From Wikipedia, the free encyclopedia. Windley Key Fossil Reef Geological State Park is a Florida State Park located at mile marker 85.5 near Islamorada. It was a former quarry used by Henry Flagler in the early 1900s to help his building of the Overseas Railroad.Following the railroad's completion, it was a source for decorative stone pieces called Keystone.t was a former quarry used by Henry Flagler in the early 1900s to help his building of the Overseas Railroad. Following the railroad's completion, it was a source for decorative stone pieces called Keystone.