Lake Passaic

[1] The lake was formed of waters released by the retreating Wisconsin Glacier, which had pushed large quantities of earth and rock ahead of its advance, blocking the previous natural drainage of the ancestral Passaic River through a gap in the central Watchung Mountains.

The effect of the lake's creation permanently altered the course of the Passaic River, forcing it to take a circuitous route through the northern Watchung Mountains before spilling out into the lower piedmont.

[2] These remnants of the ancient lake provide prime wetland habitat to a variety of plants and animals while at the same time offering recreational and outdoor opportunities to residents of northern New Jersey.

The discovery of Glacial Lake Passaic is credited to Professor George Hammell Cook, once the State Geologist of New Jersey and Vice President of Rutgers University.

[3] Twelve years later, field research conducted under State Geologist John C. Smock began to uncover wave-cut terraces and other shoreline features that more conclusively established the lake's existence.

Previously, an ancestral Passaic River flowed southeast toward Raritan Bay through the gap, but as the Hackensack lobe dammed it with ice, a lake began to form, reaching depths of nearly 200 feet.

The resulting body of water formed the Chatham Stage, which initially found an outlet to the sea via the Blue Brook Valley, between the ridges of First and Second Watchung Mountain south of the Short Hills Gap.

[1] The Hackensack lobe of the Wisconsin Glacier piled a massive moraine into the Short Hills Gap and northern Blue Brook Valley, effectively creating a permanent dam even after the ice began retreating to the north.

[1] The first major drop in the level of Glacial Lake Passaic occurred as the Wisconsin Glacier melted back to reveal the Great Notch, a significant gap present in the ridge of First Watchung Mountain in Clifton.

The gap's initial opening likely caused a flood which carved the valley of Weasel Brook, but the outflow eventually migrated further north, forming the course of the Passaic River as it exists today.

[1] 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.

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.