Monument Rocks (also Chalk Pyramids) are a series of large chalk formations in Gove County, Kansas, rich in fossils. It is a National Natural Landmark. It was the first landmark chosen by the US Department of the Interior as a national natural landmark. The chalk formations reach a height of up to 70 ft. and include formations such as buttes and arches. They are estimated to have been formed 80 million years ago.
https://en.wikipedia.org/wiki/Monument_Rocks_(Kansas)
