From the top of the cliffs overlooking Nantucket Harbor, you can see the barrier spit, Coatue, in the distance. Formed over 6000 years ago by wind, waves, and ocean currents, Coatue protects Nantucket Harbor. The harbor is a shallow embayment resulting from this protection, and home to a wide variety of sea life, including
horseshoe crabs and the
The cliffs are a result of loose materials deposited by meltwaters from the glaciers that formed Nantucket about 18,000 years ago. Nantucket Island, Martha's Vineyard, and all of Cape Cod were formed at the maximum extent - or terminal moraine - of the Laurentide ice sheet, during the Wisconsinian glacial stage. The mile-high glacier, acting like a gigantic bulldozer, scraped and pushed tons of material southward during its advance. As the climate warmed, the melting ice sheet dumped its load of sands, gravels and rocks in place, forming Cape Cod and the Islands.
With water frozen up in the giant ice sheets, sea level was 100 meters lower than it is today. Evidence for this is provided by the teeth and bones of now-extinct mastodons and mammoths that have been dredged up from the shallow seas surrounding Nantucket. Some of these fossils are only 10,000 years old.
Sea level rose rapidly as ice melted, at rates up to 15 meters per 1000 years. Nantucket and Martha’s Vineyard became islands (separated from the mainland) around 4000 years ago, although sea level continued to rise at about 1 meter per 1000 years until recently. Sea level has risen around 0.3 meters (1 foot) in the last 100 years, in part due to the increased concentrations of greenhouse gases in the atmosphere.