Over time, the sand was buried and then compressed and cemented into solid rock. The sandstone was formed mainly from sand deposited along the edge of a sea that covered the western half of Kansas about 100 million years ago during the Cretaceous Period. Known as concretions, the spheres weathered out of a sandstone layer in the Dakota Formation. Huge sandstone spheres ranging up to 20 feet in diameter cover an area roughly the size of two football fields at Rock City, about four miles south of Minneapolis in Ottawa County. Stratigraphic nomenclature: How rocks are named.Basic geology, paleontology, and fieldwork.
Eons and eonothems? Periods and systems? Understanding how geologists talk about time.
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