Monument Rocks in Northwestern Kansas
September 03, 2021
Photographer: Karen Deaver
Summary Author: Karen Deaver; Cadan Cummings
Located in northwestern Kansas, the Monument Rocks are a chain of 70 ft (21 m) tall carbonate (i.e. chalk) formations in an otherwise flat landscape. These gigantic deposits composed of chalky limestone and chalky shale are estimated to have been formed over 80 million years ago during the Cretaceous Period. During this time in geologic history, a large inland sea called the Western Interior Seaway covered the central North America from the Gulf of Mexico to north of Canada in the Arctic Ocean. After the seaway receded, these massive Niobrara Chalk structures were left behind, which suggests the sea was warm and had plentiful calcareous planktonic algae.
Recognizing their natural significance, the Monument Rocks was selected as the first National Natural Landmark in Kansas by the Department of the Interior in 1968. The site was also named as one of the 8 Wonders of Kansas in 2008.