Geology of the New York City Region

Interstate 80 Through The New Jersey Highlands

The drive west along I-80 affords numerous spectacular views of the New Jersey Highlands. The interstate crosses the Ramapo Fault just west of Parsippany, and enters the New Jersey Highlands portion of the Reading Prong. In the Highlands, the highway rolls through the hilly uplands underlain by crystalline rocks (mostly gneiss) and across valleys blanketed with the remnants of the terminal and recessional moraines of the Wisconsin glacier. Just west of Allamuchy Mountain State Park the interstate leaves the Precambrian terrane of the Reading Prong and enters the Great Meadows, a broad, rolling valley region underlain by Cambrian and Ordovician age clastics (Hardyston Sandstone) and carbonate rocks (Liethville Limestone and Allentown Dolomite). About 4 miles west of Exit 19 (Allamuchy, NJ) the highway passes through an outlier of the Precambrian age rocks in the Jenny Jump Mountains. West of the Jenny Jump Mountains near Exit 19 (Hope, NJ), the Precambrian rocks vanish beneath sedimentary cover, and are not exposed again along I-80 until the Laramie Mountains west of Cheyenne, Wyoming. The more resistant gneiss that crops out around the Jenny Jump Mountains stands above the landscape underlain by carbonate rocks of the Great Valley to the west. I-80 cross the rolling farmland of the Great Valley and continues into Pennsylvania via the Delaware Water Gap, the gorge carved by the Delaware River through the high ridge of Kittatinny Mountain (see the discussion in the Valley and Ridge Province section).

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Last modified: 3/11/2019