Quaternary alluvium in valley fiats of the Davis Mountains and adjoining lowlands in Trans-Pecos Texas contains remains of extinct horse and mammoth species as well as artifacts and other cultural material. On the basis of disconformities within the valley-fill complex, the Quaternary has been divided into three formations: Neville, Calamity, and Kokernot, in order from oldest to youngest.
The Neville consists mostly of reddish-brown to buff clay and silt. The Calamity contains relatively more sand and gravel than the Neville, although dark zones of humic silt and clay are characteristic of the Calamity. The Kokernot consists of incoherent silt, sand, and gravel. Caliche concretions and cement, abundant in the Neville, are uncommon in the Calamity and generally absent in the Kokernot.
Only the Neville contains mammoth remains. To date, no artifacts have been found in the Neville, although the overlying Calamity formation contains artifacts, buried hearths, and human skeletons. Of several cultures represented in the Calamity, one is Pecos River Cave Dweller. The Kokernot formation contains cultural material at top and bottom ; pottery in upper layers shows that these beds were deposited within the interval 1100–1400 A. D.