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Travels in Geology September 2004

The center of it all: visiting Central Park

As most native New Yorkers and visitors to New York City know, the city offers seminal architecture, world class shopping and well-known sites, such as Ellis Island, the Brooklyn Bridge and the Statue of Liberty. Among these celebrated sites is the crown jewel of Manhattan's park system, Central Park. Integral to park planners Frederick Law Olmstead and Calvert Vaux's design are deftly incorporated outcrops of the metamorphic rock schist lining the edges and dotting the landscape of the 843-acre park.

Glaciers left their mark all over New York's Central Park. To the right is a glacially scoured outcrop of gneiss and schist on the west side of the park, with the Midtown skyline behind. Photo courtesy of the U.S. Geological Survey.

Every day, thousands of people walk their dogs, jog and play sports in Central Park, casually passing the half-billion-year-old rocks that at one point were nearly 30 kilometers deep within the Earth. "The park presents a treasure trove of important geologic information," says geologist Charles Merguerian of Hofstra University and Duke Geological Laboratory, a geotechnical firm based in New York City. "We are fortunate to have such a phenomenal teaching laboratory preserved for us to enjoy."

In Merguerian's field guide to Central Park, he recommends starting at the entrance off 64th Street on the east side of the park, where the zoo is the first feature you encounter. If you turn right and walk north, passing beneath the arch with the dancing bear, the outcrops begin to appear: low ground-hugging hillocks shadowed beneath sycamores and oaks and great multistory slabs of stone bathed in bright light, ideal for climbing and exploration.

Starting at the Central Park entrance nearest the zoo — 64th Street the first outcrop you encounter is the Hartland formation, pictured here. The Hartland is one of two different schists you will see when visiting the park. Its dark gray color and easily visible layering distinguish it from outcrops of the second schist formation in the park, the Manhattan. Photo by Sarah Todd Davidson.

The rocks in the south of the park come from sediment that originally was deep in the ocean. When a volcanic arc collided with the east coast of North America, it pushed sediment along in front of the arc, driving it deep into Earth. The heat and pressure of Earth's interior altered the physical features of the once sedimentary rocks to create the rocks we can now investigate. Eventually these metamorphosed rocks were returned to the surface through uplift and erosion, resulting in our ability to observe them before a Broadway show, or following a visit to the Metropolitan Museum of Art.

The outcrops retain the dark gray color of their origins, but metamorphosis created an abundance of micas, the shiny, planar minerals that reflect like glass in the sun, as well as parallel lines within the rocks, called foliation, which impart a layered look to the outcrops. White streaks of quartz cut through the formations and are a result of molten rock forced within the cracks of the schist, creating an igneous rock called a pegmatite when the melt solidified. You can also see swirling patterns within the outcrops, a consequence of partial melting at depth, and folding features that display the effects of pressure bending the rock.

Wandering the meandering paths of the park, you can see features in one rock that distinguish it from the one before, but certain outcrops are more interesting than others. Across the baseball field from the children's carousel is on especially intriguing piece of stone. Appropriately named Umpire's Rock, it is "the most spectacular outcrop in the south of the park," Merguerian writes in the field guide. Here you can observe many of the impressions of the rock's more recent past.

To the left is Umpire's Rock, where visitors can observe signs of the park's glacial past: a glacial trough, erratics and glacial grooves. To the right you can see rocks of the Manhattan formation, the once undivided name given to all the visible schist in New York City. Its pink color and lack of layering distinguish it from the Hartland. Photos by Sarah Todd Davidson.

Not to be outdone by the effects of the Earth's interior are glacial events that made their own mark on the rocks once they reached the surface. On Umpire's Rock you can observe a glacial trough, where the ice gouged away a smooth-edged path through the outcrop, as well as glacial erratics — rocks that are not original to the outcrop, but carried and deposited in that location by the moving ice. Also visible are glacial grooves, thin lines cut out of the rock by the passage of rocks and sediment within the ice sheet, in two directions, which helps demonstrate the existence of separate glacial events.

Possessing features that would interest a geochemist or a five-year-old, a structural geologist or a grandmother, this open-air laboratory is hands-on and interactive. The proximity of Central Park to the everyday routines of New Yorkers, as well as the areas of typical interest to visitors of the city, allow fieldtrips to the city's geological spots for 10 minutes or the entire day. The park offers a unique opportunity to investigate the foundation of the island of Manhattan, the very center of New York City, breaching the surface for our aesthetic and scientific benefits.

Sarah Todd Davidson
Geotimes contributing writer

Links:

Central Park Conservancy
City of New York Parks and Recreation
USGS Central Park Geology
Merguerian's Central Park Geology Field Guide


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