Layers of Limestone and Flint: The Geology Behind the Kansas Flint Hills

The Flint Hills are named for the flint left lying on their surface. The region crosses east central Kansas from near the Nebraska border south into the Osage Hills of Oklahoma, and it is made mainly of alternating layers of limestone and shale. Many of the limestones contain chert, also called flint, a hard, dense microcrystalline quartz. As the limestone erodes, angular fragments of flint pile up at the surface. A National Park Service article on Tallgrass Prairie National Preserve explains how that rock shaped the land, its use and its buildings.

Cross section of limestone, shale and flint layering in the Kansas Flint Hills. NPS image.

Why the hills are stairsteps

The stairstep landscape comes from differential erosion. Limestone with flint resists erosion better than the softer shale between, so the tougher layers form prominent benches on the hillsides and cap the hilltops, while the shale wears away slowly and makes thin, gravelly soil. The article says these rolling slopes and rocky soils spared the Flint Hills from wide-scale plowing and helped preserve more native tallgrass prairie here than anywhere else in North America. The Pawnee, Osage, Wichita and Kansa peoples collected the exposed flint to make a wide variety of tools and weapons.

How the rock formed

The alternating layers were laid down in the Permian Period, around 280 million years ago, when a shallow ocean covered most of the surface in a tropical, warmer climate. Calcium-rich remains of marine organisms fell to the bottom and became limestone, and clay and mud became shale.

Chert occurs in the limestone either as roughly spherical nodules or as layers. Some geologists think nodules formed from silica-rich marine remains that sank and then gathered together as the limestone formed, while layers may have formed as silica recrystallized and chemically replaced the limestone. Geologists continue to research these processes.

Fossils in the stone

The rock holds many fossils, mostly marine invertebrates: corals, clams, snails, bryozoans (colonies of animals that resemble sea fans), sea urchins, crinoids (stalked animals distantly related to starfish and sea urchins) and brachiopods (clam-like animals). In some limestones, fusulinids are particularly abundant: small, one-celled animals shaped like wheat grains. They can be seen in many of the limestone blocks used for buildings at the preserve.

Soil, cattle and crops

Thin soils, limestone outcrops, vegetated shale intervals and deeply cut valleys characterize the region. Steep slopes and rocky soil limited crops to river and stream bottoms, such as along Fox Creek east of the historic ranch headquarters area, where river-deposited sediments have built thick, valuable soils.

The same rocks that made farming hard made the area ideal for grazing. Calcium from the limestone erodes into the soil and makes native prairie plants extra nutritious for grazing animals. Cattle ranching remains the dominant agricultural activity, with a legacy extending back over 150 years.

Cowboy herding cattle (top) and cattle grazing on a green grassy hillside (bottom). NPS photo.

Building with limestone

Wood was scarce when European-American settlers reached the Flint Hills in the mid-1800s, so limestone became an important building material. The Cottonwood Limestone, a thick, nearly white, even-textured, durable layer near the base of the hills in the Fox Creek valley that holds numerous fusulinids, is a common building stone in Kansas. Blocks three or more feet thick and several feet long and wide can be taken from a single ledge.

The Ranch House, portions of the Lower Fox Creek Schoolhouse and Ranch Barn, and many other structures on the preserve are built of it, as are many buildings around the state, including the Chase County Courthouse in Cottonwood Falls and most of the Kansas State Capitol in Topeka.

Chase County Courthouse, Cottonwood Falls, Kansas. NPS photo.

Sources

• National Park Service, "Flint Hills Geology," Tallgrass Prairie National Preserve, https://www.nps.gov/articles/000/flint-hills-geology.htm; rewritten in hubnx's own words. The Kansas Geological Survey's two illustrations on the original page are not reproduced here.