] birds,[13] and a variety of archaic mammals.

Mammalian groups represented include multituberculates,[14] plesiadapiforms,[15] didelphid marsupials, insectivorans, carnivorans, taeniodonts, mesonychians, condylarths, and cimolestans.

[1] Fossil remains found in the formation support the validity of the genus Thylacodon and the species T.

[9] The Puerco and Torrejon were retained as zones within the Nacimiento Formation, and their faunas became the basis of the Puercan and Torrejonian North American Land Mammal Ages.