Rocks and minerals of the Arkansas River Lowlands

Shale. A common sedimentary rock in Kansas, shale is composed of hardened, compacted clay and silt that commonly breaks along bedding planes. Its particles are too small to be seen without a microscope. Shales erode easily, and most are soft enough to be cut with a knife. Though usually gray, shale can be black, green, red, or buff.

Shale is the main component of the Wellington Formation, which underlies the Arkansas River Lowlands east of Hutchinson. Up to 700 feet thick, the Wellington is composed of mostly shale, with interspersed layers of salt, limestone, siltstone, and anhydrite. It was deposited early in the Permian Period, about 275 million years ago.

Sand. Found abundantly in Kansas, sand is a loose, unconsolidated material formed from the breaking down or weathering of older rocks and from the transportation and sorting of rock fragments by moving water or by wind. Sand particles range in size from 0.625 mm and 2 mm, larger than silt particles but smaller than pebbles.

Kansas sand is composed mostly of quartz. Sand also contains igneous and metamorphic minerals formed outside the state and transported here by running water. Sand is common throughout the Arkansas and Kansas river valleys.

In the Arkansas River Lowlands, sand dunes are common south of the river in Hamilton, Kearny, Finney, Gray, Ford, Kiowa, Edwards, Pratt, Pawnee, Stafford, Barton, Rice, and Reno counties. North of the river, however, only a few isolated areas of sand hills occur. This leads geologists to speculate that the prevailing winds were from the north during the time of deposition, which was during the Ice Age, about a million years ago. At that time, a huge ice sheet to the north may have created wind patterns that are the opposite of today's patterns, in which winds generally come from the south.

Sand along the Arkansas River is regularly dredged from the floodplain. The production of sand (and gravel) is an important industry in the state, and sand pits are common along the entire length of the river.

Gypsum. A common mineral in Kansas, gypsum is made up of calcium sulfate (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, clear, diamond-shaped crystals. The second variety 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 coarsely to finely granular, white to gray, and contains varying amounts of impurities.

In the Arkansas River Lowlands, gypsum is found in the shales of the Wellington Formation. It crops out in Wichita along Gypsum Creek, which drains the eastern part of the city. Both selenite and satin spar can be collected from these shales.

Salt. Halite, common table salt, is composed of sodium chloride (NaCl). Most salt crystals are transparent and colorless cubes, but impurities may impart a brilliant red, blue, or yellow color. Broken fragments may be very nearly cube-shaped. Halite is easy to identify because it has a salty taste and dissolves easily in water.

Salt is an evaporite, which means it is formed by the evaporation of water. (Other evaporites are gypsum and anhydrite.) Salt rarely forms outcrops because rain and groundwater dissolve salt that is exposed at the surface. In Kansas, salt is found in thick beds in Permian rocks deep underground, the largest of which is the Hutchinson salt bed, which underlies approximately 37,000 square miles in central Kansas. This salt was deposited by the evaporation of a shallow arm of the Permian sea, which was cut off from the open ocean. When that shallow arm evaporated, it left behind thick layers of gray shale, salt, and gypsum. These deposits were subsequently buried by younger sediments and remained hidden for millions of years until salt was accidentally discovered near Hutchinson in 1887 by drillers looking for oil and gas.


Buchanan, R., and McCauley, J. R., 2010, Roadside Kansas: A Traveler's Guide to its Geology and Landmarks: Lawrence, Kansas, University Press of Kansas, 392 p.

GeoFacts: Arkansas River Lowlands (pdf)