ETYMOLOGY
Fordham: named for the part of the Bronx where the rock is found; Gneiss: German for banded rock
Natural History Fordham gneiss, formed over a billion years ago, is the oldest rock in New York City forming the basement on which younger rocks were deposited. Fordham gneiss was buried and metamorphosed again with the younger rocks 450 million years ago.
Description It is medium to coarse-grained, gray, metamorphic (formed at high temperatures and pressures deep within the earth’s interior) rock and is characterized by alternating light and dark folded bands of minerals. Gneiss has a foliated texture because the minerals have been flattened under great pressure and rearranged in parallel bands. These light-colored minerals include granular quartz and feldspars, alternating with darker bands bearing flakes of biotite mica or elongated grains of hornblende. These are the same minerals that form granite, an igneous rock (formed when magma cools). Gneiss forms when tremendous pressure has been applied to granite, rearranging the granite’s minerals into alternating bands.
Common Locations Fordham gneiss can be found in many places throughout the Bronx, and in City Water Tunnel No. 3 excavations in western Queens. From Inwood Hill Park in Manhattan, look north across the Harlem River at the rocky cliff painted with the blue “C” (for Columbia University): This is Fordham gneiss. In the Bronx, gneiss forms the ridge in Riverdale. It also forms the rocky ridge in the Fordham, Tremont, and University Heights sections of the Bronx. Fordham gneiss outcroppings can be found throughout Van Cortlandt Park.
KEY POINTS
• Gneiss is characterized by light and dark bands of minerals.
• Hornblende or biotite mica often gives color to the dark bands.
• Quartz and feldspar make up the light bands of minerals.
Fordham gneiss, with its characteristic alternating light and dark bands, formed more than a billion years ago and is the oldest rock in New York City, commonly found in Van Cortlandt Park and throughout the Bronx.
ETYMOLOGY
Named for the northern Manhattan neighborhood where striking exposures of the marble can be seen.
Natural History This white-to-beige marble was transformed from sedimentary into metamorphic rock about 450 million years ago. The New York City area was then at the heart of a collision zone between two tectonic plates, where intense heat and pressure transformed the original limestone and dolomite into marble.
Description Inwood marble is white to beige in color and has a sparkling, sugary appearance. Outcrop of marble once connected Manhattan Island to the Bronx. Marble, which is much softer and more soluble than schist or gneiss, erodes readily and dissolves in water. Erosion of the Inwood Marble helped create the pathways of the Hudson, Harlem, and East rivers around the island of Manhattan. Marble is found aboveground only in northern Manhattan and the neighborhood of Marble Hill in the Bronx. However, below ground, subway and construction workers often find this beautiful, white rock.
Common Locations Isham Park, at the corner of Isham Street and Seaman Avenue, and Inwood Hill Park in Manhattan; Marble Hill in the Bronx. Quarried for over three hundred years, Inwood marble was used for headstones, such as those in the Trinity churchyard cemetery on lower Broadway at Wall Street, dated 1723, 1777, 1795, and 1796. Others may be older, but their dates and inscriptions are illegible. The New York Public Library on 42nd Street and 5th Avenue was constructed of Inwood marble. An arched gateway made of Inwood marble stands at Broadway and 216th Street. This arch, constructed in 1855, originally adorned the entrance to the Seaman-Drake estate. Huge blocks of Inwood marble from a quarry at 138th Street serve as the foundation of the old Custom House erected in 1842 and still standing on the corner of Broad and Wall streets.
KEY POINTS
• Inwood marble is a white, sparkly rock.
• Early settlers quarried this marble in northern Manhattan starting in the seventeenth century.
Outcroppings of the beautiful Inwood marble can be found in two parks at the northern tip of Manhattan Island: Isham Park and Inwood Hill Park.
ETYMOLOGY
Schist: fissile, easily split
Natural History Manhattan schist was metamorphosed from its sedimentary precursor, shale, about 450 million years ago during a plate-tectonic collision. The earth’s tectonic plates, floating over the mantle, are in constant motion; they may separate, move apart, or crash together causing earthquakes, volcanic eruptions, and pushing up mountain ranges. Manhattan schist, once largely claylike shale, was deeply buried and contorted as an “exotic” tectonic plate rode up over North America and became attached to it. Consequent continental shifts pushed the schist up to the surface, exposing some of it as large outcrops.
