The formation is a remnant of a great sand sea, or erg, that once covered an area of 140,000 square kilometres (54,000 sq mi) reaching from the present locations of the Chuska Mountains to near Albuquerque and to the southwest.

The Deza Member, to which the lowermost beds of the formation are assigned, is up to 81 meters (266 ft) thick and consists mostly of pale orange to yellow-gray sandstone (66%), claystone (16%) and sandy siltstone (16%).

Sedimentary structures are present that indicate deposition by running water, and the member fills shallow paleovalleys eroded in the underlying Mesozoic beds.

However, emydid turtles are almost exclusively aquatic, so the presence of these shell fragments provides supporting evidence that the Deza Member was deposited in an environment with permanent water bodies.

Conservative estimates of the original extent of the erg suggest it once covered an area of 140,000 square kilometres (54,000 sq mi).