Great Swamp Watershed: Geological, Natural, and Human History

Geological History

New Jersey’s Great Swamp is nestled within a 55-square-mile natural basin, just 25 miles or so from New York City. It is a quiet, undisturbed place today, but it wasn’t always that way.

Millions of years ago, the continent of Africa collided violently with North America, pushing up great mountains to the north and west. Erosion has since cut them down to size.

Later, when Africa broke away, hot molten rock flowed up from the earth’s interior, creating the Watchung Mountains to the south and east. Again, erosion has taken its toll.

Drawing of glacierFinally, about 18,000 years ago, a glacier advancing from the north ceased its forward motion and began to melt, leaving behind a great pile of rock and soil along a line from Chatham to Morristown.

The swamp basin itself was scooped out by the last glacier to pass through the area some 16,000 years ago. It stopped when it ran into the Watchung Mountains to the south. The great pile of earth and rocks it pushed before it remained behind after it melted. Today that pile is known as Basking Ridge.

Drawing of lakeAfter the glacier began to recede, the melt-off formed Glacial Lake Passaic. Eventually the water pressure forced a gap through Long Hill and formed Millington Gorge and the Passaic River. The lake emptied out (almost) leaving behind the basin that contains Great Swamp today. The basin is called a watershed, because all of its streams flow into a single body of water — the swamp itself. From Great Swamp, the water exits south through Millington Gorge and becomes the Passaic River.

Natural History

Though called a swamp, the Refuge and adjoining public lands encompass a great variety of other habitats as well: grasslands, sandy knolls, ponds, brooks, marshes, woodlands and ridges. In this rich mix, Great Swamp displays plants from both southern and northern zones, including 215 species of wildflowers.

Great Blue Heron flying with speared fish photo by Blaine Rothauser

Great Blue Heron flying with speared fish
photo by Blaine Rothauser

This varied vegetation provides a diversity of homes for the 220 bird, 33 mammal, and many reptile, amphibian, and fish species in Great Swamp. Of these species, 26 are listed by the state of New Jersey as threatened or endangered. Great Swamp also is home to one of the state’s largest breeding populations of Eastern Bluebirds, and it boasts a thriving Great Blue Heron rookery, which hatches about 80 chicks each summer.

Human History

As the glacier melted, it created a 30-mile-long lake called Glacial Lake Passaic. Over time, the lake has drained to become wetlands, part of which form Great Swamp. In the process, a great variety of plant and animal species — including humans — have come to call Great Swamp home.

There is evidence that humans lived in Great Swamp as early as 12,000 years ago, when mastodon and giant beaver still inhabited the area. When the first Europeans arrived in the 1600s, they encountered a group of Native Americans who called themselves the Lenape, which means “original people.” For a time the two groups lived peacefully together, but disease and pressures for the land eventually forced the Lenape to abandon their home. The European settlers built small towns and villages, many of which remain today: Green Village, New Vernon, Basking Ridge, Meyersville, and others.

The Great Swamp watershed figured prominently in the Revolutionary War. Continental Army troops spent eight years in the Watershed and George Wshington wintered here twice. Its high western rim provided a strategic lookout to the east and New York City, where the British troops were quartered.

In the 19th century, area residents logged the forests of Great Swamp for firewood and building materials, and tried with limited success to drain the marshlands for farming. Also during this time, the area became a retreat for wealthy New Yorkers, who often built great estates in and near the watershed.

The 20th century saw the unveiling of the most ambitious plan to put Great Swamp to human use. In 1959, the Port Authority of New York proposed to build an international jetport in the swamp, with four 10,000-foot runways. The proposal would have bulldozed many of the hills, filled in the swamp, and demolished 700 homes and other structures.

For four years, local residents fought the plan — and finally won. Thanks to their acquisitions of swampland, Great Swamp became a National Wildlife Refuge in 1964. The jetport plan was abandoned in 1968 when part of the Refuge became a Federal Wilderness Area.

Today, the 7,500-acre National Wildlife Refuge is the crown jewel in a remarkable array of protected areas in the Great Swamp watershed. Many of these are listed elsewhere on this website.



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