Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Location of bridge tower: 40o 51' 10" N, 075o 57' 31" W.
Virtually everyone in New York City is familiar with the Palisades, a long line of cliffs on the west shore of the Hudson River. Opposite Manhattan, the cliffs are in New Jersey but further north they cross the state line into New York. The Palisades are probably the most famous example of a sill, a sheet of igneous rock intruded between other rock layers.
See the labeled diagram below for the geologic history of the Palisades.
About 200 million years ago, when North America and Africa (Gondwanaland) were just beginning to break apart, the very first breaks formed a series of fault basins along what is now the east coast of the North America. As the crust spread apart, the rocks on one side of the fault tilted downward. Erosion filled the fault basins with sandstone and conglomerate. The basin in New Jersey is called the Newark Basin. Other basins extend from New Brunswick in Canada to Georgia. The sedimentary rocks in the Newark Basin are called the Newark Group and are shown in orange, below. Below them are more ancient metamorphic rocks called the Manhattan Schist, which formed during the collision of North America and Europe about 450 million years ago. By 180 million years ago the mountains formed in that earlier collision had long since been worn flat by erosion.
The fault on the west side of the Newark Basin is called the Ramapo Fault. The rocks west of the Ramapo Fault are uplifted to form the Ramapo Mountains. The rocks in the Ramapo Mountains are much older than the Manhattan Schist, about 900 million years or so.
200 million years ago, while the Ramapo Fault was active, magma (shown in purple) came up along the fault and erupted on the surface as a series of lava flows. Since the lava flows are resistant to erosion, they stand above the surrounding land as a series of ridges called the Watchung Mountains. Magma also forced its way between layers in the Newark Group to create the Palisades Sill. Since the rocks of the sill are also resistant to erosion, the Palisades remain as a line of steep cliffs.
During the Ice Age, erosion by glaciers and melt water soured away the soft Newark sedimentary rocks below the Palisades Sill to create the present channel of the Hudson River. The bottom of the Hudson River is actually below sea level for a long distance upstream. The very first exit south from the George Washington Bridge in New Jersey leads to a road beneath the bridge where the bottom of the Palisades Sill and the sedimentary rocks below can be seen.
The next fault basin to the south is in Pennsylvania, and there, too, igneous rocks form ridges because of their hardness. Some of the ridges, called Little Round Top and Cemetery Ridge, were the focus of the fighting in the Battle of Gettysburg during the Civil War.x
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Created 30 June 2009, Last Update 15 January 2020