Rib Mountain, also known as Rib Hill, is a glacially-eroded monadnock in central Wisconsin, located in the Town of Rib Mountain in Marathon County. Composed of quartzite covered with a softer syenite sheath, it was intruded about 1.5 billion years ago.Rib Mountain is near Wausau on the west side of the Wisconsin River, just west of Interstate 39 and just south of Highway 29.