Monument Rocks are a series of large chalk formations in Gove County, Kansas, rich in fossils. The formations were the first landmark in Kansas chosen by the US Department of the Interior as a National Natural Landmark. The chalk formations reach a height of up to 70 ft (21 m) and include formations such as buttes and arches. The carbonate deposits were laid down during the Cretaceous Period in what was then the Western Interior Seaway, which split the continent of North America into two landmasses. They are estimated to have been formed 80 million years ago.