The swamp basin itself was scooped out by the last glacier to pass through the area some 16

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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.

The drawing below shows the glacier at its greatest extent. (view looking north)

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 swamp.

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