A Guide to Central Park

Alicia SorrentiSpring 2010

Table of Contents Map of Central Park With Specific Sites The Purpose of This Guide Summary of Geologic Formations of New York

City Summary of the Tectonic History of New York Glaciations and Their Affect on New York

Section I Structures

Stop A Maine Monument

Stop B Restrooms

Stop C Greyshot Arch

Section II Formations and Tectonics

Stop D Outcrop 1

Stop D2 Outcrop 2

Stop E Outcrop 3 Rat Rock

Stop F Southwest of Umpire Rock

Stop F2 Southeast side of Umpire Rock

Stop F3 East side of Umpire Rock

Section III Glaciation

Stop F4 West side of Umpire Rock

Stop F5 North side of Umpire Rock

Stop G Pegmatite Boulder

Stop H Outcrop

Rouche Moutonnee

Geologic Time Scale

Diagrams of Orogenies

Conclusion

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The Purpose of This Guide

When one wants to study the natural world, New York City is the last place people think of to start investigating. This guide is designed to change peoples minds and prove NYC, like every other place in the world, has been greatly affected by natural processes.

Section one, will take a quick glance at building stones within the park. Even the seemingly un-natural aspects of the park have their own natural story.

Section two examines the outcroppings of bedrock, taking the reader for a journey through time and providing evidence of NYC’s active tectonic history.

The third section will inspect the evidence glaciers left behind as they made their voyage from the north pole, scouring the land beneath.

Summary of Geologic Formations Exposed in NYC Fordham Gneiss: Gneiss that is exposed in Fordham bronx. It formed over 1 billion years ago during the Grenville Orogeny, when a landmass (unknown) collided with Proto-North America. This was also part of the formation of Rodinia. The banding within the gneiss is often contorted, which is proof of the extreme heat and pressure.

Figure 1 Forham Gneiss

Manhattan Schist: Schist that is exposed in Manhattan. It formed when the Taconic Orogeny (Poto- North America collided with the Taconic Arcs), metamorphosed the parent rocks, shale, sands and basalt of the early Cambrian.

Figure 2 Manhattan Schist

Inwood Marble: Marble that is exposed in Inwood Manhattan. It formed when the Taconic Orogeny (Proto- North America collided with the Taconic Arcs), metamorphosed carbonate sedimentary deposits of the Cambrian and Early Ordovician.

Figure 3 Inwood Marble

Tectonic History of New York

Earth’s face is forever changing partly due to plate tectonics. When two plates converge, an orogeny is a common result, but when two plates diverge away from each other, also known as rifting, a basin is created. New York State has been affected by both of these processes.

Roughly one billion years ago the formation of Rodinia, a super continent, produced huge mountains known as the Grenville Mountains. It later eroded leaving sediment on either side of these once colossal mountains. The rifting of Rodina, which began 740 million years ago formed a proto-ocean; Iapetus. This proto-ocean became a basin where sediment was able to collect. A passive margin of the Cambrian and early Ordovician allowed for broad continental shelves to form, making shallow seas abundant, which is ideal for deposition of limestone and shale.

During the early Cambrian Proto-America began to subduct under the off shore Taconic Arcs. By the early odovician Iapetus began to close. Just as the formation of Rodinia created the Grenville Mountains, this convergence produced the Taconic orogeny. This collision caused scrapping up of continental mud and sand, as the convergence continued regional metamorphism began, turning sedimentary deposits into the metamorphic rocks schist and marble.

Another orogeny was on the horizon, as Avalon (future Eastern New England) subducted under Proto-America, this would result in the Acadian Orogeny, which began 400 million years ago. And last but not least, another supercontinent would form, when Proto-Africa began subducting under Proto-America 320 Million years ago. It would result in both the Alleghanian Orogeny and Pangea. Super continents are unstable and it was not long before volcanism and rifting began spitting it apart. The Atlantic ocean began to open 160 million years ago.

Glaciations Affects on New York

Aside from tectonics, earth’s face specifically New York City has also been modified by glaciers. Glaciers are slow moving ice sheets formed from compacted snow and the constant push of gravity. They can be several hundred feet thick and can cover several thousand square feet. There are two common types of glaciers; alpine glaciers, which form in mountains, and Continental ice sheets, which cover larger areas.

