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Washington Rock on the first ridge, looking south

The Watchung Mountains are a group of three long low ridges of volcanic origin, between 400 ft. (122 m) and 500 ft. (152 m) high, lying parallel to each other in northern New Jersey in the United States. The ridges, known as Orange or First Watchung Mountain (the southeastern ridge) and Preakness or Second Watchung Mountain (the northwestern ridge), stretch for approximately 40 miles (64 km) from Somerville (in Somerset County) in the southwest to Paterson (in Passaic County) in the northeast. The discontinuous ridge formed by Long Hill, Riker Hill, Hook Mountain and Packanack Mountain is sometimes referred to as Third Watchung Mountain and lies on the northwestern side of Second Watchung.

Trees in the Watchung Reservation between the first and second ridges

Collectively, the three ridges demarcate a geologic barrier on the western edge of the plains west of the Hudson River. The highest point in the Watchungs is High Mountain in Wayne, which is over 1,000 ft. (300 m) in elevation. Other notable summits are Garrett Mountain in Paterson and the Hilltop in Verona, Cedar Grove, and North Caldwell. All along the Watchungs there are county reservations, one of which in Union County is called the Watchung Reservation.

A section of the Hilltop in Verona — the site that used to contain a sanatorium for tuberculosis sufferers — is the highest point in Essex County; the county hospital was built there because the high elevation provided clean, mountain air away from the cities to the east, that is beneficial for people with TB.

Since the 1970s, the area has seen extensive suburban growth, but before the construction of the interstate highway system, the Watchungs actually held back urbanization.

Washington Valley in Bridgewater between the first and second ridge

During the Revolutionary War, General George Washington used the protection of the Watchung mountains to erect the first and second Middlebrook encampment. This position on the high ground also allowed him to monitor the area between Perth Amboy and New Brunswick and to identify and disturb British movements between New York City and Philadelphia.^[1]

The Newark Basin contains Traprocks, which are mineral-rich mafic volcanic rocks. Their resistance to erosion (relative to the surrounding sandstone and shale) produce elevated regions above the surrounding terrain. The volcanic rocks of the Watchung Mountains were formed as mafic volcanic material extruded on the surface as surface flows aprroximately 185 million years ago. Most of the Watchung Mountains are examples of extrusive igneous rocks, displaying characteristic columnar jointing and stacked lava flows. It has been proposed that the basalts of the Watchung Mountains are extrusive eruptions of the same magma that created the Palisades Sill several miles to the east.

The mountains have historically been the site of extensive quarrying of volcanic basalt.

• Geology of the Newark Basin and Connecticut River Basin
• U.S. Geological Survey - South Mountain Reservation

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