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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.  (For enlarged drawings of these events,
click here.)

Together, these three events created 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.



Copyright 1996- 2005. Great Swamp Watershed Association