This billion-year-old hill is one of the oldest geological formations on earth. Rib Mountain State Park is a premier day-use facility with picnicking, hiking trails and a reservable amphitheater. The top of the 60-foot observation tower offers spectacular views of the Wausau area and Wisconsin River. Granite Peak Ski Area is on the north face of the mountain and offers downhill skiing and snowboarding during winter. The park is open year-round from 6 a.m. to 11 p.m.