Geologically, the falls were formed at the end of the last ice age approximately 13,000 years ago. Formerly the Passaic had followed a shorter course through the Watchung Mountains near present-day Summit. As the glacier receded, the river's previous course was blocked by a newly formed moraine. A large lake, called Glacial Lake Passaic, formed behind the Watchungs. As the ice receded, the river found a new circuitous route around the north end of the Watchungs, carving the spectacular falls through the underlying basalt, which was formed approximately 200 million years ago.