Concretions are masses of mineral matter embedded within rock layers, including limestone, sandstone, and shale. They often take shape when minerals precipitating (settling) out of water collect around a nucleus, such as a pebble, leaf, shell, bone, or fossil. Concretions most often form in sediment before or as the sediment is compacted into rock, but they also can form in hardened sedimentary rock when minerals in groundwater are deposited in rock pores and cavities.
Generally, concretions are harder than the rocks around them. Over time, a concretion may weather out when the softer rock surrounding it erodes away. Although often rounded, concretions can be lumpy, long, oval, disk-shaped, or irregular. They vary in color, hardness, and size—from smaller than the head of a pin to several hundred pounds—and are sometimes misidentified as fossils, dinosaur eggs, or meteorites.
Concretions in Kansas are formed from any of a number of minerals, including calcite, limonite, barite, pyrite, and 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 a septarian concretion, occurs in the Cretaceous shales of the Smoky Hills region. Networks of crisscrossing ridges on the exteriors of septarians sometimes give them the appearance of turtle shells. Geologists think the ridges form when a concretion shrinks and cracks and minerals, such as calcite, are deposited in the cracks. When the concretion is exposed to weathering, the softer parts between the calcite-filled cracks erode. What were once calcite-filled cracks become ridges. A good place to find septarian concretions is just south of Hobbie Lake in Osborne County.
Kansas Rocks and Minerals, Kansas Geological Survey Educational Series 2.