Rocks and minerals of the Smoky Hills

Sandstone. A common sedimentary rock, sandstone is made up largely of quartz grains that are held together by some natural cement such as calcium carbonate, iron oxide, or silica. In the Smoky Hills, some of the sandstones in the Dakota Formation are cemented by dark-brown iron oxide and are so resistant to erosion they cap steep hills. Other sandstones—such as the giant concretions at Rock City in Ottawa County and Mushroom Rock State Park in Ellsworth County—are cemented by calcium carbonate.

Although sandstone is the most visible part of the Dakota Formation, the main components of the Dakota are clay and shale units. These clays and shales account for the red color of the roads in the eastern part of the Smoky Hills. Clays and shales are used in making bricks, tiles, pottery, chemical ware, furnace linings, and lightweight concrete aggregates. The clays found in the Smoky Hills are among the best for firing at high temperatures.

Concretions. Concretions are masses of mineral matter formed when minerals in water are deposited about a nucleus (such as a leaf or shell or other particle) forming a rounded mass whose composition or cement is usually different form the surrounding rock. This can occur at the the time of deposition, shortly thereafter, or after the sediment has hardened.

Generally, concretions are harder than the rocks around them; therefore, over time the concretions can weather out of the surrounding rocks. Concretions in Kansas are formed from any of a number of minerals, including calcite, limonite, barite, pyrite, or silica. They vary widely in shape and size, with the huge spherical concretions at Rock City in Ottawa County and Mushroom Rock State Park in Ellsworth County measuring up to 27 feet in diameter.

A special type of concretion, known as septarian concretion, occurs in the Cretaceous shales of the Smoky Hills region. The exteriors of septarian concretions are crisscrossed by a network of ridges, which gives some of them the appearance of a turtle shell. Geologists think they were formed by the shrinkage of concretions, which caused cracks to form in which minerals, such as calcite, were then deposited. When the concretion is exposed to weathering, the softer parts between the calcite-filled cracks is eroded and the cracks extend above the surface of the concretion, like ridges or little walls. A good place to find septarian concretions is in the vicinity of Hobby Lake in Osborne County.

Limestone. Limestone is another common sedimentary rock in Kansas. It is composed mostly of calcite (calcium carbonate, CaCO3). It is formed (largely in marine environments) by organic means—that is, from the remains of animals or plants—or by chemical deposition. Many animals and plants (such as oysters, corals, some sponges, sea urchins, plankton, and algae) take calcium carbonate out of the water and secrete it to form shells or skeletons. As these organisms die, they drop to the bottom of the ocean, lake, or river. Over time, the organic parts decay and the calcium carbonate accumulates to form limestone. Chemically deposited limestones are formed when calcium carbonate dissolved in water falls out of solution and settles to the bottom of the ocean, lake, or river.

The limestones that crop out in the Smoky Hills were deposited during the Cretaceous Period. A popular limestone for building, the Fencepost limestone, occurs near the top of the Greenhorn Limestone. This relatively thin limestone, up to one foot thick, is marked by a distinctive rust-colored band in the middle. In addition to widespread use as a building stone, Fencepost limestone is the source of the stone fenceposts that have made this part of the Smoky Hills known as post rock country.

Another variety of limestone, chalk dominates the landscape in the western part of the Smoky Hills. Chalk is a soft, porous, very fine grained kind of limestone that crumbles easily. In its pure form, it is white, but it may be colored by iron oxide or other impurities. Chalk forms from the seafloor accumulation of tiny marine organisms that lived near the surface. As the tiny shells piled up, a soft limy ooze formed, perfect for engulfing and preserving the remains of other animals—such as fish, sharks, turtles, clams, pterodactylsmosasaurs, and plesiosaurs—that fell to the bottom of the sea.


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: Smoky Hills (pdf)