The Narrows were most likely formed about 6,000 years ago at the end of the last ice age. Previously, Staten Island and Long Island were connected, preventing the Hudson River from terminating via the Narrows. At that time, the Hudson River emptied into the Atlantic Ocean through a more westerly course through parts of present-day northern New Jersey, along the eastern side of the Watchung Mountains to Bound Brook, New Jersey and then on into the Atlantic Ocean via Raritan Bay. A buildup of water in the Upper New York Bay eventually allowed the Hudson River to break through previous land mass that was connecting Staten Island and Brooklyn to form the Narrows as it exists today. This allowed the Hudson River to find a shorter route to the Atlantic Ocean via its present course between New Jersey and New York City.