Within the 240-acre parcel historic site are the Sanilac Petroglyphs. Michigan's only known rock carvings attributed to Native American workmanship in the Lower Peninsula of Michigan. The Sanilac Petroglyphs Historic Site is open May 24 to 26, from 10 am to 4 pm. From May 28 through August 31, the petroglyphs site is open Wednesday to Sunday, 10 am to 4 pm. You can also view the petroglyphs on Labor Day, Monday, September 1, from 10 am to 4 pm. Send any correspondence to Sleeper State Park, 6573 State Park Rd, Caseville MI 48725.