Rock City, Kansas
Rock City is a park located on hillsides overlooking the Solomon River in Ottawa County, Kansas. It is 3.6 miles south of Minneapolis, Kansas and just over 0.5 mile west of Kansas highway K-106 and the Minneapolis City County Airport on Ivy Road. In a patch of prairie about long and 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 remarkable size and spherical shape of these rock formation was first noted by Bell. Later, these boulders were either noted or described by Gould, Landes, Shaffer, Ward, and Swineford. Shaffer was the first person to map the distribution of these boulders at Rock City and investigate their petrography in detail.
Physical characteristics
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 with the average diameter being. These concretions lie apart. 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.These boulders consist of well-sorted, medium-grained sandstone, which is tightly cemented by calcite. The sandstone consists of more than 95 percent quartz sand. About 20 percent of the original sandstone, mostly feldspar grains, has been replaced by the calcite. Pyrite, which is now oxidized to goethite, occurs within the calcite cement of these concretions as microscopic crystals and very small, knobby concretions. The pyrite concretions typically are about 30 cm in diameter. Also, included within these calcite concretions are smaller calcite concretions, which have been engulfed by the growth of the larger concretions.
The host rock, which contained these spherical boulders, consists of well-sorted, medium-grained, highly porous, and friable sandstone. Being only weakly indurated by small amounts of iron oxide, sometimes seen as Liesegang rings at Rock City, it is considerably softer and very much more easily eroded than the calcite concretions. The sand comprising it accumulated within a river channel, which is part of the Dakota Sandstone, which accumulated within a low-lying coastal plain. Differential cementation and later erosion of cross-bedding inherited from the riverine sand, in which these concretions occur, created the "ornamentation", which these concretions exhibit.