The rocks of Rock City are called concretions. They are sandstone structures, some as large as houses. There are 200 of them spread out over the acreage of Rock City. They were formed when underground waters laden with minerals interacted with loose sandstone on the surface. As natural erosion occurred, giant spheres of bizarre looking stones emerged, giving the landscape as surreal and otherworldly appearance.
Fields of concretions can be found scattered throughout the world, but there are none so large as those of Rock City, Kansas.