Rocks and minerals of the Flint Hills
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Chert. Commonly known as flint, chert is found in many Kansas limestones as nodules or continuous beds. It is a sedimentary rock composed of microscopic crystals of quartz (silica, SiO2). It is opaque and ranges in color from white to dull gray or brown to black. Chert breaks with a shell-like (conchoidal) fracture, and the edges of the broken pieces are sharp. Because of this, people have used chert for thousands of years to make tools and weapons. In the Flint Hills, a bluish-gray chert is commonly seen in roadcut exposures of the Florence Limestone Member (part of the formation known as the Barneston Limestone).
Limestone. Limestone is a sedimentary rock 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.
One of the prominent limestones in the Flint Hills is the Fort Riley Limestone Member (the lowest member of the Barneston Limestone). Known in southern Kansas as the Silverdale limestone, the Fort Riley is 30 to 45 feet thick and is riddled with caves and solution cavities. At Silverdale in southern Cowley County, the Fort Riley is nearly 60 feet thick. The Cowley County courthouse in Winfield is built out of this limestone.
Another characteristic limestone in the Flint Hills is the Florence Limestone Member (of the Barneston Limestone). Ranging from 12 to 45 feet in thickness, this limestone often contains a variety of fossils, including brachiopods, pelecypods, bryozoans, and fusulinids.
Geodes. Geodes are crystal-lined cavities in rocks. They are formed by groundwater that deposits minerals in solution on the walls of rock cavities. This type of deposition usually forms good crystals. The minerals deposited may vary, but in Kansas most geodes consist mostly of quartz, chalcedony, and calcite. They can be found in many places in the Flint Hills region: near the town of Rock, along the Walnut River in Cowley County; north of the town of Douglass in Butler County; and in Riley, Marshall, and Chase counties.
Kimberlites. One of the rare examples of native igneous rock in the state, kimberlites occur at the surface in Riley and Marshall counties. Kimberlite is a soft, dull-gray rock with thin white veins of calcite and magnetite. Associated with ancient volcanic activity, kimberlites are called intrusive igneous rocks because they were forced through other rocks as they pushed to the surface of the earth. Because kimberlites are the source of most of the world's diamonds, they have generated much interest. To date, however, no diamonds have been found in association with these kimberlites. Even without diamonds, kimberlites are interesting because they provide a glimpse of the rocks that come from deep underground, perhaps as deep as 150 miles. These rocks pushed to the surface about 90 million years ago, during the Cretaceous Period. Garnet, a mineral with many faceted crystals and a glassy luster, has been found in Riley County kimberlites and in streams flowing near the kimberlites.
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: Flint Hills (pdf)