Nantucket’s barrier beaches occur at the interface between the island and the surrounding sea. Coatue, Great Point, Coskata and the Haulover collectively comprise an ever-changing fragile strip of sand that shelters Nantucket Harbor from the open waters of Nantucket Sound and the Atlantic Ocean. These beaches have been deposited and reshaped over the last 6,000 years by ocean currents moving sand northward in a process called littoral drift, forming Nantucket’s northernmost place – Great Point. An east-west current called long shore drift transported and deposited sand in a similar process to form the adjoining barrier beach known as Coatue. Smaller barrier beaches also occur at Smith Point and Eel Point, on the southwest and northwest corners of the island.
One of the unique characteristics of Coatue is its scalloped harbor shoreline. The six points, or cuspate spits, have interested geologists for years. They were produced and are maintained by wind, wave and tidal action. Because of the harbor’s orientation, the prevailing northeast winter winds and southwest summer winds hit Coatue at oblique angles. The resulting waves erode sand from the curves, or bends, and deposit it on the points. In contrast, tidal currents flowing in and out of the harbor erode sand from the points and deposit it back into the bends. These opposing forces have been in equilibrium for hundreds of years.
The processes that formed and maintain these barrier beaches are ongoing. Coatue’s westward movement is slowed by man-made jetties and tidal flow in and out of the harbor. However, long shore drift continues to deposit sand on Coatue’s north beach, causing it to slowly migrate to the northwest, into Nantucket Sound. The thin and elongated sand bridge that links Great Point to the rest of the island is known as The Galls, meaning “weak place.” It is appropriately named because it is periodically washed over and broken through by ocean storms.
Although storm events are dramatic and destructive, they are a natural part of the dynamic processes that form and maintain barrier beaches. In the Northeast, ocean waves tend to be shallow and gentle during the summer, causing sand to build up and form a wide and gently sloped beach. The stronger, more destructive waves that hit the coast during winter erode the beach, making it narrow and very steep. Thus, Nantucket’s beaches are constantly changing.
The landward side of a barrier beach is characteristically covered by a system of sand dunes deposited by wind and wave action. The primary dune is located closest to the ocean and is the most exposed to the elements. Several other distinct dune ridges behind the primary dune collectively form the more sheltered interdune habitat. A salt marsh is often located on the back side of the barrier beach dune system.
During major storms, large waves often break through and destroy a portion of the dune system, creating an overwash fan of sand. These low areas are important for dissipating wave energy, thus minimizing damage to adjacent dunes. Over time, beach grass and other pioneering vegetation will colonize the overwash fan, trapping sand and forming a new dune. In this way, the beach gradually heals from the damage done by destructive storm events.
Nantucket’s beaches provide important habitat for several species of rare and endangered shorebirds, including the least and common terns, American oystercatchers and piping plovers. These species lay their highly-camouflaged eggs in nests that consist of slight depressions in the sand. Nesting shorebird management and protection efforts are an important focus of the Foundation's science and land management staff. On Coatue, the beach and interdune habitats support the largest colony of nesting herring and greater black-backed gulls on Nantucket.
Plants that grow on the open beach are also well adapted to this harsh environment. They obtain fresh water by absorbing rain and by forming long, penetrating roots. American beach grass is the most common plant found growing on the beaches of the Northeast. It forms an extensive network of underground stems called rhizomes, which send up new shoots that hold windblown sand in place and promote the formation of new dunes. In the more protected areas behind the dunes, low shrubs capable of growing under nutrient-poor conditions serve to further anchor the sand.
Salt-spray rose, bayberry, and beach plum are salt-tolerant shrubs that shed their leaves annually, adding nutrients to the sand as they decay. As the soil becomes more stable and enriched, other species such as eastern red cedar, black huckleberry, and low-bush blueberry are able to colonize these areas, resulting in a diverse interdune plant community. Several rare and unusual plant species occur on the island’s dunes, including eastern prickly-pear cactus and pink lady’s slipper.