Sanilac Petroglyphs Park Geneology

Sanilac Petroglyphs Park Geneology. . , The Sanilac Petroglyphs are carved into an outcrop of a type of rock known as the Marshall Sandstone. About 340 million years ago, this area was a river delta. Sand, mud and lime deposited by the water hardened to form the Marshall Sandstone. Outcrops of this, formation are exposed here because the glacial soils that cover much of the Lower Peninsula are very thin in this area. Other outcrops of Marshall Sandstone can be seen as you follow the trail through the park., About 12,500 years ago, glacial ice up to a mile thick covering much of Michigan was melting and retreating to the north. Glacial melt water in this area formed a riverbed up to eight miles wide called the Ubly Channel. Here in the park the channel was about one mile wide. Water flowed westward in the channel creating what is now the Cass River valley and emptied into glacial Lake Saginaw. Rocks trapped in eddies scoured out the potholes that are found in some of the sandstone outcrops., As the glaciers disappeared, the rushing water in the Ubly Channel subsided and the Cass River was formed with its headwaters a few miles east of here. The Cass River now flows into the Shiawassee River, which merges with the Tittabawassee River to form the Saginaw River flowing into Saginaw Bay.