Description Manhattan schist is a dark gray to silvery, rusty-weathering, coarse-grained rock. Schist is easily recognized by its glittering appearance, which is caused by flecks of the mineral mica within the rock. Red garnet is commonly visible in schist.
Common Locations Manhattan schist is found in many localities and varying depths throughout its namesake island: as outcrops in Central Park, Inwood Hill Park, Marcus Garvey Park, and Morningside Park; from 18 feet below the surface in Times Square to 260 feet below the surface in Greenwich Village. Where schist runs deep below the surface, no tall buildings exist because the bedrock foundation to support skyscrapers is missing. Skyscrapers predominate in Midtown and Lower Manhattan because schist is close to the surface.
KEY POINTS
• Large outcroppings of Manhattan schist can be found in Central Park.
• Glaciers formed the grooves on the surface of the schist outcroppings.
The glittering Manhattan schist formed the bedrock of the island for which it is named. Skyscrapers are built on those parts of Manhattan where this schist is close to the surface.
ETYMOLOGY
Named after the patterns in the rock that resemble snake skin.
Natural History A broad ridge of serpentine—a metamorphic rock formed approximately five hundred million years ago when heat and pressure altered rocks from the ocean floor rich in magnesium and iron—forms the spine of Staten Island. Its mineral composition includes fibrous chrysotile (commonly known as asbestos), talc, olivine (green lava grains), as well as other ferromagnesium minerals, which also contain iron and magnesium.
Description Serpentenite is greenish-black with a glossy, glazed polish. Often mottled with light and dark, its surface has a shiny, waxy appearance and a slightly soapy texture. Serpentenite is generally fine grained and dense.
Common Locations Serpentenite is found in Staten Island. In Clove Lakes Park, outcroppings of serpentine rock are at the crest of the hills. Serpentenite is visible on an abandoned highway next to the Staten Island Expressway. Todt Hill, the highest coastal point south of the state of Maine, stands 410 feet above sea level on a bedrock of serpentenite, which can be seen south of the intersection of Todt Hill Road and Ocean Terrace. “Todt,” is a Dutch word for “dead.” The hill was named by Dutch settlers because no plants would grow here. Serpentenite’s main component is magnesium, an element that, in high concentrations, will kill most plants. The magnesium in the thin soil covering the serpentenite caused the barrenness of Todt Hill.
KEY POINTS
• Serpentenite is composed of the mineral serpentine that forms as the igneous minerals olivine and pyroxene are subjected to rising temperature and pressure.
• Serpentenite can be cut, polished, and used as ornamental building stones.
• Serpentenite is bluish to greenish gray.
The greenish rock serpentenite forms a broad ridge across parts of Staten Island.
A hillside of serpentenite on an abandoned highway is visible along northbound Staten Island Expressway on the right.
Natural History The Hartland formation consists of well-layered rocks, which are now metamorphic. Originally they were a mix of sedimentary rocks and igneous basalt flows. Hartland formation is generally considered “exotic” to ancient North America and was attached to this continent during a collision between tectonic plates about 450 million years ago. Rocks of the Hartland formation are roughly similar in age to the Inwood marble and Manhattan schist.
Description The Hartland formation contains a variety of rocks, which include quartz-feldspar gneiss, schist, amphibolites, and marble. This great variety of rocks and noticeable banding are characteristic of the Hartland formation. Quartz veins and migmatite dikes are visible as they cut across the surrounding layers of rock. Migmatite is a rock that forms when granitic melt is “mixed” with solid rock. At Orchard Beach, migmatites were produced by melting rocks 450 million years ago, and younger migmatite was produced by the injection of granitic magma into cooler host rock. The younger granites are conspicuous as coarse-grained, light-colored dikes that cut across older, folded rock units.
Common Locations In New York City, the most dramatic outcroppings of the Hartland formation lie along the shores of Pelham Bay Park in the Bronx. The Hunter Island Marine Zoology and Geology Sanctuary, north of Orchard Beach, includes the northeastern shoreline of Hunter Island, all of Twin Island, Two Trees Island, and Cat Briar Island, where, at low tide, one can walk over these dramatic and beautiful outcrops of the Hartland formation.
KEY POINTS
• Hartland formation is “exotic” to ancient North America.
• Hartland formation is well banded and contains a variety of rock types.
The Hartland formation, which forms the rocky shores of Hunter and Twin Islands, Pelham Bay Park in the Bronx, is made up of various rocks, including gneiss, schist, and marble.
