The Sanilac Petroglyphs are carved into an outcrop of Marshall Sandstone, a rock formed about 340 million years ago when this area was a river delta where sand, mud, and lime deposited by water hardened into stone. These outcrops are exposed here because the glacial soils covering much of the Lower Peninsula are very thin in this area, and other Marshall Sandstone outcrops can be seen along the park trail. About 12,500 years ago, glacial ice up to a mile thick that covered much of Michigan was melting and retreating northward, and glacial meltwater formed a riverbed up to eight miles wide called the Ubly Channel, which was about one mile wide here. Water flowed westward through the channel, creating what is now the Cass River valley and emptying into glacial Lake Saginaw, while rocks trapped in eddies scoured potholes into some of the sandstone outcrops. As the glaciers disappeared, the rushing water in the Ubly Channel subsided and the Cass River formed, with its headwaters a few miles east of here; it now flows into the Shiawassee River, which merges with the Tittabawassee River to form the Saginaw River flowing into Saginaw Bay.