The island, along with Valcour Island south of Plattsburgh, was formed 480 million years ago as a reef during the Ordovician Period in a shallow tropical sea, near where Zimbabwe is today. At that time there was no life on dry land except for a few primitive plants such as mosses and algae. Almost all of life was in the oceans. Carbon dioxide levels were 14–16 times higher than today, with high sea levels covering much of the continents, which were mostly located south of the equator. The stromatoporoid patch reef, one of the oldest known metazoan reefs, originally stretched a thousand miles from what is now Quebec to Tennessee, but only a few remnants remain today. The island's fossil reefs are part of the Chazy Fossil Reef, a National Natural Landmark dedicated in 2009.