The most upstream of three lakes, the Millington Stage was probably 50 feet deep at maximum, occupying the basin west of Long Hill, the southernmost ridge of Third Watchung Mountain. Unlike the other two lakes, the Millington Stage drained over the trap rock crest of Long Hill. But even rock could not contain one of the last pieces of Glacial Lake Passaic. By about 14,000 years ago, water spilling through a gap in Long Hill carved the Millington Gorge, which lowered the level of the lake to form the present-day Great Swamp.