Great Swamp Watershed — 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’s 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.
Finally, 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.
After 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.
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