During the Pleistocene epoch, 1.8 million years ago to about 10,000 years ago North America experienced several advances of glaciers. The most recent sheet to affect the continent was the North American Laurentide Ice Sheet, which is broken up into episodes. The last episode was the Wisconsin. This time is a keystone in human history because this is when the land bridge between Siberia and Alaska formed, Beringia Land Bridge. This allowed humans to migrate into North America and then into South America.

Evidence of the Wisconsin episode is recorded in New York and throughout North America in the form of scours, and pooling of melted ice (lakes). When a glacier advances across a continent is scours the land, these marks are called striations and grooves. If the glacier begins to retreat a moraine is formed, which is a linear mound of un-sorted sediment. When glaciers begin to thin-out and or retreat the ice can no longer hold boulders that its been transporting, and they are deposited, this boulder will likely be different than the surrounding bedrock, it would now be called a glacial erratic.

Stop A Maine Monument Architecture

History

The Maine Monument was built to honor the 258 American Sailors that lost their lives on February 15, 1898 when the Maine Battleship exploded in the Harbor of Havana. By April of 1898 Spain declared war on America, which would end with the signing of the Treaty of Paris in December 1889.Inscribed on the monument is “valiant seamen who perished on the Maine by fate unwarned, in death unafraid” .

The Maine Monument was designed by H. Van Buren Magonigle and Attilio Piccirilli with a Beaux-Arts architectural style. The bronze figure above which is part horse and part sea creature represents Americas position as a dominant world figure. Its location was debated over for sometime, originally it was going to be placed near the harbor to welcome ships, Times Square, (then Longacre Square) was also an option, but the designers decided Columbus Circle was the best location.

Building Stones of Maine Monument

The body of the monument was constructed with Tennessee Marble. This particular rock is actually a limestone, not a marble. It formed in the Ordovician. It varies in colors of pink and gray, with styolites. Limestone is a common stone used for sculptures because it can be easily carved into shapes, but repairs on the sculpture are evidence that it is easily weathered and damaged.

The base of the monument was constructed with Milford Granite. This particular rock was probably quarried from Massachusetts. This granite was formed in the Precambrian. It has a light pink color, with spots of biotite that in some cases are oriented, giving it the name “gneissic”. Granite is durable due to its interlocking crystals and high quartz content so it is often used for bases of buildings and sculptures.

Stop B Restroom Facility Building Stones

Portland Brownstone

Fordham Gneiss

Portland Brownstone- Is quarried from Connecticut. This particular sandstone formed in the Early Jurassic. This coarse sandstone is up to 65% feldspar, making it a feldspathic arenite. The reddish color comes from an iron oxide, hematite. It was likely added later for aesthetic reasons.

Fordham Gneiss- is exposed in Fordham bronx. It formed over 1 billion years ago during the Grenville Orogeny, when a landmass (unknown) collided with Proto-North American. The banding within the gneiss is often contorted, which is proof of the extreme heat and pressure. It was likely used because it was readily available and durable. Figure 1 is a large crystal of amphibole.

Figure 1

Stop C Greyshot Arch

History and Architecture

Greyshot Arch is located within the park, Central Park West between 61st and 62nd Streets. Most of the arch was completed in 1861.It is similar to Green Gap Arch, an early design by Calvert Vaux. The arch is topped with a hundred foot long sandstone balustrade, featuring fleur-de-lis design. The eighty foot arched opening often gets flooded with muddy water, and is a constant nuisance to the maintenance team of NYC parks.

Building Stones of Greyshot ArchThe face of the arch is made of Fordham Gneiss, notice the well foliated layers of amphibole and quartz below.

amphibole

Quartz

The balustrade of the arch is made of New Bruswick Sandstone, moderately grained and tan in color. This sandstone, which is mostly quartz formed in the upper Triassic.

