M. Rebekah Otto of Atlas Obscura has an article about the Sanilac Petroglyphs:
The drawings were carved into the sandstone in Sanilac County but remained hidden by dense forests until the devastating fire exposed them.
The glyphs are carved into a large rock on the ground that is forty feet long and fifteen feet wide. Carved between 300 and 1,000 years ago, the drawings were likely made by the Hopewell or Chippewa Indians. They depict flying birds, other animals, and a man with a bow and arrow – lasting testaments to a former way of life.
Archaeologists have not determined the purpose or use of the drawings, though some have speculated that they were a destination for vision quests, as the rock is isolated near the fork in river. Shaman and holy men may have used the rock as a record of their visions, depicting animals that came to them in dreams.
Today the site is often closed to the public because the soft sandstone erodes easily and the figures are slowly fading away. Call the Sanilac Petroglyphs State Historic Park to get access before visiting.
Read on for more, visit the Sanilac Petroglyph State Historic Park website at the State of Michigan, and also check out this 2011 Detroit News article about the difficulty of preserving this unique bit of Michigan history.