Watchung Mountains
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The Watchung Mountains are a pair of two 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 First Watchung Mountain (the southeastern ridge) and Second Watchung Mountain (the northwestern ridge), stretch for approximately 40 miles (64 km) from Somerville in the southwest to Paterson in the northeast. Collectively, the two 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. (304 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.
A section of the Hilltop in North Caldwell -- the site that used to contain a hospital 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.
During the Revolutionary War, General 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 and disturb British movements between New York City and Philadelphia.
Geology
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. Most of the Watchung Mountains are examples of extrusive igneous rocks, displaying characteristic columnar jointing and stacked lava flows.
The mountains have historically been the site of extensive quarrying of volcanic basalt.