Watchung Mountains

Jesse Fried • Jun 14, 2017

[need an image, preferably a geologic cross section]

The Watchung Mountains form a protective shield around the interior of Northern NJ. A band of ~2 or 3 flat-topped ridges located 15 to 20 miles inland from the Hudson River and Raritan Bay, they run roughly N-S through Passaic and Essex counties, curving west to run NE-SW through Union County, and abruptly bending NW-SE, at almost a right angle, in Bound Brook, running a few additional miles west into Somerset County. For virtually their entire length, their maximum height is between 500 and 650 feet above sea level, and their most prominent topographic feature is a steep slope near the top of the first ridge, facing outward to the east and south.

Revolutionary War general and first US president George Washington took advantage of the natural barrier provided by the Watchungs in his decision to quarter his troops at Morristown during the winter of 1779-1780. In fact a minor battle, the Battle of Springfield, occurred in the spring of 1780 when British troops encamped at Staten Island tried to reach Washington's army through Hobart Gap, near present-day Summit. A hastily-drawn British reconnaissance map, now a very cool artifact of local history, shows much detail about the roads crossing the Watchungs.

The barrier has had a great impact on the region's development throughout history. In the 19th and early 20th century, the expanding metropolis was for the most part contained to the lowlands east and south of the first ridge. Even today, the Watchungs define a barrier between denser, more urbanized suburbs and more sprawling, often newer suburbs. Some of the fanciest towns of suburban NJ take advantage of scenic, somewhat isolate upland locations in and around the range: Short Hills and North Caldwell are prime examples. In the towns that encompass both a portion of Central NJ's lowlands and a portion of the Watchungs, one usually sees the wealth of neighborhoods increase as one rises higher above the valley: Upper Montclair, Newstead in South Orange, the trend across the Wyoming section of Millburn and some cool mansions in Mountainside are good examples.

In the 19th century, mills and mill towns developed on the numerous steep rivers and streams around the Watchungs, taking advantage of proximity to large population centers. Most significantly, Paterson grew up around the mighty waterfall where the Passaic River crosses the bassalt rock layer that forms the First Watchung ridge. These mills contributed to NJ's economy, and, it might be argued, set the state's identity as an industrial powerhouse even once water power was no longer relevant.

Passes Through the Ridges

The various gaps in the Watchung Mountains have been sought as routes for travel throughout history. Here is a summary of the passages and their uses:

From north to south:

Gap/RouteLocationEarly uses19th Century Railroads NJ Transit Rail LinesInterstate HighwaysOther roads
Passaic River "Water Gap"PatersonMorris CanalDelaware, Lackawana and Western RailroadInterstate 80
Great NotchLittle FallsThe New York, Erie and Western RailroadMontclair Boonton LineRoute 46
Hobart GapSummitMinisink PathDelaware, Lackawana and Western Railroad's Morris and Essex DivisionNJ Transit's Morris and Essex LineInterstate 78Route 24
Routes dipping south to skirt the WatchungsSomervilleCentral Railroad of NJ, Lehigh Valley RailroadRaritan Valley lineRoute 22

Interstate 280 carved its own man-made pass into the ridge, a very impressive road cut that reminds us that NJ was not subdued for suburban development quite so easily.

To a large extent, development in northern New Jersey followed these paths. The routes across the Highlands are also important, but that region has always been less central.


For more detail, see the Piedmont section. For now, let's focus on the fact that the Watchung mountains are made of volcanic rock: hard, erosion resistant basalt, in contrast to the sandstone and mudstone surrounding them. The long, straight, flat-topped ridgeline with one steep side and one gradual side has to do with the geometry of the volcanic layer. It was originally a flat-lying lava flow or intrusion, now tilted downward to the west, so that it intersects the erosional surface at an angle. The steep slopes are at the point where the basalt layer has been broken through down to the sandstone beneath it. The sandstone erodes quickly, leaving the thickness of the hard basalt exposed.

Sunset view
Jesse Fried
June 7, 2017, 12:59 p.m.

From the Palisades' western edge in Hudson County, the sun sets behind the Watchung Mountains. They are just a low ridge on the horizon, in back of the meadowlands.