12 February 1909, Science, vol. 29, no. 737, pg. 280:
It is now certain, however, that the southern end of the island is not wholly underlain by Manhattan schist as formerly mapped, but that the east side is made up of the usual succession of folded Fordham gneiss, Inwood linestone and Manhattan schist.
The Catskill Water Supply of New York City:
History, Location, Sub-Surface Investigations and Construction
by Lazarus White
A very interesting formation of limestone (a few hundred feet wide), known as the "Inwood," was uncovered. This bed, found also under the Croton dam, under the Harlem River, and even as far south as Delancey Street at the lower end of Manhattan Island, separates the old Fordham gneiss from the younger Manhattan schist.
16 May 1915, New York Times, pg. SM15:
THE Island of Manhattan, as is well-known, is in the main composed of hard rock. In the upper parts pf the city it crops out in vacant lots in bold pinnacles, and even in the more densely populated parts it is continually being revealed as old buildings are torn down and foundations are sought for modern skyscrapers. In the main it is composed of what geologists term Manhattan schist, and is a hard substance containing here and there a number of garnets of low grade and little value. It is as a rule solid enough and can stand any amount of boring and blasting, and with all the tests given it affords reliable support for the heaviest of structures.
Manhattan Schist in Bennett Park
NYC Dept. of Parks & Recreation (December 2001)
Schist, which can be seen in Bennett Park, is an extremely strong and durable rock type. Deep below the buildings and busy streets of New York City, beneath the labyrinth of subway tunnels and stations, lies the geologic foundation that makes New York City unique in the world. This foundation consists of five bedrock layers: Fordham gneiss, found primarily in the Bronx; Manhattan schist, in Lower and northern Manhattan; the Hartland Formation, in central Manhattan, the Bronx, Brooklyn, and Queens; Staten Island serpentinite, in Staten Island; and Inwood marble, in Manhattan and beneath the rivers that surround it. But it is Manhattan schist, the most prevalent bedrock in Manhattan, that makes the city's famed skyline possible.
Manhattan schist was formed about 450 million years ago, making it the second oldest of New York City's bedrocks, after Fordham gneiss. At that time, the continents of the world existed as a single supercontinent, called Pangea. The continents and oceans are not anchored down in a fixed position — they rest on landmasses called tectonic plates, which float on the earth's molten core. The plates shift continuously, colliding and separating, causing earthquakes and volcanic eruptions and forming jagged mountain ranges.
A continental collision between what is now the East Coast of North America and the floor of the Atlantic Ocean, pushed a layer of shale — a sedimentary rock composed of clay and sand — from the ocean floor roughly nine miles into the molten core of the earth. There, the intense heat and pressure transformed the shale into a conglomeration of minerals, including quartz, feldspar, hornblende, and mica. The resulting metamorphic rock is known as schist. Subsequent continental shifts pushed the schist back to the surface. In some areas the schist has even been exposed. The massive rock formation that rises out of Bennett Park is a visible sign of the Manhattan schist bedrock below. Schist can be recognized by its glittering appearance, which is caused by flecks of white mica within the rock.
Manhattan schist is found at various depths — from 18 feet below the surface in Times Square to 260 feet below in Greenwich Village. Where bedrock is far below the surface, skyscrapers are not practical because it is too difficult to reach the schist that provides structural stability and support. Consequently, there are few tall buildings in Greenwich Village, but skyscrapers stand in dense clusters in midtown where schist lies close to the surface. The schist formations in Bennett Park display a rock whose importance cannot be overestimated — New York City reaches its towering heights because of this strong foundation.
(Reproduction of a Parks Department historical sign. Reprinted with permission of the City of New York/Parks & Recreation.)