Rock City is a park located on hillsides overlooking the Solomon River in Ottawa County, Kansas. In a patch of prairie about 500 meters (1600 feet) long and 40 meters (130 feet) wide, Rock City contains three clusters of large spherical boulders. These three clusters contain a total of 200 spherical boulders. It has been designated as a National Natural Landmark.
The park, owned by a non-profit corporation, has a visitor center and picnic tables. A small admission fee, which is used to maintain this park, is charged.
The large spherical boulders in Rock City are giant calcite-cemented concretions, typically called "cannonball concretions" because of their shape. They range in diameter from 3 to 6 meters (10 to 20 feet) with the average diameter being 3.6 meters (12 feet). These concretions lie 2 to 8 meters (6.6 to 26 feet) apart (McBride and others 2006). Similar giant calcite-cemented concretions have also been found in a quartzite quarry within Lincoln County and in exposures of the similar age sandstones in Utah and Wyoming (McBride and others 2003, 2006).
In the past, the origin of the spherical boulders found at Rock city had been erroneously interpreted as glacial boulders, corals, concretionary masses of limestone, and normal erosional remnants of sandstone. Shaffer (1937) was the first to recognize them as calcite-cemented concretions. From a detailed examination of the mineralogy of these concretions and the carbon and oxygen isotopes of the calcite cement comprising them, McBride and others (2006) concluded that they formed as the result of diffusion of calcium through and precipitation of calcite within the sandstone containing them after being deeply buried.