Nantucket’s ponds and bogs were formed by the retreat of the last glacier that covered the New England area, which began melting approximately 12,000 years ago in response to a gradual warming in the earth’s climate. Meltwaters from the glacier formed river valleys that ran from the glacier to the ocean, which was more than 45 miles south of Nantucket at the time. As the ice melted, sea level rose and flooded all but the highest elevations surrounding Nantucket, isolating it from the mainland. Ocean currents deposited sand into the areas where these valleys met the shore, blocking their southern ends and causing them to fill up with freshwater. Thus, a system of long, narrow ponds running perpendicular to Nantucket’s south shore was formed. The larger of these include Long Pond, Hummock and West Hummock Pond, Mioxes Pond, Miacomet Pond and Weweeder Pond.
The glacier also left behind large chunks of ice scattered across the northern portion of the island where it reached its southernmost advance. Their massive weight formed depressions in the ground that intersected the water table. Ponds formed in such a manner are called kettle holes, and are characterized by having no inflowing or outflowing streams. Instead, the pond water level fluctuates with the groundwater level. Examples of such ponds include the Pout Ponds, Gibbs Pond and Almanac Pond.
Nantucket’s ponds provide habitat for many species of wildlife. White-tailed deer use them as a source of fresh drinking water and often browse on the nearby vegetation. Many also support diverse populations of fish, amphibians, and reptiles, including yellow perch, pickerel, snapping turtles, painted turtles, spring peepers, green frogs and northern water snakes. Aquatic animals and submerged pond vegetation are important sources of food for black-crowned night herons, ospreys, hooded mergansers, buffleheads, Canada geese, mallards and many other species of birds, who use these areas for nesting, feeding, and as temporary stop-over sites along their migration routes.
Immediately after a pond is formed, nature begins to reclaim the land at its shallow edges. Emergent wetland plants that colonize this area form a thick mat of root stalks and vegetation just below the water’s surface. This becomes covered with plant debris that settles and decays, gradually replacing the shallow water with damp, organic soil. Wetland shrubs such as sweet pepperbush and highbush blueberry are then able to move into these areas, crowding out aquatic plants and thereby reducing the size of the pond.
This ongoing process causes the pond to become smaller and shallower, creating optimum conditions for Sphagnum, a genus of mosses that can hold up to twenty-five times their weight in water. As the top portions of these plants grow, the underlying layers become deprived of sunlight and die. Thick mats of dead Sphagnum gradually accumulate and become compressed by the weight of the waterlogged plants above them. Highly acidic conditions prevent this material from completely decaying, and it builds up and forms extensive layers of organic material called peat. At this point, natural succession has transformed the pond into a bog, and most of the visible areas of open water are gone. Other plants such as cotton grass and swamp azalea are then able to colonize the bog’s peaty surface.
In the early stages of this process, mats of Sphagnum, often with the roots of other bog plants entangled in them, float on the surface of the open water. Such bogs are called quaking bogs because their surfaces are unstable. In later stages, the bog depression becomes completely filled with layers of peat. Bogs that are located in hilly terrain are often entirely surrounded by a moat, where surface water from the adjacent upland collects after it runs downhill.
Peat was historically excavated from many of Nantucket’s bogs during times when firewood was unavailable. When Nantucket was blockaded during the Revolutionary War, peat was mined, dried and burned as a heating fuel. Today, bogs and other such wetland systems are protected from such activities by state and federal wetlands protection legislation.
As Sphagnum accumulates in a bog, acids that are produced during the process of plant decay build up. The thick layers of peat further serve to isolate the surface of the bog from the groundwater and its minerals and nutrients. Most plants cannot tolerate these acidic, nutrient-poor conditions, with the exception of a few species that have developed special survival adaptations.
A group of carnivorous bog plants obtain scarce nutrients from insects and other organic material that they capture. The brightly-covered leaves of the pitcher plant form a water-holding vessel with nectar glands that lure insects into this cavity, where they fall in and are “digested” by special enzymes produced by the plant. Similarly, sundews bear a rosette of leaves with sticky, glandular hairs that entrap insects that land on them.
Leatherleaf, sheep laurel, and bog rosemary are woody shrubs that belong to the heath family, a group of plants that are often found in bogs. These species conserve nutrients by having evergreen leaves, thus eliminating the need to invest energy in their annual production. They also form extensive root systems that efficiently absorb oxygen and nutrients with the assistance of beneficial fungi associated with their root hairs.
Several rare and endangered species occur in bogs and shallow ponds with peaty, sandy soils. Spotted turtles, named for their yellow polka-dot spotted shells, are regionally rare and currently under consideration for endangered species protection listing. These small turtles, which feed on insects and aquatic vegetation, lay their eggs in shallow nests excavated in sandy upland areas adjacent to ponds, bogs and other wetlands. Torrey’s beak-rush and two-flowered rush are grass-like plants that also occur in these habitats. Both of these species are listed as “Endangered” in Massachusetts.
One of North America’s native bog plants is the cranberry. Although cranberry vines occur naturally in many smaller bogs in the northern portion of the island, they were not actively cultivated here until 1857. The 234-acre, organically certified Milestone Cranberry Bog, located on the Milestone Road near Siasconset, was established around 1865 and is the only actively harvested cranberry bog on the island. The Windswept Bog, located on the Polpis Road, is a 40-acre man-made bog that was constructed at the turn of the century. Cranberry cultivation at Windswept Bog was retired in 2019; the former bogs are now in the process of being restored to naturally functional wetland habitat. Both of these properties are owned by the Foundation.