Gypcrete

gypcrete, also called Gypcrust, gypsum-cemented duricrust, an indurated, or hardened, layer formed on or in soil. It generally occurs in a hot, arid or semiarid climate in a basin that has internal drainage. It usually is composed of about 95 percent gypsum (a hydrated calcium sulfate mineral) and is initially developed in a playa as an evaporite. Gypcrete ranges from a loose, powdery deposit to massive crystalline gypsum and may be as much as 4 m (12 feet) thick.