Around 21,000 B.C., the Wisconsin Glacier covered about half of Morris County. The terminal moraine went from Hackettstown east to north of Budd Lake, east to Rockaway and Denville, then southeast to Morristown then south to the south end of Great Swamp. When the glacier melted around 13,000 B.C. the melt water created Glacial Lake Passaic. The lake extended from what is now Pompton Lakes through Parsippany south to the south end of Great Swamp. From Parsippany the lake went east to the lava flows of western Paterson. This lake was thirty miles long and ten miles wide (36 km by 12 km). The depth was about . When the Wisconsin glacier cover Morris County the ice sheet was about deep. Due to debris from the glacier, the lake was unable to drain through the Watchung Mountains near Short Hills. Instead, it drained through Moggy Hollow at the southwestern end of the lake. But when the glacier melted and receded to the New York State line, the lake drained though the Little Falls area, as this was lower in elevation than Moggy Hollow. And thus the Passaic River formed.