The Palisades (Hudson River) EarthCache

The Palisades which are also called the New Jersey Palisades or the Hudson River Palisades, are a line of steep cliffs along the west side of the lower Hudson River in Northeastern New Jersey and Southeastern New York in the United States. The cliffs stretch north from Jersey City about 20 miles to near Nyack, New York, and visible at Haverstraw, New York. They rise nearly vertically from near the edge of the river, and are about 300 feet high at Weehawken, increasing gradually to 540 feet high near their northern terminus. North of Fort Lee, the Palisades are part of Palisades Interstate Park and are a National Natural Landmark. The Palisades are among the most dramatic geologic features in the vicinity of New York City, forming a canyon of the Hudson north of the George Washington Bridge, as well as providing a vista of the Manhattan skyline. They sit in the Newark Basin, a rift basin located mostly in New Jersey.

The basalt cliffs are the margin of a diabase sill, formed about 200 million years ago, at the close of the Triassic period by the intrusion of molten magma upward into sandstone. It is then that large-scale rifting resulted in the break-up of Pangaea. That is where eastern North America began to separate from what is now north-western Africa, creating the young Atlantic Ocean. The molten material cooled and solidified before reaching the surface. Water erosion of the softer sandstone left behind the columnar structure of harder rock and the Sill that exists today. The cliffs are about 300 feet thick in sections and originally may have reached 1,000 feet).

The end-Triassic extinction event that coincided with the formation of the Hudson Palisades, Central Atlantic magmatic province, 200 million years ago ranks second in severity of the five major extinction episodes that span geologic time. The most severe extinction in the past 500 million years was the Permian–Triassic extinction event, informally known as the Great Dying, coincided with flood basalt eruptions that produced the Siberian Traps, which constituted one of the largest known volcanic events on Earth and covered over 770,000 square miles with lava. Franklyn Van Houten did trailblazing research on a rock formation known as the Newark Basin. His discovery of a consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle".

It has been observed and reported by many researchers that, generally speaking, the sill becomes progressively differentiated as one moves away from either the upper or lower contact. The "sandwich horizon" is the term given to the central region where both cooling fronts met; it is here where the diabase is the most differentiated.

The Palisades Sill is a Triassic, 200 Ma diabase intrusion. It extends through portions of New York and New Jersey. It is most noteworthy for The Palisades, the cliffs that rise steeply above the western bank of the Hudson River.

Diabase is a common rock type. It occurs mostly in shallow intrusions (dikes and sills) of basaltic composition. It grades to basalt when it solidifies rapidly and to gabbro when more time is given to the crystals to grow. The term “microgabbro” is sometimes used to refer to such rocks.

Diabase is composed of plagioclase feldspar (mostly labradorite) and pyroxene (augite). The crystals that make up dolerite are usually visible to the naked eye, but sometimes porphyritic rocks of basaltic composition with pyroxene and especially plagioclase phenocrysts are also named that way. Ophitic texture (randomly oriented plagioclase laths enclosed by pyroxene grains) is a characteristic feature of diabase. Minerals of lesser importance are magnetite, olivine, ilmenite, hornblende, biotite, chlorite, etc.

The most intriguing geological feature of the sill is a 10-meter-thick olivine-rich zone roughly 30 feet from the lower contact. The modal percent of olivine goes from 0-2% within the main body of the sill to up to 28% within this layer. It is the origin of this layer, and subsequently, the sill as a whole, that has generated much of the attention, as well as the varying origin theories proposed for the intrusion.

There is an average stratigraphic thickness of ~1000 feet, with the famous cliffs rising 300 feet on average above sea level. The intrusion dips between 10 and 15 degrees westward for most of its length. It has been determined through stratigraphic studies that the sill was intruded at a depth of approximately 3-5 kilometers. These studies also concluded that the sill was emplaced in a position nearly identical to its current one (10-to-15-degree dip); this is further confirmed by the still-vertical orientation of the columnar jointing.

It has been proposed that the Watchung basalt flows of the Watchung Mountains are extrusive eruptions of the same magma that created the Palisades Sill. Magnetic and gravity measurements have indicated the presence of a large subsurface dike between the Palisades intrusion and the Ladentown basalt, an extrusive body of Watchung basalt north of Suffern, New York. More recently, the various Watchung flows have been correlated to geochemically distinct layers within the Palisades sill, bolstering the theory that eruptions of the Palisades magma were responsible for the episodic flood basalts of the Newark Basin.

1. When glaciers passed through this area, all the loose sediments, rocks, plants, and wildlife were swept away, leaving these stark vertical cliffs. What was the effect of the bedrock here due to the weight of the ice? Hint: We are looking for the texture of the bedrock.

2. By walking along this path can you find any Diabase? Please describe what you find.

3. At this location, you can see distinct vertical layers. How wide do you estimate each layer is?

4. To prove that you were here, please post a picture of yourself or your GPS near GZ with your choice in background. Any other photos of the area are also greatly appreciated.