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 is another common sedimentary rock in Kansas.
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.