The Palisades Sill is a

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

, 200 Ma

diabase Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grain ...

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

. It extends through portions of

New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...

and

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delawa ...

. It is most noteworthy for The Palisades, the cliffs that rise steeply above the western bank of the

Hudson River The Hudson River is a river that flows from north to south primarily through eastern New York. It originates in the Adirondack Mountains of Upstate New York and flows southward through the Hudson Valley to the New York Harbor between Ne ...

. The ideal location and accessibility of the

sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, ...

, as well as its unique features, have generated much attention from nature enthusiasts, rock climbers, and geologists alike.

Location

The outcrop of the Palisades Sill is quite recognizable for its prominent cliffs above the Hudson River; it is easily seen from the western portions of

Manhattan Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state ...

. The exposure is approximately long, most of it following the

. It first emerges in

Staten Island Staten Island ( ) is a Boroughs of New York City, borough of New York City, coextensive with Richmond County, in the U.S. state of New York (state), New York. Located in the city's southwest portion, the borough is separated from New Jersey b ...

New York City New York, often called New York City or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...

. The sill then crosses the state line into New Jersey, where

Jersey City Jersey City is the second-most populous city in the U.S. state of New Jersey, after Newark.Union City, Fort Lee, and Englewood Cliffs all lie on it. The sill eventually crosses back into New York, following the Hudson River north until reaching Haverstraw. It is at this point that the sill makes a turn to the west, where it disappears near Pomona. At this turn, the sill cuts across local

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...

, making it a dike in that area, not a sill. It has been proposed that the sill reemerges in two locations in Pennsylvania (where the outcrops are also discordant with local strata), but this idea is not generally agreed upon, and discussion of the Palisades Sill is usually limited to the exposure in New York and New Jersey. Husch, J. M., 1990. Palisades sill: Origin of the olivine zone by separate magmatic injection rather than gravity settling. Geology, v. 18, p. 699 – 702. A portion of the sill is also home to the

Palisades Interstate Parkway The Palisades Interstate Parkway (PIP) is a limited-access highway in the U.S. states of New Jersey and New York. The parkway is a major commuter route into New York City from Rockland and Orange counties in New York and Bergen County in New ...

, a stretch of road that passes through the park area preserved by John D. Rockefeller to protect its natural beauty.

Geology

The end of the Triassic Period saw large-scale rifting during the break-up of

Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

. What is now eastern North America began to separate from what is now north-western Africa, creating the young

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

. Magma was generated through

decompression melting Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma ...

, and a portion of it was intruded into the

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

and

arkose Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. Quartz is c ...

s of the Stockton Formation within the

Newark Basin The Newark Basin is a sediment-filled rift basin located mainly in northern New Jersey but also stretching into south-eastern Pennsylvania and southern New York. It is part of the system of Eastern North America Rift Basins. Geology Approximat ...

, one of the Eastern North America Rift Basins. The magma would eventually solidify and, after millions of years, the overlying rocks would be uplifted and eroded, exposing the Palisades Sill as we know it today. The composition of the sill is that of

, although its

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

assemblage is not uniform throughout the thickness of the body. The mineralogy of the sill consists principally of

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more p ...

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

, several varieties of

pyroxenes The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (F ...

, and

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...

, with minor

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more ...

titanite Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals ...

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

and

oxides An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

. Most researchers report 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

is the most differentiated. The most intriguing geological feature of the sill is a 10-meter-thick

-rich zone roughly 10 meters (30 ft) 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 (see below). There is an average

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...

thickness of 300 meters (~1000 feet), with the famous cliffs rising 100 meters (300 ft) 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 Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing occurs in many types of igneo ...

. It has been proposed that the Watchung

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

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.Kodama, K.P., 1983. ''Magnetic and gravity evidence for a subsurface connection between the Palisades sill and the Ladentown basalts''. Geological Society of America Bulletin, v. 94, no. 1, p. 151-158. 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

.Puffer, J.H., Block, K.A., Steiner, J.C., 2009. ''Transmission of Flood Basalts through a Shallow Crustal Sill and the Correlation of Sill Layers with Extrusive Flows: The Palisades Intrusive System and the Basalts of the Newark Basin, New Jersey, U.S.A.'' The Journal of Geology, volume 117, p. 139–155.

Origin

Due to the presence of the olivine-rich zone, the normally difficult task of determining the history of an

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

body becomes even tougher. *The original studies concluded that the sill was the result of a single injection of magma. The variety in mineralogy was credited to simple crystal fractionation. *The next model introduced whole rock

geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing th ...

data and determined that there were at least two separate injections; an olivine-rich magma was followed by normal tholeiitic basalt. *The third hypothesis instead proposed at least three, but probably four separate pulses, with the olivine-rich magma being the final one. This was also conjectured through the use of whole-rock chemistry. Each of these theories supported the idea of crystal fractionation playing a significant, if not total, role in the differentiation of the sill. A single source for the magma was assumed. *The latest conclusion reached for the origin of the intrusion was that the olivine in the olivine-rich zone could not have been in equilibrium with the rest of the body, indicating more than one source for the magma. This was determined through

mass balance In physics, a mass balance, also called a material balance, is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have b ...

equations. It was also suggested that lateral flow within the still-liquid body played as important a role in differentiation as fractionation. Due to the interest in the subject, and a lack of a satisfactory conclusion, research on the Palisades Sill is currently ongoing.

Notes

References

#Gorring, M. L., Naslund, H. R., 1995. Geochemical reversals within the lower 100 m of the Palisades sill, New Jersey. ''Contributions to Mineralogy and Petrology'', v. 119, p. 263 – 276. #Husch, J. M., 1990. Palisades sill: Origin of the olivine zone by separate magmatic injection rather than gravity settling. ''Geology'', v. 18, p. 699 – 702. #Lewis, J. V., 1907. The origin and relations of the Newark rocks. ''New Jersey Geological Survey Annual Report'', 1906, p. 99 – 129. #Lewis, J. V., 1908a. Petrography of the Newark Igneous Rocks. ''New Jersey Geological Survey Annual Report'', 1907, p. 99 – 167. #Lewis, J. V., 1908b. The Palisade diabase of New Jersey. ''American Journal of Science'', 4th ser., v. 26, p. 155 – 162. #Shirley, D. N., 1987. Differentiation and Compaction of the Palisades Sill, New Jersey. ''Journal of Petrology'', v. 28, part 5, p. 835 – 865. #Steiner, J. C., et al., 1992. A cumulus-transport-deposition model for the differentiation of the Palisades sill. "Geological Society of America Special Paper 268", p. 193-217. #Walker, F., 1940. The differentiation of the Palisade diabase, New Jersey. ''Geological Society of American Bulletin'', v. 51, p. 1059 – 1106. #Walker, F., 1952. Late magmatic ores and the Palisade sheet. ''Economic Geology'', V. 47, p. 349 – 351. #Walker, F., 1956. The magmatic properties and differentiation of dolerite sills – A critical discussion. ''American Journal of Science'', v. 254, p. 433 – 443. #Walker, K. R., 1969a. The Palisades Sill, New Jersey: A Reinvestigation. ''Geological Society of America Special Paper 111'', 178 p. #Walker, K. R., 1969b. A Mineralogical, Petrological, and Geochemical Investigation of the Palisades Sill, New Jersey. ''Geological Society of America Memoir'', v. 115, p. 175 – 187. {{Geology of the Newark Basin Igneous petrology of New Jersey Igneous petrology of New York (state) Triassic magmatism Sills (geology)

Location

Geology

Origin

See also

Notes

References