Mackinac Island's dramatic rock formations and shoreline are made of brecciated limestone that originated more than 350 million years ago, when the Straits of Mackinac were underlain with a thick layer of rock salt several hundred feet below the surface. Swirling underground waters slowly dissolved the rock salt, leaving large caverns below the surface. Eventually the cavern roofs collapsed, leaving chimneys or stacks of broken rock. Circulating water cemented the rocks of the stacks together, producing formations harder than the surrounding unbroken rock. The erosive power of the ancient Great Lakes washed away the softer stone and exposed the breccia features.