It preserves fossils dating back to the late Pennsylvanian to early Permian periods.

[1] The formation consists mostly of sandstone and conglomerate (50%) with lesser amounts of limestone (21%) and shale and siltstone (29%).

[2] The formation contains fossils ranging in age from middle Desmoinesian (Moscovian) to early Wolfcampian (Asselian).

Sutherland in 1963, who considered it correlative with the upper part of the Madera Formation.

[2] However, in 2004, Kues and Giles recommended restricting the Madera Group to shelf and marginal basin beds of Desmoinean (upper Moscovian) to early Virgilian age, which excluded the Alamitos Formation.