Map - Warren County, New Jersey (Warren County)

Warren County is a county located in the U.S. state of New Jersey. As of the 2020 U.S. census, the county's population was 109,632, representing an increase of 940 (0.9%) from the 108,692 residents counted at the 2010 census. The county borders the Delaware River and Easton, Pennsylvania in the Lehigh Valley to its west, the New York City metropolitan area to its east, and The Poconos to its northwest.

Warren County is the only county in New Jersey which is not part of the combined statistical areas of New York City or Philadelphia. The most populous place is Phillipsburg, with 14,950 residents at the time of the 2010 census while Hardwick Township had both the largest area 37.92 sqmi and the fewest people with 1,696 residents. Its county seat is Belvidere.

Warren County was incorporated by an act of the New Jersey Legislature on November 20, 1824, from portions of Sussex County. At its creation, the county consisted of the townships of Greenwich, Independence, Knowlton, Mansfield, Oxford, and the now defunct Pahaquarry. The county is part of the North Jersey region.

According to the 2010 census, the county had a total area of 362.86 sqmi, including 356.92 sqmi of land (98.4%) and 5.94 sqmi of water (1.6%).

Warren County has rolling hills, with the Kittatinny Ridge in the west. Allamuchy Mountain and Jenny Jump Mountain are part of the New York – New Jersey Highlands, also known as the Reading Prong. Around 450 million years ago, a chain of volcanic islands collided with proto North America. The chain of islands went over the North American plate, thus the Highlands were created from the island rock and so was the Great Appalachian Valley. The Highlands is Allamuchy Mountains and the Jenny Jump Mountains. Thus, Warren County was born.

Around 400 million years ago, a small continent that was long and thin collided with proto North America. This created the Kittatinny Mountains as land compressed from the collision. The quartzite that was lying in a shallow sea over top of the Martinsburg shale, folded and faulted due to pressure and heat. The quartzite lifted, thus the Kittatinny Mountain was born.

The final collision was when the African plate collided with the North American plate. This was the final episode of the building of the Appalachian Mountains. Then the African plate tore away from North America.

Then the Wisconsin Glacier covered the northern part of the county from 21,000 to 13,000 BC. This glacier covered the top of Kittatinny Mountain and carved the terrain in the northern part of the county. The terminal moraine runs from north of Belvidere to the south of Great Meadows to north of Hackettstown, to the north of Budd Lake. Blairstown Township, Hope Township, half of Independence Township, part of White Township, and all of Allamuchy Township was covered by the Glacier. When the glacier melted, a lake was formed at Great Meadows. Slowly the lake drained leaving a large flat area filled with organic material.

The county is drained by three rivers. All three rivers are shallow and narrow. They are fresh water rivers that are excellent for fishing. The Paulins Kill drains the western portion of the county. The river flows from Newton to Blairstown Township, and then through Knowlton Township where it drains into the Delaware River. The Pequest River drains the middle of the county flowing from Andover Township through Allamuchy, then to Independence Township where it turns west and flows through White Township and then empties into the Delaware River at Belvidere. The third river is the Musconetcong. Starting at Lake Musconetcong, the river divides the county from Morris and Hunterdon. This river drains the southern portion of the county and empties into the Delaware River near Warren Glen.

Warren County is located in two valleys of the Great Appalachian Valley. The first is the Kittatinny Valley, which is in the northern part of the county, and the Lehigh Valley, which borders the southern part of the county.