Sea stacks are somewhat common geological phenomena in the Straits area. Arch Rock is probably the most popular and recognizable example from Mackinac Island. These stacks are usually made of limestone. As the water rushed around them, the softer material washed away, leaving the harder surfaces intact. So yes, that means that the water used to be 75-100 feet higher than it is right now! There are several other sea stacks in the area, including Castle Rock in St. Ignace. This pretty little plate is a souvenir someone might have taken home after clambering up the rocks and gazing at the gorgeous views. This is what you bought to commemorate a trip in the days before selfies!