South Killdeer Mountain is a butte formed because the top consists of an erosionally resistant caprock that kept it from eroding as fast as the surrounding lower prairie. The Airkaree Formation forms the top layer of the caprock and is Miocene (about 25 million years old). It consists of volcanic ash that was deposited in a fresh water lake. The interaction with the chemistry of the lake formed the ash into something akin to concrete in hardness which explains its resistance to erosion.
References
Page 142 - Dicka, A. B., 2012, 101 American Geo-sites you've gotta see: Mountain Press Publishing Co., 250p.
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