NEW YORK'S LANDSCAPE
In detail, the New York City region is highly varied topographically (Fig. 15.16). However, two linear topographic trends predominate: one that runs north-northeast, the other northeast (Fig. 15.17). The northnortheast trend is manifest in Manhattan and the Bronx by a series of more or less parallel, elongate ridges and valleys. Also, for much of their length, the Hudson, Harlem, and East rivers, the Palisades cliffs along the west shore of the Hudson, and Todt Hill, the central, high spine of Staten Island, follow this trend. The other, northeast trend is formed by an abrupt junction between the billy sections of northern Brooklyn and Queens and the gently sloping, almost flat areas to the southeast. It is also present, but less clearly, in the southeast margin of the hills of southeast Staten Island.
The north-northeast trend is, for the most part, the result of differential erosion of tilted or folded rocks whose general orientation parallels that of the Appalachian Mountains and whose configuration is the result of the deforming stresses that created the Appalachians over hundreds of millions of years (Fig. 15.18). Gently plunging folds composed of gneisses, schists, and marble underlie Manhattan and the Bronx (Fig. 15.19). The marble is much less resistant to erosion than the gneisses and schists, and underlies valleys, lowlands, and river channels (the Jerome Avenue Valley, the low area of Harlem, the East and Harlem rivers). The gneisses and schists form highlands or ridges: the main spine of Manhattan, Washington Heights, Riverdale, and Fordham Heights. The Palisades are the result of differential erosion of tilted diabase, a tough, dark, igneous rock that forms the cliffs, and less resistant shales and sandstones above and beneath the diabase. The serpentine which composes Todt Hill, the highest point on Staten Island, is similarly tougher and more resistant to erosion than are adjacent sedimentary rocks.
In several places in Manhattan and the Bronx, valleys and portions of river channels that run north to northwest interrupt the trend. These features follow differentially eroded fault zones. In fault zones, the rock is highly fractured, and thus much more accessible to weathering and erosion than nearby, unfaulted rocks. Examples (Fig.
15.19) include the Dyckman Street fault valley in northern Manhattan, which cuts through Washington Heights; the 125th Street fault, which underlies the center of the valley between Columbia University on Morningside Heights and the College of the City of New York on Saint Nicholas Heights; the north-south portion of the channel of the Harlem River; and the channel which separates Randalls Island from the Bronx.
The northeast trend roughly parallels the orientation of the length of Long Island and the strike of the sedimentary layers and the glacial moraines that compose the island. Brooklyn and Queens at the western end of Long Island are underlain by sedimentary layers that strike northeast and are inclined gently to the southeast (Figs. 15.18 and 15.20). These layers appear at or near the surface in the vicinity of Long Island Sound, where differential erosion has left relatively tough sands and clays at elevations of more than 60 feet above sea level. Streams flowing northward into the Sound carved a series of short, steep valleys which, subsequently widened and deepened by glacial erosion and flooded by rising sea level, formed a series of embayments (Flushing Bay, Little Neck Bay). Resting on top of these sands and clays and forming the highest elevations is a belt of glacially deposited debris composed of an unsorted, unstratified mixture of boulders, sand, silt, and clay. This debris was deposited in the interval between 75,000 and 17,000 years ago when the area was covered by a massive sheet of glacial ice. In the vicinity of New York, the ice was moving in a generally southerly direction, bringing with it a huge load of detached bedrock, sediment, and soil that it had scoured from more northerly regions. This rocky debris was dumped as the periphery of the glacier melted, forming a belt of hills known as a terminal moraine. Localities such as Forest Hills, Kew Gardens Hills, Park Slope, Prospect Park, Ridgewood and Bay Ridge rest on the terminal moraine. A continuation of the moraine and the underlying inclined sedimentary layers forms the southernmost hills of Staten Island.
Sloping gently southeastward from the edge of the terminal moraine in Brooklyn and Queens is an apron of sediment (outwash plain) that slopes very gently toward the Atlantic Ocean. This rests on the underlying inclined sedimentary layers, and was formed through the accumulation of sand, silt, and mud deposited by streams carrying away meltwaters from the glacial ice. The sharp edge between terminal moraine and outwash plain constitutes the major element of the northeast trend.
Along the Atlantic shore, loose sediment has been thrown shoreward by waves and carried westward by longshore cur-rents to form a series of barrier islands and spits, most notably Rockaway Peninsula and Coney Island. These sandy bars protect bodies of quiet, lagoonal water such as Jamaica Bay.
An understanding of the broad geomorphic framework upon which New York is built-the comparative resistance to erosion of the different rock and sediment types, their structural (geometric) configuration, the processes which operate on them, the general sequence of geologic eventsprovides the observer with a new sense of environment, another means of "finding one's way around." Rocky outcrops are not necessary as signposts: the slope of the land penetrates consciousness through the thickest layers of concrete, bricks, and asphalt. A walk or drive "uptown" (north) or "downtown" (south) in Manhattan or the Bronx is almost inevitably along or across ridges and valleys. Where the drop of the ground is pronounced, as at the 125th Street fault-valley, the subway lines, seeking to remain level, emerge from underground and proceed along trestles. In Brooklyn, the "F" train's elevated route is interrupted as it plunges into a tunnel that burrows through the terminal moraine. Many parks and cemeteries are located where the land was too steep for easy farming or subsequent urbanization. Momingside, St. Nicholas, Colonial, High Bridge, Fort Tryon, and part of Inwood parks occupy steep ground where the schists of the Manhattan Formation rise above the weak Inwood Marble (Fig. 15.19). In Brooklyn and Queens, Greenwood Cemetery, the northern parts of Prospect Park and the Brooklyn Botanic Gardens, Cypress Hills and adjacent cemeteries, Forest Park, and the southern parts of Cunningham and Alley parks all lie along the crest of the terminal moraine (Fig. 15.20). Natural creeks, subsequently much modified, have also become the sites of parks (Flushing Creek, Alley Creek, Bronx River) as have outlying marshy areas reclaimed through sanitary landfill: for example, Marine, Canarsie, and Spring Creek parks around the edge of Jamaica Bay.
The precipitous cliffs of the Palisades across the Hudson River are highly visible from many places in Manhattan and the Bronx: when looking westward down crosstown (east-west) streets; from Riverside Park; from tall buildings; when driving along highways on the east side of the Hudson River (Fig. 15.19).
Topographic barriers result in departures from the grid pattern of streets. Note how Dyckman Street, 125th Street, and Broadway are adjusted to the exigencies of the terrain (Fig. 15.21).
A host of small details of New York's geological landscape are ubiquitous, especially in the city's numerous parks. Glacial erratics (large boulders deposited by glaciers) suggest the enormous power of moving ice (Fig. 15.22A). Glacial striations and grooves and lopsided, glacially carved bed- rock hills indicate the direction of glacial movement (Fig. 15.22B). In Pelham Bay Park in the Bronx is found the southernmost example in eastern North America of a "rockbound" coast, replete with storm-eroded headlands and tiny, sheltered bays. Sand dunes, beaches, and tidal inlets and flats at Plumb Beach in brooklyn present another type of coastal expression.
The geologic and geomorphic setting of New York is clearly manifest in numerous ways: directly visible in parks and along many waterfronts; directly felt and seen in the changing slope of the land; indirectly experienced in land-use decisions and the geometry of the city plan. Knowledge of the geology of this area provides a constant stream of opportunities for identification with larger natural frameworks of space and time.
- David Leveson.