An unnamed irregular natural arch, Ottawa County, Kansas
Photo by Jay Wilbur
Example Irregular Natural ArchNABSQNO 14S-609400-4327600. This unnamed miniature natural arch has eroded through a remnant calcite concretion of Dakota sandstone. The boulder it penetrates is one of several roughly-spherical concretions that are the attraction of Rock City, a private park that charges the public a modest fee to access these geological oddities. Rock City is located about 3 miles southwest of Minneapolis, in Ottawa County, Kansas. Because the process of erosion that formed the arch is atypical and probably unique, this natural arch is classified as an irregular natural arch. It is sometimes called "Donut Rock".