Niobrara chalk in Logan County
Niobrara chalk in Logan County


Chalk, a sedimentary rock, is a soft form of limestone that is not well cemented and thus is often powdery and brittle. It usually ranges in color from white to light gray to buff and forms from sediment deposited in a saltwater environment. Composed mostly of the mineral calcite and formed mainly from the remains of floating microorganisms and algae, chalk deposits often contain fossils of marine animals of various sizes.

The Cretaceous-age Niobrara Chalk in western Kansas was deposited in a massive inland sea that ran north to south across west-central North America about 80 million years ago.

Monument Rocks and Castle Rock, two sets of chalk spires that rise above the plain in Gove County, are composed of chalk formed from the enormous amount of ooze that settled on the floor of a Cretaceous sea floor. Subsequently buried under layers of sediment and compressed into rock, the Niobrara Chalk was eventually exposed at the surface when overlying rock layers eroded. As the exposed chalk began to erode, the remnants at Monument Rocks and Castle Rock were spared when more resistant, localized beds above them helped shield the softer layers below.

The western Kansas chalk beds became famous in the 19th century for largely complete fossils of giant swimming and flying reptiles known as mosasaurs, plesiosaurs, and pterosaurs as well as fossils of aquatic birds with teeth, 20-foot-long fish, and clams up to six feet in diameter.


Brosius, L., McCauley, J., Sawin, B., and Buchanan, R., 2003, Geology and paleontology of northwestern Kansas: Public Field Trip: Kansas Geological Survey, Open-File Report 2003-25, 13 p.

Buchanan, R., 2010, Kansas Geology: An Introduction to Landscapes, Rocks, Minerals, and Fossils (2nd ed.): Lawrence, Kansas, University Press of Kansas, 240 p.

Buchanan, R., and McCauley, J. R., 2010, Roadside Kansas: A Traveler's Guide to Its Geology and Landmarks (2nd ed.): Lawrence, Kansas, University Press of Kansas, 392 p.

Everhart, M. J., 2005, Oceans of Kansas—A natural history of the Western Interior Sea: Indiana University Press, 322 p.

Hattin, D. E., 1982, Stratigraphy and depositional environment of Smoky Hill Chalk Member—Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas: Kansas Geological Survey, Bulletin 225, 108 p.

Kansas Rocks and Minerals, Kansas Geological Survey Educational Series 2