[2] The formation consists of up to 300 feet (91 m)[3] of gypsum or anhydrite with a few thin beds of limestone.

[2] The Castile Formation, and the overlying Salado Formation, form an evaporite sequence that formed in a very deep basin (over 500 metres (1,600 ft)) from increasingly saline waters.

The presence of fine laminations in the formation, which can be traced over great distances, indicate deposition well below wave base.

[2] The unit was first named by George Burr Richardson in 1904 for exposures in west Texas.

[3] The formation was subsequently trace north into the Pecos River valley[5] and is extensive in the subsurface.