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.
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