Stop D Outcrop

This was the first stop that we looked at outcroping of the bedrock. We determined it as bedrock and not a boulder because of its size, shape, all sides under ground, and the fact that there were several outcroppings of the same kind of rock.We identified it as schist because of its coarse interlocking crystals of quartz, mica, amphibole and feldspar, moderate foliation and overall dark color. We took the direction of foliation, it is oriented 40˚ NE; we did this so we could compare it to the other exposures and see if they are truly of the same formation.

Muscovite. The large amount of mica in these schist exposures causes it to be referred to as mica-schist

The large quartz lenses that we see throughout the outcrops of Manhattan Schist exist because there was not only shale being metamorphosed during the Taconic Orogeny, but sandstone as well. Had it not been metamorphosed, Manhattan Schist would have still looked like figure 1; which is a picture of sediment that was deposited underwater, then an orogeny uplifted it, allowing us to observe it at the surface. This outcrop was not metamorphosed, as it is still the sedimentary rocks sandstone and shale. It is known as the Martinsburg Formation, which can be seen in New Paltz New York. Martinsburg is located beneath an unconformity that is believed to be the Taconic Orogeny, meaning the deposit existed before the orogeny took place. The red arrow is sandstone, and the blue arrow is shale. Figure two on the other hand is what is seen in central park, Manhattan Schist, which was metamorphosed during the Taconic Orogney, converting the shale into schist and the sandstone into lenses of quartz, figure 2.

Figure 1. Martinsburg Formation in New Paltz, NY (Not metamorphosed)

Figure 2. A two foot long quartz lens within Manhattan Schist

Quartz lenses

Sandstone

Shale

Quartz lens

Schist, which is formed from the metamorphism of the parent rock shale

Stop D2 Out Crop

As we approached this outcrop, it looked as the first did, dark and submerged in the ground on all sides. After taking a closer look we decided it was the same, Manhattan schist. It has coarse grains of quartz, amphibole, micas and feldspar. More impressing is the fact that the foliation was in the same 40˚ NE direction, further adding to the idea that it is an exposure of Manhattan Schist.Additional investigations led us to find both quartz and pegmatitic dikes, which are vertical intrusions. The bedrock also has signs of once being ductile, in the form of folding, as seen in the picture below. The red arrow is pointing to one of the folded ridges. These folds could have only been made my extreme pressure, such as converging plates.

Stop E Rat Rock

Similar to the earlier outcrops, this stop was an outcropping of Manhattan Schist. The foliation is also oriented to the NE, about 40˚. The most interesting part of this stop was the 3D view of a small pegmatitic dike. One side of the rock is cut for a stairwell which gives a unique cross section of the outcrop, and you can see the vertical sheet intrusion (dike) from two sides the stairs and the top (bird’s eye-view).

Stop F1 Southwest Umpire Rock

This outcropping of Manhattan Schist posed a new question for Geo 13. Are we seeing evidence of glaciations in the form of grooves or is this differential erosion?We performed two tests, first we took the orientation of both the grooves and the foliation, they were a match, 38˚NE. This supported differential erosion because it is unlikely that the grooves would perfectly form parallel to the foliation. The second test we performed was a hardness test, using a nail we scratched both the highest point of the groove and the lowest point, the ridge was harder; this also supported the theory that what we were seeing was differential erosion not glacier created scours, striations. The area in question can be seen below in figure 1.

Figure 1This is an example of differential erosion, due to the foliation of the schist. When a rock becomes foliated, different minerals make up each band, and in this case you can see that the stronger minerals are more resistant to weathering and therefore are more pronounced. The red arrow is pointing to where the stronger minerals are forming a small ridge.

Stop F2 Southeast Umpire Rock

At this location we saw signs of metamorphism that took place after the initial case of metamorphism that formed the schist (metamorphic rock) to begin with. This coincides with research that NY has experienced more than one mountain building period. This folding could be evidence of the Acadian or Allegahnian orogenies. If you follow the red line over the picture below, it would take you about 20 steps to walk over it, but if you follow the yellow line that cuts across the fold, it would only take you 2 or 3 steps. There was so much pressure it reduced these layers to nearly 10% of the original length.

