Gypsum is a mineral commonly found in Kansas. When salty seawater evaporates, dissolved salts, including the mineral gypsum, are left behind. If conditions are right, large quantities of gypsum build up into thick beds of sedimentary rock.

The gypsum throughout the state was deposited during the Permian, when an arm of the inland sea was cut off from the main body of the ocean. That sea evaporated, leaving behind thick layers of sodium chloride (salt) and calcium sulfate (gypsum and anhydrite). These deposits are called evaporites because they formed from the evaporation of water.

Rock gypsum, common in the Red Hills, is coarsely to finely granular, white to gray, and contains varying amounts of impurities. A good outcrop of rock gypsum, part of the Blaine Formation, can be seen near milepost 213 on U.S. Highway 160, about 10 miles west of Medicine Lodge. The Blaine Formation, which includes layers of gypsum, dolomite, and red shale, is the source of gypsum mined near Sun City in northwestern Barber County.

The Red Hills are home to numerous caves that have formed in the gypsum. In Barber County alone, there are 117 such caves. These caves are among the youngest in the state, and because they are formed in soft gypsum, they will be among the first to crumble and wash away. Closely associated with the caves are features called natural bridges. In the Red Hills, natural bridges form when a cave collapses but a section of the cave roof remains standing in the shape of a bridge. A natural bridge south of Sun City that collapsed in 1964 was 35 feet wide and 55 feet long and stood 12 feet above the stream below.