The Castile and Rustler formations have a combined thickness of 4,450 feet and compose one of the thickest evaporite sections in the world.

Continual accumulation of concentrated saline water beneath the surface water in the Delaware basin raised the level of the concentrated water to the point where it caused epoch-making events in upper Permian history. The inflow of marine water was less than evaporation and Capitan reef building and associated petroleum deposition was stopped. Further excess evaporation caused deposition of evaporites in the Delaware basin. This event marks the close of Capitan time and the beginning of Castile.

The lower Castile is confined to the Delaware basin. In places its thickness attains 2,000 feet. It is composed of banded anhydrite, white anhydrite, and white halite, but lacks potassium minerals.

Subsurface study discloses many hundred feet of depositional relief on the basin floor near the end of lower Castile time. This condition is responsible for unsuccessful attempts to make a structural correlation across the Delaware basin on the base of the upper Castile ("Main Salt").

At present potash is the mineral product of greatest economic importance in the Castile formation. Two mines are producing from the upper Castile. The deposits near Carlsbad are among the finest in the world. The known potash reserve plus the large unexplored area favorable for commercial deposits indicates that the United States need have no fear of future potash shortage.

A study of two subsurface cross sections shows many interesting features about evaporite sedimentation.