NEW YORK CITY BEDROCK

 

FIELD TRIP STOP – NEW YORK CITY BEDROCK (Lower Paleozoic Metamorphics)

 

LOCATION: Outcrops are found in the northernmost tip of Manhattan between 161st St. and Fort Tryon and Inwood Hill Parks, as well as in the southernmost Bronx between 161st and 181st Streets. More detailed locations can be found in Schuberth (1968).

 

GEOLOGIC FEATURES: Lower Paleozoic Metamorphics, Granitic Dikes, Foliation, Folds, Boudinage, Glacial Striations and Potholes.

 

DESCRIPTION:  There are three major rock formations that make-up the bedrock of New York City.  In stratigraphic order, these are the Fordham Gneiss, Inwood Marble, and Manhattan Schist.  These three metamorphic rock types are referred to as the New York City Group.  Each of these are thought to be the result of intense heat and pressure (metamorphism) possibly of Cambrian and Ordovician sedimentary rocks that accumulated in a marginal basin prior to mountain building events (the Taconic and Acadian Orogenies.)  Others have suggested a late PreCambrian origin for the Fordam Gneiss and Manhattan Schist (Brock, P.C. and P.W.G. Brock).

 

The Fordham Gneiss is well-foliated and banded. Light colored bands are mainly composed of quartz and orthoclase (orange) feldspar while the darker bands are predominantly Biotite Mica (black and flakey) with some plagioclase feldspar and hornblende. These bands are characteristically folded and contorted. The heat and pressure responsible for the initial foliation also is responsible for the subsequent putty-like (plastic) flowing of the bands.  Granite pegmatite intrusions are scattered within the Fordham Gneiss, having originated from the melting of rock deep within earth. The Fordham Gneiss is thought to have resulted from the metamorphism of mainly sandstones in a sedimentary basin prior to mountain building.

 

The Inwood Marble is non-foliated and composed of the mineral calcite with some dolomite (magnesium-rich calcite). Some minor minerals (such as mica) can provide a foliated look to the marble. Some narrow veins of quartz and feldspar can also be present.  Outcrops of marble are not common due to its tendency to chemically weather. The Inwood Marble is thought to have formed largely from the metamorphism of sedimentary limestones and dolomites.

 

The Manhattan Schist is a well-foliated mica schist. In places, layers of granite, follow the foliation, often pinching and swelling along their lengths.   This formation forms the majority of the bedrock in Manhattan.  It is commonly thought to have been the result of the metamorphism of predominantly sedimentary shales.

 

Rocks of the New York City Group are thought to be time-equivalent to sedimentary rocks found to the west that were not affected by the mountain building tectonics of the present day NYC area.

 

STUDENT QUESTIONS:

(1) Describe the process of Metamorphism.

(2) How can minerals involved in metamorphism change in composition and/or crystal size.

(3) What is foliation?

(4) How does the direction of regianal pressure affect the direction of foliation?

(5) Why does marble weather faster than Gneiss?

(6) What is Boudinage and how does it form?

(7) Describe the variations in regional pressure that could account for the rock structure observed in Figure 3.

(8) How can glaciers polish and striate the underlying bedrock?

(9) When did glaciers cover the NYC area?

SELECTED REFERENCES:

-Brock, P.C. and P.W.G. Brock. 2001. Field Guide for Long Island Geologists Field Trip, Oct. 27, 2001. School of Earth and Environmental Sciences, Queens College, Flushing N.Y. 2001.

-Schuberth, Christopher J. 1968, The Geology of New York City and Environs. The Natural History Press, Garden City, New York. 304 pp.

PHOTOS:

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Figure 1 - The Fordam Gneiss, lowest member of the New York City Group.  Note both the near-vertical foliation throughout and the presence of boudinage in the right side of the photo.

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Figure 2 - Large coarsely crystalline pegmatite dike in the Fordham Gneiss crossing the foliation at a high angle.

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Figure 3 - The Fordham Gneiss forming the crest of a large-scale anticline. Regional pressures occurring subsequent to the formation of the foliation have then folded the foliation.

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Figure 4 - Outcropping of the light-colored Inwood Marble.

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Figure 5 - Boudinage and foliation of mica in the Manhattan Schist.

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Figure 6 - Foliation in the Manhattan Schist along with a small dike cross-cutting the foliation.

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Figure 7 - Vertical wall of the Manhattan Schist. Notice the Normal Fault oriented at about 20 degrees to the vertical causing an offset and accompanying drag fold in the Injection Dike.

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Figure 8 - Near vertical foliation in the Manhattan Schist

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Figure 9 - Glacially polished surface with striations caused by glacial advance over an outcrop of the Manhattan Schist.  Direction of glacial advance can be determined by rubbing your hand along the striations. The rock surface will feel rough up-glacier and smooth in the direction of glacial advance,

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Figure 10 - A sub-glacial pothole in the Manhattan Schist formed by the flow of water at the bottom of the glacier.