A prehistoric lake once swallowed the heart of Morris County

Countries & Cities · Published 14 October 2026 Swipe more like this →

In short: Before it was a hub of New Jersey suburbs, much of this county sat beneath Glacial Lake Passaic. This massive body of water was trapped by glacial debris, creating a landscape that would eventually give rise to some of the region's most iconic swamps and rivers.

Around 13,000 BC, as the Wisconsin Glacier began to recede, it left behind a massive volume of meltwater. This water pooled to form Glacial Lake Passaic, an enormous reservoir stretching 30 miles long and 10 miles wide, reaching depths of about 200 feet. The lake was so vast that it covered what are now the Pompton Lakes, Parsippany, and extended as far east as the lava flows in western Paterson.

The lake's drainage history is a study in geological obstruction. Initially, debris from the glacier blocked the water from flowing through the Watchung Mountains near Short Hills. Instead, it was forced to drain through Moggy Hollow at its southwestern end. However, as the glacier continued to melt and retreat toward the New York State line, the lake eventually found a lower path through the Little Falls area. This shift in drainage is what ultimately led to the formation of the Passaic River.

The legacy of this prehistoric lake remains visible today in the county's geography. Following the drainage of the water, various notable wetlands emerged, such as Troy Meadows, Lee Meadows, Hatfield Swamp, Great Piece Meadows, and the Great Swamp. These areas were essentially the former floor of Lake Passaic, preserved as marshes after the waters vanished millennia ago.

Beyond these glacial features, Morris County's water systems are defined by a complex network of rivers that form most of its borders. The Pequannock River drains the northern boundary, while the Pompton and Passaic Rivers define the eastern sections. To the west, the Musconetcong River serves as a natural border, creating a region where hydrology and history are inextricably linked.

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