During the late Triassic and early Jurassic periods, when the North American plate separated from the African plate, an aborted rift system was created. The resulting rift valley, known as the Newark Basin, was filled with alternating layers of red bed sediment and flood basalts. Over millions of years, the rift valley was faulted, tilted, and eroded, until the edges of the hard flood basalt layers formed ridges. Prior to 20,000 years before the present, an ancestral Passaic River flowed through a gap in these ridges. This changed when the Wisconsin Glacier, a massive continental ice sheet which formed during the last ice age, advanced on the region and permanently plugged the gap with glacial rubble. As the glacier eventually melted back, water pooled behind the ridges (known today as the Watchung Mountains), forming Glacial Lake Passaic. After thousands of years, the lake drained leaving behind many swamps, including Lee Meadows.