Complex sentence: Mackinac Island, in the Straits of Mackinac to the northeast, appeared to have the shape of a turtle; which led the Native Americans here to believe that the turtle contributed to life's beginnings.
Split into simpler sentences: Mackinac Island, which lies within the Straits of Mackinac to the northeast, appeared to have the shape of a turtle. This image led the Native Americans here to believe that the turtle contributed to life's beginnings.