Geology of the New York City Region

42. South Mountain Reservation

Interstate 78 crosses the southern end of the Watchung Mountains. West of Newark, it passes through the Oranges, then passes through a gap in the ridges of the 1st and 2nd Watchung Mountains. The Milburn Gap was carved by an ancient river whose course was changed by the advance of the Pleistocene glaciers. West of Milburn the interstate rises through a series of road cuts carved through a thick section of the Preakness Basalt of the 2nd Watchung. Once again, it is not safe or legal to stop and examine cuts along the highway. Fortunately, there are numerous locations to examine volcanic rocks and the intervening red beds in scenic settings on public lands nearby, including the South Mountain Reservation and the Watchung Reservation.

South Mountain Reservation is a 2,048 acre multiple use area (recreation, watershed, and nature preserve) that encompasses a portion of 1st Watchung Mountain in Milburn, New Jersey (Figure 96). The land encompasses the traprock hillsides along the Rahway River. Washington's Rock, the hilltop overlooking Milburn, was used as a lookout by Washington's Continental Army to watch the movement of British troops in the Newark valley to the east. At the time of the Revolution, most of the valley to the east of the Watchung Mountains had been stripped of forests for use as lumber and for agriculture, so that sentries had a good view from the top of the 500 foot high ridge. The forests on the ridges were heavily lumbered in the late 19th Century for firewood and paper making. At the turn of the century, the Essex Count Park Commission organized the purchase of the land. Restoration of the forests included the replanting of rhododendron and mountain laurel groves in the 1910s, and the planting of red and white pine by CCC projects in the 1930s. The park has an extensive system of trails that wind along the Rahway River and along the hilltop of 1st Watchung Mountain.

Map of South Mountain Reservation, 1st Watchung Mountain, Milburn, New Jersey
Figure 96. Map of South Mountain Reservation (1st Watchung) near Milburn, New Jersey.

In terms of distance from Manhattan, this is one of the closest scenic natural areas to the city. A hike on South Mountain in the late spring or fall is highly recommended. The preserve is accessible via the Milburn Station on the NJ Transit - Morris & Essex Lines from Penn Station in Manhattan. The Locust Grove trailhead parking area is located just across the road from the train station. By car, take I-78 west to Exit 49. Head east on Springfield Avenue about 1 mile and turn left on Milburn Avenue. In about two miles turn right on Locust Grove. Go through the underpass beneath the train. The trailhead parking area is in the wooded area directly across the street. A suggested route is as follows (see the trail on Figure 96). From the trailhead parking area, walk northward along a gravel road, keeping the Rahway River on your left. Numerous paths cross the road, but keep on the paths that follow the valley. You will pass a couple ponds where the river has been dammed. About two miles into the hike you will come to a path to scenic stone bridge across the stream. Do not cross the bridge, but continue following the river trail northward about a quarter mile. A trail to the right leads to Hemlock Falls, a falls at the base of a long cascade that tumbles through a ravine carved in the lower flow Orange Mountain Basalt (Figure 97). The trail to the top of the falls leads to a expansive barren surface where the tops of columnar joints in the basalt are well exposed. A large glacial erratic rests along the stream just upstream from the lower falls (Figure 98). This is a very scenic destination for a rest or a picnic! The return route involves walking uphill along a wooded path on the south side of the falls. Follow any of the broad paths southward up to the top of the ridge. All the trails eventually intersects Crest Drive, a closed park road that runs along the southeast side of the mountain. Walk south road to the overlooks on the south end of South Mountain overlooking Milburn and the Newark Valley beyond. The Manhattan skyline dominates the eastern horizon.

Hemlock Falls on clumnar-jointed Orange Mountain Basalt, South Moutain Reservation
Figure 97. Hemlock Falls cascades off the columnar-jointed basalt (Orange Mountain Basalt of the 1st Watchung) in the South Mountain Reservation.

Glacial erratic and columnar joints in Orange Mountain Basalt
Figure 98. Columnar joints in the Orange Mountain Basalt just above Hemlock Falls in the South Mountain Reservation. The large rounded boulder to the right is a large glacial erratic.

South Orange Avenue leads eastward uphill through a pine forest planted by the Civilian Conservation Corp, then crosses a foot bridge over the road. The falls are a short walk down to the base of the hill. Like most parks in New Jersey, deer are plentiful, so are the deer ticks that carry lyme disease. Be sure to check for ticks after hiking.

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Last modified: 3/11/2019