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 Minneapolis is a city in and the county seat of Ottawa County, Kansas, United States. As of the 2020 census, the population of the city was 1,946. History The community was originally called Markley's Mills, and under the latter name was l ...

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 National Natural Landmarks (NNL) Program recognizes and encourages the conservation of outstanding examples of the natural history of the United States. It is the only national natural areas program that identifies and recognizes the best e ...

. 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 A rock formation is an isolated, scenic, or spectacular surface rock (geology), rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock Geological formation, formation ...

was first noted by Bell.Bell, W.T., 1901, ''The remarkable concretions of Ottawa County, Kansas'', American Journal of Science, 4th Series, v. 11, p. 315-316. Later, these boulders were either noted or described by Gould,Gould, C.N., 1901, ''The Dakota Cretaceous of Kansas and Nebraska'', Kansas Academy of Science, v. 17, p. 122-178. Landes,Landes, K.K., 1935, ''Scenic Kansas'', Geological Survey of Kansas Bulletin, n. 36, 55 p. Shaffer,Shaffer, H.L., 1937, ''Concretions in the Dakota Sandstone'', Compass, v. 17, p. 87-90. Ward,Ward, H.K., 1938, ''Concretions of Rock City. Mineralogist'', v. 6, p. 23-24. and Swineford. Shaffer was the first person to map the distribution of these boulders at Rock City and investigate their

petrography Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The clas ...

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 Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, which is tightly cemented by

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

. The sandstone consists of more than 95 percent

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

. About 20 percent of the original sandstone, mostly

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

grains, has been replaced by the calcite.

Pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

, which is now oxidized to

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

, occurs within the calcite cement of these concretions as microscopic crystals and very small, knobby concretions. The pyrite concretions typically are about 30 cm (1-foot) 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 (banding) 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 In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

inherited from the riverine sand, in which these concretions occur, created the "ornamentation", which these concretions exhibit.

Origin

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 was the first person to recognize them as calcite-cemented concretions. From a detailed examination of the

mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...

of these concretions and the carbon and oxygen isotopes of the calcite cement comprising them, McBride and others concluded that they formed as the result of diffusion of

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

through and precipitation of calcite within the sandstone containing them after being deeply buried. The

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

and

comprising these concretions came either from marine

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

, shells, anhydrite, or some combination of these in addition to

bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial bioche ...

derived from oxidized

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

from

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

outside of, but hydrologically connected to, the Dakota Sandstone. After the formation of the concretions, differential erosion of the considerably softer sandstone surrounding them exposed as free-standing boulders.

Gallery

Image:RockCityE21.jpg, Tree at Rock City Image:RockCityE23.jpg, A turtle-shaped rock Image:RockCityE24.jpg, A large, round rock Image:RockCityE25.jpg, Trees at Rock City Image:RockCityE22.jpg, Hesperis matronalis growing by a rock in May Image:RockCityE20.jpg, Looking towards Minneapolis, KS across rippling wheat fields in May

References

External links

Rock City Park, Kansas
Hours, prices, history *Kansas Geological Survey, nd
Rock City, Minneapolis, Kansas
*Biek, B., 2002
Concretions and Nodules in North Dakota
North Dakota Geological Survey, Bismarck, North Dakota. Explains how concretions are created. *Dietrich, R.V., 2002

The Wayback Machine. an
PDF file of ''Carbonate Concretions--A Bibliography''
CMU Online Digital Object Repository,

Central Michigan University Central Michigan University (CMU) is a Public university, public research university in Mount Pleasant, Michigan, United States. It was established in 1892 as a private normal school and became a state institution in 1895. CMU is one of the eigh ...

, Mount Pleasant, Michigan. *Hansen, M.C., 1994
''Ohio Shale Concretions'' PDF version, 270 KB
Ohio Division of Geological Survey GeoFacts n. 4, pp. 1–2. *Heinrich, P.V., 2007
''The Giant Concretions of Rock City Kansas'' PDF version, 836 KB
BackBender's Gazette. vol. 38, no. 8, pp. 6–12. *Kansas Geological Survey, 2004
Educational Resources, Photos from Ottawa CountyOttawa County Map
KDOT
Rock Climbing at Rock City
{{authority control Geography of Kansas National Natural Landmarks in Kansas Protected areas of Ottawa County, Kansas

Physical characteristics

Origin

Gallery

See also

References

External links