Arch Rock is a geological formation on Mackinac Island in Michigan. It is a natural limestone arch formed during the Nipissing post-glacial period, a period of high Lake Huron levels following the end of the Wisconsin glaciation. To this day Arch Rock stands on the Lake Huron shoreline 146 feet above the water. Limestone breccia is not an ideal material for natural bridges, and this type of formation is quite rare in the North American Great Lakes region.