Septarian concretions are a special type of concretion. 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 from the surrounding rock. This can occur at 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. The smallest are oolites, which can be smaller than the head of a pin. At the other end of the spectrum are the huge spherical concretions at Rock City in Ottawa County and Mushroom Rock State Park in Ellsworth County, the largest of which have diameters of 27 feet.
The exteriors of septarian concretions are crisscrossed by a network of ridges, giving some of them the appearance of a turtle shell. Geologists think they were formed by the shrinkage of concretions, which caused cracks to form, followed by the deposition of such minerals as calcite. When the concretion is exposed to weathering, the softer parts between the calcite-filled cracks are eroded and the cracks extend above the surface of the concretion, like ridges or little walls.
Read more about concretions in Rocks and Minerals of the Smoky Hills.
The sample pictured above is from Jewell County, Kansas
Buchanan, Rex C., Tolsted, Laura L., and Swineford, Ada, 1986, Kansas Rocks and Minerals: Kansas Geological Survey, Educational Series 2, 60 p.
Klein, Cornelis, 1993, Manual of Mineralogy (after James D. Dana), 21st Edition: New York, Wiley, 681 p.
Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey; text by Liz Brosius, Kansas Geological Survey.
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