Monsignor Kett Playground

New York City sits atop a foundation comprised of five distinct layers of bedrock: Fordham gneiss, Manhattan schist, the Hartland Formation, Staten Island serpentinite, and Inwood marble. Of these layers, Inwood marble is perhaps the most aesthetically distinctive with its pure, crystalline white appearance. The white rock below is marble that was excavated from Inwood Hill Park and placed in Monsignor Kett Playground to serve as a symbol of this area’s geological past.

Inwood marble, named for the northern Manhattan neighborhood in which it is most visible at the surface, came into being about 450 million years ago, during the formation of the ancient supercontinent Pangea. Continents are not anchored down—they rest on plates along with pieces of the ocean floor, which in turn float on the earth’s molten core. These plates shift continuously, colliding and separating, causing earthquakes and volcanic eruptions, often forming mountain ranges with rocky peaks. It was during the coalescence of continents into Pangea that the East Coast of North America collided with a string of volcanic islands, and Inwood marble was formed. Inwood marble was originally composed of limestone and dolomite, minerals that had accumulated over the underlying gneiss foundation. When the two plates collided, the limestone and dolomite were crushed under the intense heat and pressure, forming a white, metamorphic marble.

Inwood marble, much softer than schist and gneiss, is easily eroded, particularly by acid rain. Marble’s softness is responsible for much of the geography of New York City. Veins of marble once connected Manhattan to the mainland. Over time, the marble washed away, creating pathways for the Hudson, Harlem, and East Rivers around the newly-formed island of Manhattan. Marble is now rarely found above ground. Below ground, however, miners and subway workers often find themselves surrounded by the dazzling white rock.