Location: Fremont CountySize: 449 acresDesignated: September 2001Landowner: The State Land Board
The unusual "Devil's Dunce Cap" formations are the eroded remnants of a 1200-square-mile volcanic ash flow. This Gribbles Park Tuff was deposited 29 million years ago during the Oligocene. Although located within the Thirtynine Mountain Volcanic Field, the tuff's source is thought to be outside of the field to the west. The exact source is unknown, and research on this may be useful in determining the timing of the opening of the upper Arkansas valley. The more well-known Wheeler Geologic site in Mineral County has higher formations (and is from a different volcanic source), but this site covers a larger area than Wheeler.