Red Hills—Rocks and Minerals

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Download fact sheet on the rocks and minerals of the Red Hills.

Gypsum. A common mineral in Kansas, gypsum is calcium sulfate with two molecules of water (CaSO4•2H2O). Calcium sulfate without water is the mineral anhydrite (CaSO4). It is colorless or white to light gray (or, rarely, bright red), and is so soft that it can be scratched by a fingernail.

Gypsum is divided into three varieties. The first, selenite, consists of flat, transparent, diamond-shaped crystals. Excellent selenite crystals can be found throughout the Red Hills. Another variety of gypsum is called satin spar. It is white or pink, fibrous, and has a silky luster. It is found as thin layers in beds of rock gypsum and in certain shales. The third variety, called massive or rock gypsum, is common in the Red Hills. Rock gypsum 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 the massive gypsum that is mined near Sun City in northwestern Barber County. The Gyp Hills near Medicine Lodge take their name from the gypsum in the Blaine Formation.

drawing of two selenite crystals   color photo of white gypsum sample with parallel fibers, aligned  vertically

Two forms of gypsum: selenite crystals on the left, satin spar on the right.

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. South of Sun City are the remains of a natural bridge that collapsed in 1964. Before its collapse, the bridge was 35 feet wide and 55 feet long and stood 12 feet above the stream that cut through it.

The gypsum in the Red Hills (and 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.

Anhydrite. Made up of calcium sulfate (CaSO4), anhydrite occurs commonly as light-gray crystalline masses, though some has a fibrous structure. It also occurs as individual crystals, particularly in dolomite. It has a glassy luster, and is translucent. It is harder and heavier than gypsum; it can be scratched with a knife but not with a fingernail. Anhydrite may change into gypsum if it comes into contact with water, a common occurrence in near-surface exposures.

In the Red Hills, anhydrite is found in Permian deposits associated with gypsum, dolomite, and red silt. It is mined along with gypsum near Sun City in northwestern Barber County.

Dolomite. A sedimentary rock, closely related to limestone, dolomite is made up of calcium magnesium carbonate, CaMg(CO3)2, whereas pure limestone is made up of calcium carbonate without the magnesium. Dolomite is a fine- to coarse-grained rock that is generally gray or light in color in its unweathered state; it weathers to a buff or tan color when exposed to the elements. It is used in much the same way as limestone.

In Kansas, dolomite is not nearly so common at the surface as limestone. In the Red Hills it is found in the Day Creek Dolomite, a formation that is about two-and-a-half feet thick in Clark County. This dolomite, which crops out at Clark County State Lake, formed in the enclosed evaporating basin of the Permian sea.

Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.

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