Stop F3 East Umpire Rock

On the east side of Umpire Rock we saw a great example of cross cutting relationships, which helps geologists give an age to rock layers. There were two dikes cutting through the bedrock, one was a small pegmatitic dike and the other was a larger fine grained granite dike. In one section of the bedrock these two dikes crossed each other. The large dike cut through the smaller one, which means the smaller one is older because it would have already had to be there in order to be intersected by the large granite dike. This out crop and set of intrusions record different periods of tectonic activity first the initial metamorphism of the schist, the metamorphism of the first intrusion, and last the intrusion of the second dike.

Above the large granite dike (red arrow) cuts through the other, smaller pegmatitc dike (blue arrow). The large dike is also not as metamorphosed which is another indicator that it came after the pegmatitic dike.

Stop F4 West Umpire RockThis is the first stop we were able to determine true evidence of glaciers, scouring called striations were present. Unlike F1 the grooves were perpendicular to the foliation (differential erosion would be parallel to the foliation.) We took a measurement of 330˚and 150˚ which coincides with research, that the glaciers came from the northwest, and traveled southeast. The blue arrows show the faint grooves and their direction, the red arrow is the foliation which is in a different direction.

Stop F5 North Umpire RockOn the north side of Umpire Rock we studied three boulders. One was the same material as the bedrock, Manhattan Schist, but the others were different. Although they to were dark in color, they were not foliated and had different crystal sizes. The boulders were noticeably more rounded than those of Manhattan Schist, which indicates traveling. The picture below is of a weathered rounded boulder. A freshly broken piece allowed one to see the fine crystals and It became more obvious that this was basalt, an extrusive igneous rock (figure 1). We asked ourselves, how did these boulders of basalt and gabbro get here? Someone could have put them there, but the fact that they are the same material of outcroppings northwest of Central Park, and again well rounded (traveled) suggests they may be erratic boulders. As the glaciers traveled, they carried sediment and boulders with them, and deposited them when they thinned out and or retracted. These boulders are believed to be from the Palisades Sill in NJ.

Figure 1. An erratic boulder, Basalt.

Stop G BoulderCentral Park was completed in 1873, before the usage of motor vehicles. Therefore this boulder that stands approximately 1.8 meters off the ground and weighs several tons, was probably not brought in during the time of design. An alternative theory of origin is that it was deposited by a glacier.

See next page for composition/ close ups

plagioclase feldspar, Approximately 1 inch in length

Muscovite

Quartz

Composition of Stop G Boulder

Stop H Outcrop

This stop was again looking at Manhattan Schist, this specific location had clear glacier scours, striations. We took a measurement of the direction of the stiations,145˚. Which is similar to that of stop F4, indicating the striations were made at the same time.

Roche Moutonnee

Rouche Moutonnee is a feature left behind by glaciers. The smooth mound as seen in the picture below is formed as a glacier flows over a rock surface, water seeps into cracks, which than freezes. Water expands when it freezes so pieces of the rock will break off, the pieces will than freeze to the bottom of the glacier. As the glacier continues to flow across the surface it takes the newly attached pieces of rock with it, in a process called “plucking”.

Direction the glacier flowed

Area that was “plucked”

Geologic Time Scale

Glaciations (Wisconsin Episode)

Alleghanian Orogeny

Acadian Orogeny

Taconic Orogeny

Deposition of the mud that would become Manhattan Schist

Grenville Orogeny

Diagram of Orogenies

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Conclusion

By the end of this trip, one should be certain that New York City has been part of an amazing journey, and still is. The world is forever changing and sometimes we are lucky enough to find clues that help piece together our world’s history.

We learned that a broad continental shelf formed on a passive margin, allowed for deposits of mud to accumulate. The mud was then transformed into a metamorphic rock (Manhattan Schist), because of the heat and pressure caused by the Taconic Orogeny. Further pressure from later orogenies caused folding to occur within the schist.

Heat and pressure would not be the last ground shaping event that New York Formations would endure, glaciers left their mark as well. Long island would not exist had the glaciers not deposited enormous amounts of sediment.

The next time you take a trip through New York City, make sure you appreciate the natural history that is preserved in Central Park.