Geography
Mackinac Island was formed as the glaciers of the last ice age began to melt around 13,000 BC. The bedrock strata that underlie the island are much older, dating to Late Silurian and Early Devonian time, about 400 to 420 million years ago. Subsurface deposits of halite (rock salt) dissolved, allowing the collapse of overlying limestones; these once-broken but now solidified rocks comprise the Mackinac Breccia.
The melting glaciers formed the Great Lakes, and the receding lakewaters eroded the limestone bedrock, forming the island's steep cliffs and rock formations. At least three previous lake levels are known, two of them higher than the present shore: Algonquin level lakeshores date to about 13,000 years ago, and the Nipissing level shorelines formed 4,000 to 6,000 years ago. During an intermediate period of low water between these two high-water stages, the Straits of Mackinac shrank to a narrow gorge which discharged its water into Lake Huron through Mackinac Falls, located just east of Mackinac Island.
As the Great Lakes assumed their present levels, Mackinac Island took on its current size. The steep cliffs were one of the primary reasons for the British army's choice of the island for a fortification; their decision differed from that of the French army, which had built Fort Michilimackinac about 1715 near present-day Mackinaw City. The limestone formations are still part of the island's appeal. However, tourists are attracted by the natural beauty rather than the strategic value. One of the most popular geologic formations is Arch Rock, a natural limestone arch, 146 feet (45 m) above the ground. Other popular geologic formations include Devil's Kitchen, Skull Cave, and Sugar Loaf.
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